Physics: Newton's Third Law
Action and reaction forces in everyday situations
Physics: Newton's Third Law
Action and reaction forces in everyday situations
Physics - Grade 9-12
- 1
State Newton's Third Law in your own words. Include what is meant by an action-reaction force pair.
Focus on the two objects interacting, not on two forces acting on the same object.
Newton's Third Law says that when one object exerts a force on a second object, the second object exerts an equal-magnitude force in the opposite direction on the first object. An action-reaction force pair always acts on two different objects. - 2
A student pushes on a wall with a force of 80 N to the east. What force does the wall exert on the student?
The wall exerts a force of 80 N to the west on the student. The forces are equal in magnitude and opposite in direction. - 3
A rocket engine pushes hot gases downward. Explain how Newton's Third Law helps the rocket move upward.
Identify the two objects as the rocket and the gases.
The rocket pushes the gases downward, and the gases push the rocket upward with an equal and opposite force. This upward force is the thrust that accelerates the rocket. - 4
A 0.20 kg tennis ball hits a racket. During contact, the racket exerts a 120 N force on the ball. What force does the ball exert on the racket?
The ball exerts a 120 N force on the racket in the opposite direction. Newton's Third Law says the forces in the interaction are equal in magnitude and opposite in direction. - 5
Two ice skaters push off from each other. Skater A has a mass of 50 kg, and Skater B has a mass of 75 kg. During the push, which skater experiences the larger force? Explain.
Do not confuse force with acceleration.
Both skaters experience forces of equal magnitude. Skater A may have a larger acceleration because the same force acts on a smaller mass, but the interaction forces are equal. - 6
In the same ice skater situation, why does the 50 kg skater accelerate more than the 75 kg skater if the forces are equal?
Use Newton's Second Law, a = F divided by m.
The 50 kg skater accelerates more because acceleration is force divided by mass. With the same force, the smaller mass has the greater acceleration. - 7
A book rests on a table. Identify the Newton's Third Law partner force for the force of the table pushing upward on the book.
The partner force is the book pushing downward on the table. These two forces act on different objects and are equal in magnitude and opposite in direction. - 8
A book rests on a table. A student says, 'The upward normal force and the downward weight of the book are a Newton's Third Law pair.' Explain why this statement is incorrect.
Check whether the two forces act on the same object or on different objects.
The statement is incorrect because both the normal force and the weight act on the same object, the book. A Newton's Third Law pair must act on two different objects. - 9
When you walk forward, your foot pushes backward on the ground. What force helps you move forward?
The ground pushes forward on your foot through static friction. This forward friction force helps accelerate your body forward. - 10
A swimmer pushes water backward with her hands. Use Newton's Third Law to explain how she moves forward.
The action force is on the water, and the reaction force is on the swimmer.
The swimmer pushes the water backward, and the water pushes the swimmer forward with an equal and opposite force. That forward force moves the swimmer through the water. - 11
A car tire pushes backward on the road as the car accelerates forward. Identify the reaction force and explain its role.
The reaction force is the road pushing forward on the tire. This forward static friction force provides the external force that accelerates the car forward. - 12
A 60 kg astronaut in space pushes a 10 kg tool with a force of 18 N to the right. What force does the tool exert on the astronaut, and in what direction?
The masses affect acceleration, not the size of the Third Law force pair.
The tool exerts an 18 N force on the astronaut to the left. The forces are equal in magnitude and opposite in direction. - 13
For the astronaut and tool in the previous problem, calculate the acceleration of the astronaut caused by the tool's force.
The astronaut's acceleration is 0.30 m/s² to the left because a = F divided by m = 18 N divided by 60 kg = 0.30 m/s². - 14
A bug hits the windshield of a moving truck. Compare the force of the bug on the truck with the force of the truck on the bug.
Equal force does not mean equal effect on motion.
The bug and the truck exert equal-magnitude forces on each other in opposite directions. The bug has a much larger acceleration because its mass is much smaller. - 15
A student fires a ball from a spring launcher. The launcher pushes the ball forward with a force of 35 N. Identify the action-reaction pair and describe why the launcher recoils.
The launcher pushes the ball forward with 35 N, and the ball pushes the launcher backward with 35 N. The launcher recoils because the backward force from the ball accelerates the launcher in the opposite direction.