91Ó°ÊÓ

Only one force acts on an object. Can the object have zero acceleration? Can it have zero velocity? Explain.

Superman must stop a 120-km/h train in 150 m to keep it from hitting a stalled car on the tracks. If the train’s mass is$\mathbf{3}\mathbf{.}\mathbf{6}\mathbf{×}{\mathbf{10}}^{\mathbf{5}}\mathbf{kg}$, how much force must he exert? Compare to the weight of the train (give as %). How much force does the train exert on Superman?

What causes the boat in Fig. 4-41 to move forward?

(a) The force that the man exerts on the paddle

(b) The force that the paddle exerts on the water

(c) The force that the water exerts on the paddle

(d) The motion of the water itself

A child on a sled reaches the bottom of a hill with a velocity of$\mathbf{10}\mathbf{.}\mathbf{0}\mathbf{m}\mathbf{/}\mathbf{s}$and travels 25.0 m along a horizontal straightaway to a stop. If the child and sled together have a mass of 60.0 kg, what is the average retarding force on the sled on the horizontal straightaway?

(a) A box sits at rest on a rough 33° inclined plane. Draw the free-body diagram, showing all the forces acting on the box. (b) How would the diagram change if the box were sliding down the plane? (c) How would it change if the box were sliding up the plane after an initial shove?

A wet bar of soap slides down a ramp 9.0 m long inclined at 8.0°. How long does it take to reach the bottom? Assume${\mathrm{μ}}_k=\mathbf{0}\mathbf{.}\mathbf{060}$.

A skateboarder, with an initial speed of$\mathbf{2}\mathbf{.}\mathbf{0}\mathbf{m}\mathbf{/}\mathbf{s}$rolls virtually friction free down a straight incline of length 18 m in 3.3 s. At what angle is the incline oriented above the horizontal?

Question: (II) Uphill escape ramps are sometimes provided at the side of steep downhill highways for trucks with overheated brakes. For a simple 11° upward ramp, what minimum length would be needed for a runaway truck traveling at 140 km/h. Note the large size of your calculated length. (If sand is used for the bed of the ramp, its length can be reduced by a factor of about 2.)

A person pushes a 14.0-kg lawn mower at constant speed with a force of$F=\mathbf{88}\mathbf{.}\mathbf{0}\mathbf{N}$directed along the handle, which is at an angle of 45.0° to the horizontal (Fig. 4–58). (a) Draw the free-body diagram showing all forces acting on the mower. Calculate (b) the horizontal friction force on the mower, then (c) the normal force exerted vertically upward on the mower by the ground. (d) What force must the person exert on the lawn mower to accelerate it from rest to 1.5 m/s in 2.5 seconds, assuming the same friction force?

FIGURE 4-58 Problem 50.

Question: (II) The block shown in Fig. 4–59 has mass$m=7.0\mathrm{kg}$and lies on a fixed, smooth, frictionless plane tilted at an angle of$\mathrm{θ}=22.0°$to the horizontal. (a) Determine the acceleration of the block as it slides down the plane. (b) If the block starts from rest at 12.0 m above the plane from its base, what will be the block's speed when it reaches the bottom of the incline?