Exercises: Newton's Third Law: Action-Reaction Force Pairs

Emma pushes on a wall with her hand. State Newton's Third Law in your own words, and then name the reaction force that goes with Emma's push on the wall.

A soccer player kicks a ball with a force of $20\ \text{N}$ to the right. What is the size and direction of the force the ball exerts on the player's foot? Do these two forces act on the same object or on different objects?

A box on a table has two horizontal forces on it: $15\ \text{N}$ to the right and $15\ \text{N}$ to the left. Are the forces balanced or unbalanced? What is the net force on the box?

Jayden stands on a skateboard and pushes against a wall. He rolls backward away from the wall. Identify the action-reaction pair in this interaction. Then explain why Jayden moves but the wall does not appear to move.

Three horizontal forces act on a crate being dragged across a floor: a $50\ \text{N}$ pull to the right, a $12\ \text{N}$ friction force to the left, and a $8\ \text{N}$ push from a second person to the left. Calculate the net force on the crate, give its direction, and say whether the forces are balanced or unbalanced.

A book rests on a desk. Gravity pulls the book down with $10\ \text{N}$, and the desk pushes up on the book with $10\ \text{N}$. A student says, "These are an action-reaction pair because they are equal and opposite." Decide whether the student is correct. Name the true reaction force to the desk's push on the book.

A rocket in space expels hot gas out of its engine toward the back. There is no air for the rocket to push against. Using Newton's Third Law, explain how the rocket still speeds up. Identify the two forces in the action-reaction pair and the object each force acts on. Then explain why the two equal forces do not cancel each other.

A $2\ \text{kg}$ backpack hangs at rest from a spring scale. Later, Malik pulls the backpack upward and it accelerates upward. (a) For the backpack at rest, list the vertical forces on it and decide whether they are balanced. (b) While it accelerates upward, compare the upward force from the scale with the downward force of gravity, and say whether the forces are balanced. (c) Name the reaction force to the scale's pull on the backpack.