Newton's Laws of Motion
Grade 8 · Science · Study Guide
Understand the three fundamental laws that govern how objects move and interact with forces.
Newton's First Law states that an object at rest stays at rest, and an object in motion stays in motion at the same speed and direction unless acted on by an unbalanced force. This tendency to resist changes in motion is called inertia.
Newton's Second Law describes the relationship between force, mass, and acceleration: F = ma. A larger force produces greater acceleration, while a larger mass requires more force to achieve the same acceleration.
Newton's Third Law states that for every action there is an equal and opposite reaction. When you push on a wall, the wall pushes back on you with the same force in the opposite direction.
| Law | Statement | Example |
|---|---|---|
| 1st (Inertia) | Objects resist changes in motion | Seatbelts stop passengers when a car brakes |
| 2nd (F=ma) | Force = mass × acceleration | Pushing a heavy cart vs. a light cart |
| 3rd (Action-Reaction) | Equal and opposite reactions | Rocket exhaust pushes rocket upward |
| Mass (kg) | Force (N) | Acceleration (m/s²) |
|---|---|---|
| 2 | 10 | 5 |
| 5 | 10 | 2 |
| 10 | 10 | 1 |
Key terms
- Inertia
- The tendency of an object to resist changes in its state of motion.
- Net Force
- The overall force on an object after all forces are combined.
- Acceleration
- The rate of change of velocity over time.
- Newton (N)
- The SI unit of force, equal to 1 kg·m/s².
Practice questions
A hockey puck sliding on ice eventually stops. Which law best explains why it stops?
- Newton's First Law — friction is an unbalanced force acting on it
- Newton's Second Law — the puck's mass increases over time
- Newton's Third Law — the ice pushes the puck upward
- None of Newton's laws apply here
Show answer
The First Law says an object in motion stays in motion unless acted on by an unbalanced force. Friction is that unbalanced force.
A 4 kg object accelerates at 3 m/s². What is the net force acting on it?
- 7 N
- 12 N
- 1.33 N
- 0.75 N
Show answer
Using F = ma: F = 4 kg × 3 m/s² = 12 N.
According to Newton's Third Law, the force a bat exerts on a baseball is greater than the force the baseball exerts on the bat.
Show answer
Action-reaction pairs are always equal in magnitude. The baseball pushes back on the bat with the same force.
The formula for Newton's Second Law is F = ___.
Show answer
Force equals mass times acceleration (F = ma).
Explain why astronauts in a spacecraft orbiting Earth feel weightless even though gravity is still acting on them.
Show answer
This demonstrates inertia and the effect of gravity as a centripetal force in orbit.
Fun facts
- Newton developed his laws of motion in 1687, and they accurately predicted spacecraft trajectories over 300 years later.
- You experience Newton's Third Law every time you walk — your foot pushes backward on the ground, and the ground pushes you forward.
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