Newton's Third Law

Newton's third law of motion deals with the forces between objects. The third law states that if an object applies a force on a second object, the second object applies a force of equal value but opposite in direction to the first. It is important to remember that action forces act on one object and reaction forces act on a second object. To solve third law problems, a free body diagram must be drawn for each object in the problem. Once the diagrams are drawn we can apply Newton's second law to each. Because the objects are joined together they will have the same acceleration. Please watch the following presentation that will introduce Newton's Third Law of Motion.

Newton's Third Law Practice

The third law of motion introduces ways to relate the forces between objects. We still need to draw free body diagrams and apply the second law of motion to those diagrams. The third law allows us to do this with multiple objects. Please watch the following presentation of some example problems.

Newton's Third Law Self-Assessment

A bulldozer pushes on a large 600kg rock. The rock is pushed up against another rock with a mass of 800kg. The bulldozer driver decides to see if he can push both so he uses the bulldozer to apply a horizontal force of 950N to the first rock. Assume that the rocks are on a frictionless surface and that they move horizontally.

Bulldozer pushing on one rock, which is pushing on another rock

Draw the free body diagram for each of the rocks.
What is the acceleration of the rocks?
What is the force between the two rocks?

SOLUTIONS

A 40kg box accelerates at 0.54 m/s² to the right across a horizontal table when a force of 372N @ 20^o is applied to it. This box is tied to a second box by a rope that passes over a massless frictionless pulley. That box hangs suspended in the air over the side of the table.

Problem 2 Image

Draw the free body diagram for the boxes.
What is the mass of the box hanging from the rope?
Determine the tension on the rope connecting the two boxes.
The rope is now cut, what is the magnitude and direction of the acceleration of each box?

SOLUTIONS

Look at the three boxes in the diagram below. Box A has a mass of 15kg, B has a mass of 50kg, and C has a mass of 25kg.

Box A is hanging off the table to the left, Box is on the table and Box C is hanging on to the right.

Draw the free body diagrams for all three objects.
Determine the acceleration of the system.
Calculate the tension on each rope.

SOLUTIONS

Newton's Third Law Practice Problems

Download Newton's Third Law Practice Problems for more practice.

Download solutions to the practice problems.

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