Investigation 4

Newton’s Third Law of Motion, Impulse and Momentum

Newton’s Third Law of Motion:“When one object exerts a force on a second, the second object exerts an equal and oppositely directedforce on the first.;” or “To every action there is an equal and opposite reaction.”

Answer the following questions are clearly and concisely as you can.

  1. A physics student holds a ball in her hand at rest.

a.The two forces that are acting on the ball are:

1)gravitational force (gravity) – the earth pulling downward on the ball (noncontact)

2)her hand exerts a force on the ball upwards (contact)

  1. Name the reaction forces to the two forces above and indicate the directions of the forces.

1)gravitational force of the ball pulling on the earth (upward)

2)ball exerts a force on her hand (downward)

2.The physics student releases the ball (lets the ball drop). Consider the ball as it falls toward the ground without air resistance. The force that is acting on the ball as it is falling is the gravitational force (gravity) pulling downward. Name the reaction force and indicate its direction.

gravitational force of the ball pulling on the earth upward

3.A physics student who weighs 800 Newtons (about 170 lbs) stands on a bathroom scale in an elevator that is not moving (at rest).

a.The two forces that are acting on him are;

1)gravitational force (gravity) pulling downward with a force of 800 Newtons

2)the bathroom scale pushing upward on him with a force of 800 Newtons

b.Name the reactions forces and indicate their strengths (in Newtons) and their directions.

1)gravitational force of the student pulling upward on the earth with a force of 800 N

2)the student pushes downward on the scale with a force of 800 N

4.A physics textbook is at rest on a table. A calculator is placed on top of the book. Name the three forces that are acting on the book. Name the corresponding reaction forces.

action force / reaction force
table pushing upward on the book / book pushing downward on the table
calculator pushing downward on the book / book pushing upward on the calculator
earth pulling downward on the book (gravity) / book pulling upward on the earth

5.A physics student sitting in a stationary Lamborghini (car), is holding onto the steering wheel, and is strapped in with her seat belt. She knows that in the vertical direction, 1) the gravitational force pulls her downward and 2) the seat pushes her upward and that the net vertical force is zero, thus resulting in no upward or downward acceleration. In the horizontal direction, 3) the back of the seat exerts force on her and pushes her forward, and 4) the steering wheel exerts a force on her and pushes her backward.

Name the reactions forces to each of the four forces described above, and indicate their directions.

reaction force / direction
1) she exerts a gravitational force on the earth / upward
2) she exerts a force on the seat / dowward
3) she exerts a force on the back of the seat / backward
4) she exerts a force on the steering wheel / forward