Motion and Forces: 40-Question Lesson Test

Answer: C

Correct answer: 10 m. Distance is the total length of the path followed, so the two legs are added: 6 m + 4 m = 10 m. Direction does not matter for distance.

• A: Multiplying the two legs has no physical meaning; path lengths are added.
• B: This is the size of the displacement (6 m minus 4 m), not the path length.
• C (correct): Distance adds every meter of the path regardless of direction: 6 m + 4 m = 10 m.
• D: This counts only the first leg and ignores the 4 m traveled on the way back.

Answer: A

Correct answer: Velocity. A vector has magnitude and direction. Velocity tells both how fast and which way, while speed, mass and time have size only.

• A (correct): Velocity has both a size and a direction, so it is a vector.
• B: Temperature is a number with a unit and no direction, so it is a scalar.
• C: Speed tells only how fast, with no direction, so it is a scalar.
• D: Mass is an amount of matter with no direction, so it is a scalar.

Answer: C

Correct answer: Newton. Newton's second law, F = ma, defines force as mass times acceleration, so the unit of force is a kilogram multiplied by a meter per second squared. One newton is the force that gives a 1 kg mass an acceleration of 1 m/s², and it is named for Isaac Newton. Keep related units separate: the kilogram measures mass, meters per second measure speed, and the joule, a newton times a meter, measures energy.

• A: Meters per second measure speed or velocity.
• B: The kilogram measures mass.
• C (correct): A newton is kg·$\frac{\mathrm{m}}{{\mathrm{s}}^2}$, the unit of force.
• D: A joule measures energy or work.

Answer: B

Correct answer: It can continue at constant velocity. Newton's first law says an object keeps its velocity unless a net force acts, so zero net force means zero acceleration, not zero motion. A moving object with balanced forces keeps the same speed and direction, and a resting one stays at rest. The common misconception is that motion needs a constant push, but friction is what usually stops things. Any speeding up, slowing down or turning would require an unbalanced force.

• A: Net-force balance does not remove mass.
• B (correct): Newton’s first law allows uniform straight-line motion.
• C: A zero net force does not require zero velocity, and slowing down would need a net force.
• D: Increasing speed needs nonzero acceleration and therefore a net force.

Answer: B

Correct answer: 15 m/s. One kilometer is 1,000 m and one hour is 3,600 s, so divide km/h by 3.6.

• A: This comes from multiplying by 3.6, which converts m/s to km/h instead.
• B (correct): Dividing by 3.6 converts km/h to m/s: 54 / 3.6 = 15 m/s.
• C: Dividing by 60 changes hours to minutes only and ignores the kilometer-to-meter change.
• D: Dividing by 10 is not a valid conversion between km/h and m/s.

Answer: A

Correct answer: Toward Earth’s center. Gravity is an attraction between masses, so the ball is pulled toward the center of Earth's mass, which is what down means everywhere on the planet. The direction depends only on where the ball is, not on how it moves. A ball thrown upward still feels a downward pull, which is why it slows, stops and falls back. Its path may curve, but the gravitational force keeps pointing toward Earth's center.

• A (correct): The gravitational acceleration is toward Earth’s center.
• B: Near Earth gravity acts down, not up.
• C: Gravity is not controlled by the ball’s travel direction.
• D: A tangential direction would not describe gravity generally.

Answer: D

Correct answer: 0 m/s². Acceleration is the change in velocity divided by time. The train keeps the same speed and the same direction, so the change in velocity is zero and the acceleration is 0 m/s².

• A: Nothing in the situation changes the velocity, so there is no acceleration at all.
• B: This confuses speed with acceleration; a high speed does not mean the velocity is changing.
• C: This is the free-fall acceleration due to gravity, which does not apply to a train on level track.
• D (correct): Neither the speed nor the direction changes, so the velocity is constant and the acceleration is zero.

Answer: A

Correct answer: On two interacting objects. Newton's third law says forces come in pairs: when your hand pushes on a wall, the wall pushes back on your hand with equal strength in the opposite direction. Because the two forces act on different bodies, they never cancel each other. That is the key to telling a third-law pair from balanced forces, which act on the same object. The law holds whether the objects are moving, at rest or not touching at all, as with gravity.

• A (correct): The paired equal-and-opposite forces act on different interacting bodies.
• B: Forces on one object can balance but are not a third-law pair.
• C: The law applies to moving objects as well.
• D: Interactions occur without touching ground.

Answer: A

Correct answer: They land at the same time. Gravity pulls harder on the heavier ball, but that ball also has proportionally more inertia, so the two effects cancel. With air resistance negligible, both accelerate at g and hit the ground together.

• A (correct): Both fall with the same acceleration g because the heavier ball's larger weight is matched by its larger inertia.
• B: Lighter objects do not fall faster; without air resistance every object falls with the same acceleration.
• C: Fall time does not depend on mass at all, so it cannot shrink in proportion to mass.
• D: This is the common misconception that heavier objects fall faster; extra weight is offset by extra inertia.

Answer: D

Correct answer: Its mass in kilograms. Mass measures how much matter an object contains and how strongly it resists changes in motion, so it depends only on the object itself. Weight is the gravitational force on that mass, W = mg, and changes with location. On the Moon, g is about one-sixth of Earth's, so a 60 kg astronaut still has 60 kg of mass but weighs only about one-sixth as many newtons. Confusing mass with weight is the usual mistake.

• A: Gravitational acceleration differs between Earth and Moon.
• B: Weight depends on the local gravitational field.
• C: Support force can change with gravity and conditions.
• D (correct): The object’s intrinsic mass remains the same.

Answer: B

Correct answer: 15 km/h. Total distance is 9 km + 6 km = 15 km. Total time is

or 1 hour. Average speed is 15 km / 1 h = 15 km/h.

• A: This uses only the first leg's distance over the full hour instead of the total 15 km.
• B (correct): Total distance 15 km divided by total time 60 min (1 h), including the rest, gives 15 km/h.
• C: This is 15 km divided by 60 minutes, which is 0.25 km per minute, mislabeled as km per hour.
• D: This leaves out the 15-minute rest; average speed counts every second that passes, including stops.

Answer: C

Correct answer: 2 m/s². Acceleration is the change in velocity divided by the time taken.

• A: This divides time by the velocity change, which flips the formula.
• B: This is the change in velocity; it still has to be divided by the 6 s it took.
• C (correct): The change in velocity is 16 - 4 = 12 m/s, and 12 m/s divided by 6 s is 2 m/s².
• D: This divides the final speed by the time and forgets to subtract the starting speed.

Answer: B

Correct answer: Staying at rest. The slope of a position versus time graph is the velocity. A horizontal line has zero slope, so the position stays the same and the object is at rest.

• A: Steady negative velocity would show as a straight line sloping downward.
• B (correct): A flat line means the position is not changing, so the slope, which is the velocity, is zero.
• C: Steady positive velocity would show as a straight line rising upward, not a flat one.
• D: Speeding up shows as a curved position graph whose slope keeps increasing.

Answer: C

Correct answer: 30 m. Displacement equals the area between a velocity graph and the time axis. Here the region is a triangle.

• A: This is the slope of the line, which gives the acceleration in m/s², not the displacement.
• B: This treats the area as a full rectangle, as if the object moved at 6 m/s the whole time.
• C (correct): The area under the line is a triangle: one half times 10 s times 6 m/s equals 30 m.
• D: This reads the height of the line as a position; the height is the velocity at the end.

Answer: D

Correct answer: 19 m/s. For constant acceleration the final velocity is the starting velocity plus acceleration times time.

• A: This is only the gain from acceleration; the starting speed of 4 m/s must be added.
• B: This adds the acceleration to the starting speed without multiplying it by the 6 s.
• C: This mixes in the displacement formula, adding one half times a times t squared to the starting speed.
• D (correct): Using v = v0 + at gives 4 + 2.5 × 6 = 4 + 15 = 19 m/s.

Answer: B

Correct answer: 3 m/s². First find the net force, then divide by mass.

• A: This adds friction to the push instead of subtracting it, though friction acts in the opposite direction.
• B (correct): The net force is 60 - 24 = 36 N, and 36 N divided by 12 kg is 3 m/s².
• C: This divides mass by net force, which turns the relationship upside down.
• D: This uses the 60 N push alone and ignores friction; acceleration depends on the net force.

Answer: D

Correct answer: 40 N. Weight is the force of gravity on a mass, found with W = mg: mass in kilograms times gravitational acceleration in meters per second squared. Multiplying kg by m/s² gives kg·m/s², which is the newton, so the units confirm the setup. Here 4 kg × 10 m/s² gives 40 N. The common slip is to report the mass, 4, as the weight; kilograms measure matter, while newtons measure force.

• A: Division by g would not give weight.
• B: Four is mass in kilograms, not force in newtons.
• C: Adding 4 and 10 is dimensionally wrong.
• D (correct): W=mg=4×10=40 N.

Answer: D

Correct answer: 20 m. From rest, the drop distance is one half times g times the time squared.

• A: This leaves out the one-half factor in the displacement formula.
• B: This forgets to square the time, using 0.5 × 10 × 2.
• C: This is only the distance fallen during the second second, not the total from the start.
• D (correct): Starting from rest, the distance is one half times g times t squared: 0.5 × 10 × 4 = 20 m.

Answer: C

Correct answer: Exactly 80 N. The bookcase stays at rest, so the net horizontal force is zero. Static friction adjusts to match the 80 N push, up to its maximum, so it is exactly 80 N in the opposite direction.

• A: If friction were larger than the push, the bookcase would accelerate backward, which it does not.
• B: Static friction acts even without sliding; it is what keeps the bookcase from moving.
• C (correct): The bookcase is not accelerating, so static friction matches the push exactly, pointing the other way.
• D: If friction were smaller than the push, a net force would remain and the bookcase would start moving.

Answer: D

Correct answer: Drag equals her weight. Terminal speed means her velocity is no longer changing, so her acceleration is zero. Zero acceleration requires zero net force, so drag up must equal weight down.

• A: If drag were larger, the net force would point up and she would be slowing down.
• B: Drag grows with speed, so at terminal speed it is large enough to balance her weight, not zero.
• C: If drag were smaller, a downward net force would remain and she would keep speeding up.
• D (correct): At constant velocity the net force is zero, so the upward drag exactly balances the downward weight.

Answer: B

Correct answer: The book's upward gravitational pull on Earth. Third-law partners act on two different objects and are the same kind of force. Earth's gravity pulls down on the book, so the partner is the book's gravity pulling up on Earth.

• A: This is the partner of the table's normal force on the book, not of the book's weight.
• B (correct): A third-law pair involves the same two objects and the same type of force, so Earth pulls the book and the book pulls Earth.
• C: This force balances the weight, but it acts on the same object and is a different type of force, so it is not the pair.
• D: Friction acts parallel to the surface and has nothing to do with pairing the book's weight.

Answer: D

Correct answer: 6 kg·m/s. Momentum is mass times velocity.

• A: This divides velocity by mass instead of multiplying the two.
• B: This takes half of mass times velocity; momentum has no one-half factor.
• C: This is one half times m times v squared, which is the kinetic energy calculation, not momentum.
• D (correct): Momentum is mass times velocity: 0.15 kg × 40 m/s = 6 kg·m/s.

Answer: A

Correct answer: The net external force is zero. Constant velocity means unchanging speed and direction, so the acceleration is zero, and Newton's second law, F = ma, then requires zero net force. The cart keeps moving because of its inertia, not because something keeps pushing it. The common misconception is that motion needs a forward force larger than the others; such a force would make the cart speed up. Its momentum is not zero, since it has both mass and velocity.

• A (correct): Constant velocity in an inertial frame implies zero net force.
• B: Moving mass has nonzero momentum.
• C: Velocity is not changing, so acceleration is zero.
• D: A larger forward resultant would accelerate the cart.

Answer: D

Correct answer: A straight line tangent to the circle. Circular motion needs a net inward force, here supplied by string tension. When the string breaks, that force disappears, and by the first law the ball continues in a straight line in the direction it was moving, tangent to the circle.

• A: Moving toward the center would also require an inward force, which is gone.
• B: No outward force acts on the ball; it simply stops being pulled inward.
• C: Circling requires an inward force, and the string can no longer supply it.
• D (correct): With no inward force, the ball keeps the velocity it had, which points along the tangent.

Answer: C

Correct answer: Tension is less than the weight. Moving up and slowing down means the velocity is up but the acceleration is down. The net force must point down, so the weight is larger than the cable tension.

• A: The cable still supports the elevator; it only needs to pull a bit less than the weight to slow it.
• B: Equal forces would mean zero acceleration and constant speed, but the elevator is slowing.
• C (correct): Slowing while rising means the acceleration points down, so the downward weight must exceed the upward tension.
• D: That would give an upward acceleration, which happens when the elevator speeds up while rising.

Answer: C

Correct answer: The water’s force on swimmer. By Newton's third law, when the swimmer's hands and feet push water backward, the water pushes the swimmer forward with an equal force. The two forces act on different objects, and only the force on the swimmer can move the swimmer. A body cannot propel itself with its own internal forces; it needs something external to push against. Rowing a boat and walking on the ground work the same way.

• A: The other object, water, exerts the paired force.
• B: Gravity points generally downward.
• C (correct): Water exerts an equal-and-opposite forward force on the swimmer.
• D: This is the backward force acting on water, not the forward reaction on swimmer.

Answer: D

Correct answer: 15 N. Perpendicular forces combine with the Pythagorean theorem.

• A: Subtracting works only for forces in opposite directions.
• B: Simply adding works only for forces in the same direction, not for forces at right angles.
• C: Averaging the two forces has no physical meaning for combining them.
• D (correct): For forces at right angles, use the Pythagorean theorem: the square root of 81 + 144 is the square root of 225, which is 15 N.

Answer: C

Correct answer: 25 N upward. The book is at rest, so the upward normal force must equal the total downward force. Weight is

plus the 5 N push gives 25 N upward.

• A: This balances only the hand's push and leaves out the book's weight.
• B: This balances only the weight and ignores the student's extra 5 N downward push.
• C (correct): The table must balance both the 20 N weight and the 5 N push, so it pushes up with 25 N.
• D: This subtracts the push instead of adding it, but the push is downward like the weight.

Answer: A

Correct answer: It is speeding up while moving west. Compare signs: both the velocity and the acceleration are negative, so both point west. When acceleration points the same way as the motion, the object speeds up, even though the acceleration is negative.

• A (correct): Velocity and acceleration have the same sign, so they point the same way and the car gains speed westward.
• B: Constant speed in a straight line would require zero acceleration.
• C: The negative velocity means the car is moving west, not east.
• D: Slowing happens only when acceleration and velocity have opposite signs.

Answer: B

Correct answer: They provide the force needed to change passenger velocity. By Newton's first law, a passenger's body keeps moving forward at the car's original speed when the car brakes. Without a belt, nothing slows the body until it strikes the dashboard or windshield, producing a large force over a very short time. The belt applies a restraining force across the strong bones of the chest and hips, slowing the passenger along with the car. Inertia is not removed; it is managed.

• A: Inertia persists during the stop; the belt helps manage its consequence.
• B (correct): The belt exerts a restraining force as the passenger tends to continue moving.
• C: Gravity remains downward, not backward, whether or not a belt is worn.
• D: Mass does not disappear during a stop.

Answer: B

Correct answer: 72 m. Stopping distance is

With the same deceleration, doubling $v_0$ makes $v_0^2$ four times larger, so the distance becomes

• A: Braking distance grows with speed; it does not stay the same when the speed doubles.
• B (correct): Braking distance depends on speed squared, so doubling the speed multiplies the distance by 4: 18 × 4 = 72 m.
• C: This assumes braking distance is proportional to speed, but the speed is squared in the formula.
• D: This multiplies by 8, as if distance depended on speed cubed rather than squared.

Answer: A

Correct answer: 40 km/h. Find each time: 30 km / 60 km/h = 0.5 h, and 30 km / 30 km/h = 1 h. Average speed is total distance over total time: 60 km / 1.5 h = 40 km/h.

• A (correct): The legs take 0.5 h and 1 h, so 60 km divided by 1.5 h is 40 km/h.
• B: Averaging the two speeds works only for equal times; here more time is spent at the slower speed.
• C: This divides the total distance by only the second leg's time of 1 hour.
• D: This divides the total distance by the first leg's time only, leaving out the hour spent on the second leg.

Answer: D

Correct answer: 144 m. Treat each stage separately.

Stage 1:

Stage 2:

Total:

• A: This uses the final 12 m/s for the whole 14 s, ignoring the slower speeds during the first stage.
• B: This counts only the cruising stage and leaves out the 24 m covered while speeding up.
• C: This applies the acceleration for all 14 s, though the tram stopped accelerating after 4 s.
• D (correct): Stage 1 covers 24 m and ends at 12 m/s; stage 2 covers 12 × 10 = 120 m; the total is 144 m.

Answer: B

Correct answer: 200 N. Force times time equals the change in momentum.

• A: This is the change in momentum in kg·m/s; it still has to be divided by the stopping time.
• B (correct): The change in momentum is 0.5 × 20 = 10 kg·m/s, and 10 divided by 0.05 s is 200 N.
• C: This multiplies the momentum change by the time instead of dividing by it.
• D: This doubles the momentum change as if the ball bounced back, but it simply stops.

Answer: A

Correct answer: 4 m/s. Momentum is conserved because no significant outside force acts along the track.

• A (correct): Total momentum is 2 × 6 = 12 kg·m/s, shared by 3 kg, so the speed is 12 / 3 = 4 m/s.
• B: This averages the two starting speeds, which ignores the different masses.
• C: Keeping the same speed would increase the total momentum, since more mass is now moving.
• D: This divides the momentum by the 1 kg cart alone instead of the combined 3 kg.

Answer: A

Correct answer: 6,400 N. First find the centripetal acceleration, then use

• A (correct): The inward acceleration is 20² / 50 = 8 m/s², and 800 kg × 8 m/s² = 6,400 N.
• B: This multiplies mass by speed, which gives momentum, not the inward force.
• C: This stops at the acceleration of 8 m/s² and gives it a force unit instead of multiplying by mass.
• D: This forgets to square the speed, using 800 × 20 / 50.

Answer: C

Correct answer: 1.2 m. The vertical and horizontal motions are independent. Vertically the marble falls from rest:

Horizontally it keeps 3 m/s, so it travels

• A: The horizontal landing distance does not simply equal the table height.
• B: This uses 0.8 s as the fall time, mixing up the table height with the time.
• C (correct): The fall time from 0.8 m is 0.4 s, and the steady 3 m/s horizontal speed carries it 3 × 0.4 = 1.2 m.
• D: This uses t squared (0.16) as if it were the time; the square root gives 0.4 s.

Answer: A

Correct answer: Both decrease. On a ramp the weight splits into a part along the slope and a part into the surface. Steepening the ramp shrinks the into-surface part, so the normal force falls, and with less pressing the maximum friction falls too. Meanwhile the along-slope part grows, which is why the block eventually slides.

• A (correct): A steeper slope puts less of the weight into the surface, so the normal force and the friction limit both shrink.
• B: The into-surface part of the weight gets smaller, not larger, as the slope steepens.
• C: The normal force equals the weight only on a level surface; on a ramp it is smaller.
• D: Maximum friction depends on how hard the surfaces press together, so it drops along with the normal force.

Answer: C

Correct answer: 36 m. For constant acceleration, add the distance from the starting speed and the extra from acceleration.

• A: This counts only the acceleration term, as if she had started from rest.
• B: This leaves out the one-half factor on the acceleration term, giving 20 + 32.
• C (correct): Using v0 t plus one half a t squared gives 5 × 4 + 0.5 × 2 × 16 = 20 + 16 = 36 m.
• D: This counts only the distance from the starting speed and leaves out the extra due to acceleration.

Answer: B

Correct answer: 1 m/s east. Average velocity is displacement divided by time. The guard ends 200 m east of the start, so the average velocity is 200 m / 200

east.

• A: This uses the 400 m total distance, which gives the average speed, not the average velocity.
• B (correct): The displacement is 300 - 100 = 200 m east, and 200 m divided by 200 s is 1 m/s east.
• C: The guard ends up east of the start, so the displacement and average velocity point east.
• D: This uses only the first 300 m leg and ignores walking back 100 m.

Answer: B

Correct answer: At rest. Motion is always measured relative to a reference point. Inside the transport, the soldier's position does not change, so she is at rest relative to the vehicle, even though both move at 20 m/s relative to the road.

• A: That is the soldier's motion relative to the road, not relative to the vehicle she is riding in.
• B (correct): The soldier keeps the same position inside the transport, so relative to the transport she is not moving.
• C: The road appears to move backward relative to the transport, but the seated soldier does not.
• D: The transport moves at a steady speed in a straight line, and the soldier stays in her seat, so nothing here shows acceleration.

Answer: A

Correct answer: 150 s. For constant speed, use

Here

• A (correct): Time equals distance divided by speed: 1,800 m / 12 m/s = 150 s.
• B: This drops a zero; 12 m/s for 15 s covers only 180 m.
• C: This adds a zero; 12 m/s for 1,500 s would cover 18,000 m.
• D: This multiplies distance by speed instead of dividing, which does not give a time.

Answer: D

Correct answer: 15 kg. Newton's second law is

• A: This multiplies force by acceleration; mass is force divided by acceleration.
• B: This divides acceleration by force, which flips the relationship upside down.
• C: Adding force and acceleration mixes two different units and has no physical meaning.
• D (correct): Rearranging the second law gives mass = net force / acceleration = 45 / 3 = 15 kg.

Answer: A

Correct answer: 32 m. For steady braking to rest,

Check: stopping takes

s at an average of 8 m/s, and 8 x 4 = 32 m.

• A (correct): Using the braking formula, 16 squared is 256, and 256 divided by 2 x 4 = 8 gives 32 m.
• B: This divides 256 by 4 and leaves out the 2 in the denominator of the braking formula.
• C: This is 16 divided by 4, which is the stopping time in seconds, not a distance.
• D: This divides 256 by 2 only, forgetting to include the 4 m/s² deceleration.

Answer: D

Correct answer: 1 m/s backward. Before firing, the total momentum is zero, so afterward the two momenta must cancel. The bullet carries 0.010 kg x 400

·m/s forward, so the rifle carries 4 kg·m/s backward: 4 / 4 kg = 1 m/s backward.

• A: The size is right, but the rifle must move opposite to the bullet so the total momentum stays zero.
• B: This gives the rifle the bullet's speed, ignoring that the rifle is 400 times more massive.
• C: Conserving momentum means the total stays zero, not that each object stays still; the bullet's forward momentum must be balanced.
• D (correct): The bullet's momentum is 0.010 x 400 = 4 kg·m/s forward, so the 4 kg rifle needs 4 kg·m/s backward, which is 1 m/s.

Answer: D

Correct answer: Mass. A scalar has size only, while a vector has size and direction. Displacement, acceleration and force all need a direction to be fully described; mass does not.

• A: Displacement includes the direction of the change in position, so it is a vector.
• B: Acceleration has a direction, such as toward the center of a curve, so it is a vector.
• C: A force is a push or pull in a particular direction, so it is a vector.
• D (correct): Mass is just an amount with a unit, such as 5 kg, with no direction, so it is a scalar.

Answer: C

Correct answer: 2 s; 20 m. Gravity slows the ball by 10 m/s every second, so it reaches 0 m/s at the top after

Its speed changes evenly from 20 to 0 m/s, so its average speed is 10 m/s, and 10 m/s x 2 s = 20 m.

• A: The time is right, but 20 m/s x 2 s assumes the ball kept its full launch speed the whole way up; its average speed is only 10 m/s.
• B: The height is right, but 4 s is the time for the whole trip up and back down, not just the rise.
• C (correct): The ball loses 10 m/s each second, so it stops after 2 s, and its average speed of 10 m/s over 2 s gives 20 m.
• D: This uses the round-trip time and multiplies it by the launch speed, making both values too large.

Answer: C

Correct answer: 8 times. Use

Doubling v multiplies $v^2$ by 4, and halving r doubles the result again, so the acceleration grows by 4 x 2 = 8 times.

• A: This doubles for the speed only and ignores both the squaring of speed and the smaller radius.
• B: This accounts for doubling the speed (squared gives 4) but ignores the halved radius, which doubles it again.
• C (correct): Doubling speed multiplies the acceleration by 4, and halving the radius multiplies it by 2, for 4 x 2 = 8.
• D: Constant speed on each curve does not make the two accelerations equal; the size depends on speed squared over radius.

Answer: B

Correct answer: Inertia, as described by Newton's first law. Newton's first law says an object at rest stays at rest unless a net force acts on it. The cloth moves so quickly that friction acts only briefly, so the plates' inertia keeps them nearly in place.

• A: The third law describes paired forces between two objects; it does not explain why the plates stay in place.
• B (correct): The plates tend to stay at rest, and the brief, small friction force from the fast-moving cloth is not enough to change their motion much.
• C: Air pressure does not explain why the plates resist being moved sideways.
• D: Gravity on the plates stays the same; it does not change because the cloth is pulled.

Answer: C

Correct answer: Moving at a constant velocity in the negative direction. On a position versus time graph, the slope equals the velocity. A straight line has a single slope, so the velocity is constant, and a downward slope means that velocity is negative.

• A: A straight line has a constant slope, so the velocity is not changing; slowing down would show as a curve.
• B: Rest appears as a flat, horizontal line, because the position does not change.
• C (correct): The slope gives the velocity: a constant negative slope means a steady velocity in the negative direction.
• D: Speeding up would make the slope get steeper, which would show as a curve, not a straight line.

Answer: A

Correct answer: 160 N. Steady speed in a straight line means balanced forces, so the kinetic friction equals the 160 N push. The 220 N was needed only to overcome the larger maximum static friction and start the cabinet moving.

• A (correct): At steady speed the net force is zero, so sliding friction exactly matches the 160 N push.
• B: This is the maximum static friction needed to start the cabinet moving, not the friction while it slides.
• C: This is the difference between the two pushes, which is not a force acting on the sliding cabinet.
• D: Adding the two pushes does not describe any single force acting on the cabinet.