Newton's Third Law

Question 1

Alex and Maya each stand on separate, frictionless sleds on a frozen lake. They lean forward and push straight against each other’s hands. After the push, Alex’s sled accelerates to the right and Maya’s sled accelerates to the left. Which statement correctly identifies an action‑reaction pair and a pair of balanced forces in this situation?

Accepted Answer

The force Alex exerts on Maya’s sled and the force Maya exerts on Alex’s sled are an action‑reaction pair; the weight of Alex and the normal force from his sled on Alex are balanced forces acting on the same object.

Answer

The weight of Alex and the normal force from his sled on Alex are an action‑reaction pair; the push forces between the sleds are balanced forces acting on the same object.

Answer

Both the push forces between the sleds and the weight–normal forces on Alex are action‑reaction pairs.

Answer

The push forces between the sleds are balanced forces, and the weight–normal forces on Alex are an action‑reaction pair.

Question 2

When a 0.012 kg bullet is fired from a pistol, the expanding gases exert a forward force of 400 N on the bullet. According to Newton’s Third Law, why does the pistol experience a recoil?

Accepted Answer

The gases push the bullet forward with 400 N, and the bullet pushes back on the gases with an equal 400 N; the gases then push the pistol backward with that same 400 N, causing recoil.

Answer

The explosion creates extra mass in the bullet, so the pistol must move backward to keep the total mass of the system unchanged.

Answer

When the bullet leaves the barrel, the pistol loses weight, so it tips backward as a result.

Answer

The force on the bullet is larger than the force on the pistol, so the pistol accelerates backward faster than the bullet moves forward.

Question 3

Two identical carts, each weighing 0.5 kg, roll toward each other on a frictionless track and collide. During the collision each cart exerts a force on the other. Which statement best describes the forces the carts exert on each other?

Accepted Answer

The force each cart exerts on the other has the same magnitude and opposite direction.

Answer

Cart A exerts a force on Cart B, but Cart B does not exert any force on Cart A.

Answer

The forces are equal in magnitude only if the carts have the same speed at the moment of impact.

Answer

Cart A exerts a larger force on Cart B than Cart B exerts on Cart A.

Try three questions

When a 0.012 kg bullet is fired from a pistol, the expanding gases exert a forward force of 400 N on the bullet. According to Newton’s Third Law, why does the pistol experience a recoil?

Two identical carts, each weighing 0.5 kg, roll toward each other on a frictionless track and collide. During the collision each cart exerts a force on the other. Which statement best describes the forces the carts exert on each other?

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