Collisions and Newton's Third Law

Two cars collide head-on. At every moment during the collision, the magnitude of the force the first car exerts on the second is exactly equal to the magnitude of the force the second car exerts on the first.

How is this collision an example of Newton’s 3rd Law of Motion?

• After the collision, neither car has any velocity so no momentum is present. It would take an unbalanced force to make the cars move from the location.

• We do not have enough information to determine the force of the impact as we do not know the mass or the acceleration of the cars before the collision.

• Since the collision was equal and opposite, we can say that an equal force was exerted on both cars.

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The diagram below shows a 5.00 kg block at rest on a horizontal, friction-less table. Gravity is acting on this block, and the acceleration of gravity on Earth is 9.807 m/s².

Which of the following diagrams best represents the force exerted on the block by the table?

• 
• 
• 

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The magnitude of the force that a baseball bat exerts on a ball is 50 Newtons

The magnitude of the force that the ball exerts on the bat is...?

• 5.0 N

• 50 N

• 250 N

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A man is pushing a wheelbarrow full of dirt as he works on a project in his back yard.

Compared to the magnitude of the force exerted on the wheelbarrow by the man, the magnitude of the force exerted on the man by the wheelbarrow is...?

• larger

• zero

• the same

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Both dogs in the picture have the same mass, and they both are exerting the same amount of force on the rope toy.

What is most likely to happen when the rope toy breaks in the middle as a result of the forces the dogs are exerting?

• The dogs will be thrown backwards, in opposite directions from each other, the same distance.

• One dog will be thrown backwards while the other dog will be able to move forwards.

• As the two dogs are the same mass, neither dog will be thrown backwards.

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A science teacher did a demonstration by dropping two balls together at the same time. The two balls had different volumes and mass. When the larger ball impacted the surface of the floor, the smaller ball was launched upwards at a high rate of speed (see diagrams)

Two-Part Question

Part 1: What caused the smaller ball to accelerate at such a high rate?

Part 2: Would this same phenomena occur if you used two identical basketballs?

• Part 1: The smaller ball traveled with a higher velocity downwards.

  Part 2: The basketballs would not reach the same velocity.

• Part 1: The difference in momentum caused the smaller ball to accelerate.

  Part 2: Basketballs have a greater mass. Therefore it would have more momentum

• Part 1: The difference in mass and momentum caused the smaller ball to accelerate.

  Part 2: The basketball would not reach the same velocity that the small ball did.

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A spring scale reads 5 Newtons as it pulls a 75 gram coffee cup across a table.

What is the magnitude of the force exerted by the coffee cup on the spring scale?

• 15 N

• 75 N

• 5 N

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Two carts are pushed apart by an expanding spring as shown in the diagram.

If the average force on the 1-kilogram cart is 1 Newton, what is the average force on the 2-kilogram cart?

• 0.5 N

• 1 N

• 4 N

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While driving down the interstate on your way to school, you hit a bug that splatters on your windshield. Newton’s 3rd Law states that for every action there is an equal and opposite reaction.

If actions and reactions are equal and opposite, why is the bug obliterated but your car continues on a stable velocity?

• The bug has much less mass than the car, the bug has more momentum since it is may have been moving faster before the collision.

• The bug has much less mass than the car, it also has much less momentum, causing the destruction of the bug.

• The windshield is made of a much harder substance than the bug causing the death of the bug on impact.

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A mosquito flying over a highway strikes the windshield of a truck traveling at a speed of 100 kilometers per hour (62 miles per hour).

Compared to the force magnitude of the truck on the mosquito during this collision, the force magnitude of the mosquito on the truck is...?

• the magnitude is significantly less.

• the magnitude is exactly the same.

• the magnitude is significantly greater.

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