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physics Advanced Subsidiary (AS) Level Physics.
 
Solved problems for the chapter:
 

Newton's Third Law of Motion

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Detailed solution steps are available at:
 
https://en.openprof.com/wb/chapter:newton_039%3Bs_third_law_of_motion/5133/


Example 1.

A car is driven into a sharp bend in a horizontal road. The radius of the bend is , and the coefficient of friction between the tires and the ground is .


Driving a car in a bend .



  • Draw the external forces on the car and name them.

  • What does the maximum speed of a car depend on when driving safely through a bend in a horizontal road?

  • What is the maximum speed with which the car can drive safely around the bend?

  • On normal road traffic, only the right half of the road should be used. Based on the equation obtained in part b above, explain on which side of the bend the car can drive at a higher speed.

    
Example 2.

A rope attached to the ceiling hangs a 2 kg weight. A second rope is attached to this weight, on which hangs a 5 kg weight. What are the tension forces on both ropes and where are they directed?

    
3.

Two skaters are holding a rope in their hands as shown in the picture. The weight of the first skater is 40 kg and the second 60 kg. The first skater pulls the second with a force of 60 N. What are the speeds of both skaters after two seconds of pulling? Friction is ignored.


    
4.

Are the following statements correct?


  • If we act on a body with a certain force, the body also acts on us with an equal and opposite force.

  • Bodies interact only with direct contact.

  • A body acts on another body with a force F, and the second body on the first with the opposite same force. Since the sum of both forces is zero, the body is at rest or moving evenly and in a straight line.

  • The law of interaction also applies when a body acts with force on another body and accelerates it.

  • The earth acts on the Moon with the same force as the Moon on the Earth.

    
5.

Karl and John are standing on skateboards. John has a rope tied around his waist, with which Karl pulls him towards himself with a force of 30 N. Karl notices that during the pulling, he also started moving towards Jan. Explain why.


    
6.

A professor is standing on the stage. His weight is 700 N. With what force does the stage act on the professor?


    
7.

Zane and Kelvin stand on skateboards and push off with their hands. Kelvin acts on Zane with a force of 40 N. With what force does Zane act on Kelvin? How will they move after the push?


    
8.

A ball falls to the ground. Describe this using Newton's 3rd law, a pair of equal and opposite forces acting on the ball.

    

9.

A lightweight spring balance hangs on a stand with a 2 N weight suspended on it. Determine at least three pairs of forces according to Newton's third law acting on the stand, spring, and weight.


    
10.

A car accelerates on a flat road. The motion of the car is constantly inhibited by the force of air resistance. Describe what applies to air according to Newton's 3rd law.

    
11.

A log placed on a slope with an inclination of to the ground has a weight of 80 N. With what force does the slope act on the log?

    
12.

A box is lying on the table. The box is pushed by a finger with a force of 6 N. The box then begins to move. The motion of the box is hindered by a frictional force of 4 N. With what force does the box act on the finger? Does this force depend on the frictional force?

    
13.

A magnet attracts an iron box with a force of 6 N.

With what force does the box attract the magnet?

    
14.

A boat sails uniformly over the sea. It is subjected to an engine force of 20 kN.

  • What is the force of water resistance?

  • Does the boat also have a force on the water?

  • How do you know?

    
15.

On a slope with an inclination of , John holds a barrel with a force of 300 N with his hand. The dynamic component of the force on the slope is 400 N. How will the barrel move? How much force will the barrel exert on John's hand?


    
16.

A box is shown resting on the ground. Newton’s third law implies that four forces of equal magnitude are involved. These forces are labelled P, Q, R and S. Forces P and Q act on the box. Forces R and S act on the Earth. For clarity, the forces are shown slightly separated.




Which statement about the forces is correct?


  • P is the equal and opposite force to Q and both are forces of contact.


  • Q is the equal and opposite force to P and both are gravitational forces.


  • R is the equal and opposite force to S and both are forces of contact.


  • S is the equal and opposite force to Q and both are gravitational forces.

    
17.

Newton’s third law of motion is often summarized as ‘Every action (force) has an equal and opposite reaction.’ A book rests on a table. If the weight of the book is the ‘action’ force, what is the ‘reaction’ force?


  • the pull of the book on the Earth


  • the pull of the Earth on the book


  • the push of the book on the table


  • the push of the table on the book