Question

What causes objects to move or stay still?
Claim:
Evidence:

Answer 1

Claim:
- An object moves, stops, or changes its motion only when a net external force acts on it. If the forces on an object are balanced (net force = 0) it will remain at rest or continue moving at constant velocity (Newton’s first and second laws; Fnet = m·a).

Evidence:
- Everyday balances: a book sits still on a table because gravity (weight) is balanced by the table’s normal force (net force = 0 → no motion).
- Push tests: pushing a stationary box causes it to start moving because your push adds an unbalanced force. If you stop pushing and friction provides an opposite force, the box slows and stops (unbalanced force in opposite direction).
- Low-friction demonstration: an air-hockey puck or puck on ice glides nearly unchanged for a long time — with almost no net horizontal force it keeps moving at nearly constant speed, showing inertia.
- Galileo’s inclined-plane observations: objects slow less on shallower slopes, suggesting that without forces (friction) an object would continue to move indefinitely.
- Quantitative law (Newton’s second law): experiments with a spring scale and different masses show acceleration is proportional to applied force and inversely proportional to mass (a = F/m).
- Newton’s cradle and collisions: momentum and forces transferred at impact cause some balls to move and others to remain still, illustrating that changes in motion occur where forces are exchanged.
- Free fall and rockets: objects accelerate toward Earth because gravity is an unbalanced force; rockets accelerate because expelling gas produces an unbalanced thrust force (action–reaction).

Simple test you can do:
- Pull a toy cart with a spring scale and measure acceleration for a light and a heavier load while keeping the pull force the same. You’ll see the heavier cart accelerates less, confirming F = m·a.