Practice forces, equilibrium, friction, connected objects, and motion under forces. Quiz one topic at a time or take the whole unit.
The idea each topic turns on, and the mistake students make most often.
Key ideaZero net force means zero acceleration, so the object is at rest or moving at constant velocity.
Common mistakeThinking a moving object needs a net force to keep moving. Constant velocity needs no net force.
Key ideaA free-body diagram shows only the forces exerted on one object. Newton’s third-law pairs act on two different objects, so they never cancel each other.
Common mistakeDrawing a force the object exerts on something else, or calling weight and the normal force a third-law pair.
Key ideaWeight is the gravitational force, mg. The normal force adjusts to whatever the surface must push; it equals mg only when nothing else acts vertically and there is no vertical acceleration.
Common mistakeAssuming the normal force always equals mg, for example in an accelerating elevator or when another force pushes down or pulls up.
Key ideaIn equilibrium, the force components cancel separately along each axis: ΣFx = 0 and ΣFy = 0.
Common mistakeBalancing whole forces instead of components, or choosing sine where cosine belongs for an angled force.
Key ideaKinetic friction is μk·N. Static friction is only as large as it needs to be, up to a maximum of μs·N.
Common mistakeUsing μs·N for static friction when the object isn’t about to slip, or forgetting that the normal force changes when other forces have vertical components.
Key ideaTilt your axes along the incline. Gravity’s component along the slope is mg sin θ and its component into the slope is mg cos θ.
Common mistakeSwapping sin θ and cos θ. Check with θ = 0, where the component along a flat surface must be zero.
Key ideaTreat connected objects as one system to find the acceleration from the external forces, then apply Newton’s second law to a single object to find an internal force such as tension.
Common mistakeIncluding internal forces in the system’s net force, or assuming the tension equals the hanging weight when the system accelerates.
Key ideaMoving in a circle requires a net inward (centripetal) force of size mv²/r; in uniform circular motion this is the entire net force. It is supplied by real forces such as tension, gravity, friction or the normal force.
Common mistakeAdding an outward “centrifugal” force to the free-body diagram, or listing centripetal force as an extra force.
Key ideaGravitational force follows an inverse square law, F = Gm₁m₂/r², with r measured from center to center. Doubling the distance cuts the force to a quarter.
Common mistakeMeasuring r from the surface instead of the center, or forgetting to square it.
This is one of the 40 questions in the Unit 2 quiz.
Which situation describes an object in equilibrium?
Answer: A. It is at rest or moves in a straight line at constant speed
An object in equilibrium is at rest or moves in a straight line at constant speed. Its net force is zero.
AP-style FRQs for this unit. Students score themselves point by point against a rubric and see a full solution.
