Motion and Stability: Forces and Interactions
What you must know
This domain links motion description to the causes of motion. Be fluent with speed = distance/time, velocity as speed with direction, and acceleration as a change in velocity (speeding up, slowing down, or turning). Interpret simple distance-time and speed-time graphs: slope on a distance-time graph is speed; a flat line means at rest.
Understand Newton's laws at a conceptual level. An object at rest or moving at constant velocity has balanced (net zero) forces. Unbalanced forces cause acceleration, and greater mass means less acceleration for the same force (F = ma). Every force has an equal and opposite reaction force acting on a different object. Know gravity as a force toward Earth's center, friction as a force opposing sliding, and normal force as a surface push. Also recognize that forces are pushes or pulls with magnitude and direction, and that they add as vectors.
Traps to watch
- Constant speed does not mean zero acceleration if direction changes (circular motion).
- "No motion" does not mean "no forces" - it may mean balanced forces.
- The action-reaction pair never cancels on the same object; they act on two objects.
- Heavier objects do not always fall faster; ignoring air resistance, they accelerate equally.
Rule of thumb
Draw a free-body diagram: list forces, then find the net force. Net force zero → constant velocity (or rest). Net force nonzero → acceleration in the net-force direction.
Worked example
A 10 kg cart pushed with 30 N net force: a = F/m = 30 N ÷ 10 kg = 3 m/s². If friction is 10 N, net force is 20 N and a = 2 m/s².