21) You push downward on a box at an angle 25° below the horizontal with a force of 750 N. If the box is on a flat horizontal surface for which the coefficient of static friction with the box is 0.76, what is the mass of the heaviest box you will be able to move?

A) 59 kg

B) 68 kg

C) 54 kg

D) 82 kg

22) A person is dragging a packing crate of mass 100 kg across a rough horizontal floor where the coefficient of kinetic friction is 0.400.  He exerts a force F sufficient to accelerate the crate forward. At what angle above horizontal should his pulling force be directed in order to achieve the maximum acceleration?

A) 21.8°

B) 27.7°

C) 30°

D) 34.5°

E) 45°

23) A packing crate rests on a horizontal surface. It is acted on by three horizontal forces: 600 N to the left, 200 N to the right, and friction. The weight of the crate is 400 N. If the 600-N force is removed, the resultant force acting on the crate is

A) zero.

B) 200 N to the right.

C) 200 N to the left.

D) 400 N to the left.

E) impossible to determine from the information given.

24) A 6.0 kg box slides down an inclined plane that makes an angle of 39° with the horizontal. If the coefficient of kinetic friction is 0.19, at what rate does the box accelerate down the slope?

A) 4.7 m/s2

B) 5.2 m/s2

C) 5.5 m/s2

D) 6.2 m/s2

25) A 200 g hockey puck is launched up a metal ramp that is inclined at a 30° angle. The coefficients of static and kinetic friction between the hockey puck and the metal ramp are μs = 0.40 and μk = 0.30, respectively. The puck’s initial speed is 63 m/s.  What vertical height does the puck reach above its starting point?

A) 130 m

B) 270 m

C) 200 m

D) 66 m

26) A 200 g hockey puck is launched up a metal ramp that is inclined at a 30° angle. The coefficients of static and kinetic friction between the hockey puck and the metal ramp are μs = 0.40 and μk = 0.30, respectively. The puck’s initial speed is 4.9 m/s. What speed does it have when it slides back down to its starting point?

A) 2.8 m/s

B) 3.5 m/s

C) 4.2 m/s

D) 4.9 m/s

27) A 4.00-kg block rests on a 30.0° incline as shown in the figure.  If the coefficient of static friction between the block and the incline is 0.700, what magnitude horizontal force F must act on the block to start it moving up the incline?

A) 34.0 N

B) 51.1 N

C) 54.7 N

D) 84.0 N

E) 76.4 N

28) A box is sliding down an incline tilted at a 12.0° angle above horizontal.  The box is initially sliding down the incline at a speed of 1.50 m/s.  The coefficient of kinetic friction between the box and the incline is 0.340.  How far does the box slide down the incline before coming to rest?

A) 0.920 m

B) 2.33 m

C) 1.78 m

D) 0.720 m

E) The box does not stop.  It accelerates down the plane.

29) A 50.0-kg block is being pulled up a 16.0° slope by a force of 250 N which is parallel to the slope. The coefficient of kinetic friction between the block and the slope is 0.200. What is the magnitude of the acceleration of the block?

A) 0.528 m/s2

B) 0.158 m/s2

C) 0.412 m/s2

D) 0.983 m/s2

E) 0.260 m/s2

30) In the figure, a T-bar ski tow pulls a skier up a hill inclined at 10° above horizontal. The skier starts from rest and is pulled by a cable that exerts a tension T at an angle of 30° above the surface of the hill. The mass of the skier is 60 kg and the effective coefficient of kinetic friction between the skis and the snow is 0.100. What is the maximum tension in the cable if the starting acceleration is not to exceed 0.400 g?

A) 246 N

B) 366 N

C) 431 N

D) 187 N

E) 535 N