Newton's Third Law

Newton's Third Law states that...

• ... an object in motion continues to accelerate.

• ... for every action there is an equal and opposite reaction.

• ...every force causes another force to accelerate.

• ... when mass increases, force decreases.

A bowling ball hits the pins. Which drawing correctly shows the force on the ball and the pins? Bigger arrows mean bigger forces.

• 
• 
• 
• 

Answer explanation

The forces are equal and opposite, so the arrows should be the same size (equal force) pointing in opposite directions.

When the ball swings down to hit the others, it stops, and the ball on the other end of the line flies up to the same height. Why?

• Only the balls at the very end can move.

• The reaction of the impact with the other balls stops the one on the left, and the action of its force travels from ball to ball to the other end.

• The reaction makes the force stronger and stronger as it travels from one side to the other.

• Gravity controls the movement of the balls, so they swing back and forth because they keep falling down from the force of gravity.

The car is driving down the street. What object in this picture creates the force that pushes the car forward?

• The engine

• The wheels

• The exhaust

• The ground

Answer explanation

The engine of the car spins the wheels, causing them to push against the ground with the help of friction. The reaction force of the ground pushing back on the wheels makes the car move forward.

You and your friend are both wearing roller skates. You push your friend. What might happen?

• Your friend moves back, but you don't.

• You move back, but your friend doesn't.

• You and your friend both move back.

• Neither of you move back.

You kick a soccer ball. What is true about the force on you and the ball?

• The force on the ball is bigger.

• The force on you is bigger.

• Both forces are the same.

The Moon's gravity pulls on the Earth and the Earth's pulls on the Moon. Which force is stronger?

• The Earth's pull is stronger.

• The Moon's pull is stronger.

• The two forces are the same.

Answer explanation

The Moon's pull on the Earth and the Earth's pull on the Moon are a force pair, so they are equal.

A small car and a large car get into an accident. The small car is much more seriously damaged than the large car. Why might this happen?

• The big car has more mass, so it exerts a larger force than the small car.

• The big car has a larger acceleration than the smaller car, so it stays safe.

• The forces are the same, so the lower mass car has a larger acceleration, causing more damage.

• Big cars are built with stronger materials compared to small cars.

You kick a ball. If kicking the ball creates two equal and opposite forces, why does the ball move?

• One force acts forward on the ball and one force acts backward on your foot.

• You create a larger force on the ball because you have more mass.

• The ball moves more than you do because it has less mass.

• Both forces act on your foot, not the ball. The ball has no force pair.

A car runs into a wall. The driver is thrown forward, but their seatbelt stops them from hitting the steering wheel. How is this related to Newton's laws?

• They continue moving forward because of inertia until they hit the seatbelt, which exerts an equal and opposite force on them to stop them.

• They start moving forward because the force of the crash causes them to accelerate towards the front of the car.

• The force of the crash has a smaller impact on the car and a larger impact on the driver because the driver has less mass.

• The seatbelt exerts a force on the driver to keep them in their seat, and the driver exerts no force on the seatbelt.