A box of mass M=0.2kgM0.2kg starts out at rest on a horizontal table. The surfac

Question

A box of mass M=0.2kgM0.2kg starts out at rest on a horizontal table. The surface is notfrictionless. At t=0.75st0.75s you start pushing on the box horizontally in the +xx direction, causing it to speed up. The force you push with is not necessarily constant. You stop pushing on the box at t=2st2s, after which it slides on its own as it slows down and comes to rest at t=4st4s. Between t=2st2s and t=4st4sthe acceleration of the box is 2m/s22ms2.

A

B

Both graphs are possible.

2) What is the coefficient of kinetic friction between the box and the table ?

0.100.10

0.150.15

0.200.20

0.250.25

Not enough information is provided to determine this.

3) What is the speed of the box at t=2st2s?

Not enough information is provided to determine this.

1 m/s1 ms

2 m/s2 ms

3 m/s3 ms

4 m/s4 ms

4) Instead of pushing on the box to get it moving, suppose you slowly lifted the left side of the table and noticed that the box started to slide when the angle between the surface of the table and horizontal reached 15 degrees. What is the coefficient of static friction between the box and the table?

5) For the final two questions suppose you are using the same box (M=0.2kgM0.2kg) but a different horizontal surface so that the coefficient of friction is not necessarily the same as you found above.

0.100.10

0.150.15

0.200.20

0.250.25

0.300.30

6) Use the data shown above to determine which of the following answers is closest to the coefficient of static friction between the box and the table?

0.100.10

0.150.15

0.200.20

0.250.25

0.300.30

Explanation / Answer

1)    option A plots describe the xx component of the acceleration .

2)   coefficient of kinetic friction = 2/9.8

                                                     = 0.200

3)    speed of the box at t=2s = 2 * 2

                                                 = 4 m/sec

4)    coefficient of static friction   = tan15

                                                     = 0.2679

5)    coefficient of kinetic friction = 0.3/1.96

                                                      = 0.15

6) coefficient of static friction = 0.4/1.96

                                                      = 0.20

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