Tutorials in introductory physics forces

Newton Second & Third Law

Tutorials in Introductory Physics ©Pearson Custom Publishing McDermott, Shaffer, & P.E.G., U. Wash. Updated Preliminary Second Edition, 2011

Mech HW–39

1. A block initially at rest is given a quick push by a hand. The block slides across the floor, gradually slows down, and comes to rest.

a. In the spaces provided, draw and label separate free-body diagrams for the block at each of the three instants shown.

A quick push by a hand…

1. (Initially at rest)

the sliding block slows…

2.

v

and is finally at rest.

3.

b. Rank the magnitudes of all the horizontal forces in the diagram for instant 1. Explain.

c. Are any of the forces that you drew for instant 1 missing from your diagram for instant 2?

If so, for each force that is missing, explain how you knew to include the force on the first diagram but not on the second.

d. Are any of the forces that you drew for instant 1 missing from your diagram for instant 3? If so, for each force that is missing, explain how you knew to include the force on the first diagram but not on the third.

NEWTON’S SECOND AND THIRD LAWS

Newton’s second and third laws

Tutorials in Introductory Physics ©Pearson Custom Publishing McDermott, Shaffer, & P.E.G., U. Wash. Updated Preliminary Second Edition, 2011

Mech HW–40

2. Two crates, A and B, are in an elevator as shown. The mass of crate A is greater than the mass of crate B.

a. The elevator moves downward at constant speed.

i. How does the acceleration of crate A compare to that of crate B? Explain.

ii. In the spaces provided below, draw and label separate free-body diagrams for the crates.

Free-body diagram for crate A

Free-body diagram for crate B

iii. Rank the forces on the crates according to magnitude, from largest to smallest. Explain your reasoning, including how you used Newton’s second and third laws.

iv. In the spaces provided at right, draw arrows to indicate the direction of the net force on each crate. If the net force on either crate is zero, state so explicitly. Explain.

Is the magnitude of the net force on crate A greater than, less than, or equal to that on crate B? Explain.

Elevator (moving down

at constant speed)

A

B

Cable

Crate A Crate B

Direction of net force

Newton’s second and third laws Name

Tutorials in Introductory Physics ©Pearson Custom Publishing McDermott, Shaffer, & P.E.G., U. Wash. Updated Preliminary Second Edition, 2011

Mech HW–41

b. As the elevator approaches its destination, its speed decreases. (It continues to move downward.)

i. How does the acceleration of crate A compare to that of crate B? Explain.

ii. In the spaces provided below, draw and label separate free-body diagrams for the crates in this case.

Free-body diagram for crate A

Free-body diagram for crate B

iii. Rank the forces on the crates according to magnitude, from largest to smallest. Explain your reasoning, including how you used Newton’s second and third laws.

iv. In the spaces provided at right, draw arrows to indicate the direction of the net force on each crate. If the net force on either crate is zero, state so explicitly. Explain.

Is the magnitude of the net force on crate A greater than, less than, or equal to that on crate B? Explain.

Crate A Crate B

Direction of net force

Newton’s second and third laws

Tutorials in Introductory Physics ©Pearson Custom Publishing McDermott, Shaffer, & P.E.G., U. Wash. Updated Preliminary Second Edition, 2011

Mech HW–42

3. A hand pushes three identical bricks as shown. The bricks are moving to the left and speeding up. System A consists of two bricks stacked together. System B consists of a single brick. System C consists of all three bricks. There is friction between the bricks and the table. a. In the spaces

provided at right, draw and label separate free-body diagrams for systems A and B.

b. The vector representing the acceleration of system A is shown at right. Draw the acceleration vectors for systems B and C using the same scale. Explain.

c. The vector representing the net force on system A is shown at right. Draw the net force vectors for systems B and C using the same scale. Explain.

d. The vector representing the frictional force on system A is shown below. Draw the remaining force vectors using the same scale.

N BH

N AB

N BA

f AT

f BT

Explain how you knew to draw the force vectors as you did.

A

B

Free-body diagram for system A

Free-body diagram for system B

Acceleration of A

Acceleration of B

Acceleration of C

Net force on A

Net force on B

Net force on C