BuildPhysics

AP Physics 1 · Unit 2, Part 1 · Forces

Newton’s Third Law Practice Problems

Identify the two objects in an interaction and calculate how connected or touching objects accelerate. Practice contact pushes, strings, several blocks, and thrust without surrounding air.

12 questions6 conceptual · 6 calculation

Start practice

Free · No account required · Self-paced

Simulation scene from the Newton’s Third Law lesson.Open interactive practice
A scene from the related lesson. Open the practice to work through the questions.

What you’ll practice

Name the interaction before writing the equation

Identify the force pair

Name which object exerts each force and which receives it. Separate interaction partners from forces that happen to balance on one object.

Connect equal forces with different accelerations

Use each object’s mass and full net force to explain how equal interaction forces can produce different motion.

Calculate forces between objects

Analyze two and three touching blocks, a cord pulling a middle block, a crate on a platform, and a short thrust interval.

A question from this set

Name both objects in the pair

Block 1 pushes block 2 to the right as two touching blocks accelerate along a frictionless track. Which force is the third-law partner of that contact push?

  • The external push that drives block 1 to the right.
  • The contact push of block 2 on block 1 to the left.
  • Earth’s downward gravitational force on block 2.
Try it in the practice set
Name both objects in the pairBlock 1 pushes block 2 to the right as two touching blocks accelerate along a frictionless track. Which force is the third-law partner of that contact push?Block 1Block 21 pushes 2Both blocks accelerate right
Block 1 pushes block 2 to the right as both accelerate.

Key ideas

Equal and opposite forces act on different objects

Switch the two object names

If block 1 pushes block 2, its partner is block 2 pushing block 1. Both forces belong to the same contact interaction and exist together.

The system choice changes which forces are internal

Contact forces between included blocks cancel in the whole-system equation. To find a particular contact force, isolate a block or smaller collection.

See the physics

Follow contact arrows on each object

Check your response, then watch interacting blocks move together. Compare the opposite contact arrows and use individual objects to check a force calculated from the system acceleration.

Use a hint for a starting point or open a worked solution to follow the reasoning. Your progress saves in this browser, so you can return to unfinished questions.

Ready for newton’s third law?

Begin with a conceptual question, then work through the calculations.

Start practice