Newton's three laws describe how forces affect motion. The first law (inertia) states an object at rest stays at rest, and an object in motion stays in motion, unless acted on by an unbalanced force. The second law states force equals mass times acceleration (F = ma) — heavier objects need more force to accelerate at the same rate. The third law states every action has an equal and opposite reaction.
Example
When a rocket launches, it pushes hot gas downward (action), and the gas pushes the rocket upward with equal force (reaction) — demonstrating Newton's third law directly powering the rocket's motion.
Key terms
Inertia:
An object's tendency to resist changes in its motion.
Force:
A push or pull that can change an object's motion.
Acceleration:
The rate at which an object's velocity changes.
Questions
1. Newton's first law states an object in motion:
Stays in motion unless acted on by an unbalanced force
Always immediately stops on its own
Speeds up automatically with no force needed
Reverses direction automatically
2. Newton's second law is written as:
F = ma
F = m + a
F = m/a
F = m - a
3. Newton's third law states every action has:
An equal and opposite reaction
No reaction at all
A reaction in the same direction only
A reaction that is always weaker
4. Inertia is:
An object's tendency to resist changes in motion
A type of force applied externally
A measurement of colour
A type of energy only
5. Force is best described as:
A push or pull that can change motion
A type of colour
A measurement of temperature only
Something that never affects motion
6. Acceleration is the rate at which:
An object's velocity changes
An object's colour changes
An object's mass changes on its own
Nothing changes at all
7. A rocket launching demonstrates:
Newton's third law
No law of motion at all
Only the first law, with no other law involved
A law unrelated to motion
8. Using F = ma, if mass is 10kg and acceleration is 2m/s², the force is:
20N
12N
5N
8N
9. Using F = ma, if force is 50N and mass is 10kg, the acceleration is:
5m/s²
50m/s²
10m/s²
500m/s²
10. Why does a heavier object require more force to achieve the same acceleration as a lighter one?
Force needed is proportional to mass when acceleration is held constant, per F = ma
Mass has no relationship to the force required for acceleration
Heavier objects always accelerate faster with less force
F = ma only applies to very light objects
11. A book resting motionless on a table demonstrates:
Newton's first law, since no unbalanced force is acting on it
A violation of all three of Newton's laws
Only the third law, with no connection to the first
No law of motion applies to stationary objects
12. Why does pushing on a wall not make the wall move, despite you applying a force?
The wall exerts an equal and opposite reaction force back on you, and the wall is fixed in place
The wall applies no force back on you at all
Newton's third law does not apply to stationary objects like walls
Applying force to any object always makes it move
13. Why does a seatbelt help protect a passenger during a sudden stop, in terms of inertia?
Without it, the passenger's body would continue moving forward due to inertia even as the car stops
Inertia only affects objects that are already stationary
A moving passenger has no tendency to continue moving when a car stops
Seatbelts have no connection to any of Newton's laws
14. Using F = ma, if a 5kg object accelerates at 4m/s², the force required is:
20N
9N
1.25N
45N
15. Why is Newton's first law sometimes called the "law of inertia"?
It describes an object's natural tendency to resist changes to its state of motion without an external force
Inertia has no connection to this particular law
The first law describes only forces, not motion tendencies
This law only applies to objects that are already accelerating
16. Why do astronauts experience apparent weightlessness in orbit, even though gravity is still acting on them?
They are in continuous free-fall around Earth, so gravity is providing centripetal force rather than a felt sensation of weight
Gravity completely disappears once an object reaches orbit
Astronauts in orbit are not subject to any of Newton's laws
Weightlessness in orbit has no connection to gravitational force at all
17. Why might understanding F = ma be essential for engineers designing vehicle safety features like crumple zones?
Extending the time over which a collision force acts can reduce the peak force experienced, protecting occupants
Force, mass and acceleration have no practical application in vehicle design
Crumple zones work by increasing the force experienced during a crash
F = ma only applies to objects moving at a constant velocity
18. Why does a swimmer pushing water backward propel themselves forward, illustrating Newton's third law?
The reaction force from the water pushes the swimmer forward with equal and opposite force to their push
Water never exerts any force back on a swimmer
Swimming involves no interaction between force and motion
This example only demonstrates Newton's first law, not the third
19. Why are all three of Newton's laws often needed together to fully explain the motion of a complex system, like a moving car?
Each law explains a different aspect — resistance to change, the effect of force, and paired reaction forces — that together describe real motion
Only one of Newton's three laws is ever needed to explain any motion
The three laws contradict each other and cannot be applied together
Complex systems like cars are not subject to any of Newton's laws
20. Why might a rocket in the vacuum of space still be able to accelerate, despite having nothing to "push against"?
It expels mass (exhaust gas) backward, and the reaction force from that expulsion pushes the rocket forward, per Newton's third law
Rockets cannot accelerate at all once they leave the atmosphere
Newton's laws only apply within Earth's atmosphere, not in space
A rocket needs solid ground or air to push against to accelerate
21. Why might engineers need to account for both an object's mass and its desired acceleration when calculating the force a machine (like an elevator motor) must produce?
F = ma shows that the required force depends on both how much mass is being moved and how quickly it needs to accelerate
Only mass, and never acceleration, needs to be considered when calculating required force
Only acceleration, and never mass, needs to be considered when calculating required force
Force calculations for machinery have no connection to Newton's second law
Answer key (parent copy)
1. Stays in motion unless acted on by an unbalanced force
2. F = ma
3. An equal and opposite reaction
4. An object's tendency to resist changes in motion
5. A push or pull that can change motion
6. An object's velocity changes
7. Newton's third law
8. 20N
9. 5m/s²
10. Force needed is proportional to mass when acceleration is held constant, per F = ma
11. Newton's first law, since no unbalanced force is acting on it
12. The wall exerts an equal and opposite reaction force back on you, and the wall is fixed in place
13. Without it, the passenger's body would continue moving forward due to inertia even as the car stops
14. 20N
15. It describes an object's natural tendency to resist changes to its state of motion without an external force
16. They are in continuous free-fall around Earth, so gravity is providing centripetal force rather than a felt sensation of weight
17. Extending the time over which a collision force acts can reduce the peak force experienced, protecting occupants
18. The reaction force from the water pushes the swimmer forward with equal and opposite force to their push
19. Each law explains a different aspect — resistance to change, the effect of force, and paired reaction forces — that together describe real motion
20. It expels mass (exhaust gas) backward, and the reaction force from that expulsion pushes the rocket forward, per Newton's third law
21. F = ma shows that the required force depends on both how much mass is being moved and how quickly it needs to accelerate