Forces and Equilibrium flashcards

$W=mg$: weight W in newtons, mass m in kg, g about 9.8 m/s² (often 10 on tests). Mass never changes with location; weight does. An 8 kg toolbox weighs about 78.4 N.

$F_{net}=ma$: net force (N) equals mass (kg) times acceleration (m/s²); acceleration points along the net force. Example: 3,000 N on a 1,500 kg car gives 2 m/s².

They are equal and opposite but act on different objects. Only forces acting on the same object go in its free-body diagram and can cancel.

$R=\sqrt{F_1^2+F_2^2}$. Example: 6 N east and 8 N north give 10 N. Opposite forces subtract; only same-direction forces add directly.

$\tau =Fd\sin \theta$: force F (N) times distance d (m) from pivot, giving N·m. Largest when the push is perpendicular (90°); zero when the force points through the pivot.

Net force is zero and net torque is zero. Equal, opposite but offset forces (a couple) give zero net force yet still rotate the object.

Static friction matches the push up to a maximum ${\mu }_{s}N$; it is not always at that maximum. Once sliding, kinetic friction is ${\mu }_{k}N$, usually smaller than the static maximum.

Down-slope part $W\sin \theta$; into-slope part $W\cos \theta$, which equals the normal force with no other forces. A block stays put if $\tan \theta \le {\mu }_s$.