20)
A loaded penguin sled with weight W rests on a plane inclined at angle θ to the horizontal (Fig. 6-28). Between the sled and the plane, the coefficient of static friction is µs, and the coefficient of kinetic friction is µk. (a) What is the minimum magnitude of the force 
parallel to the plane, that will prevent the sled from slipping down the plane? (b) What is the minimum magnitude F that will start the sled moving up the plane? (c) What value of F is required to move the sled up the plane at constant velocity? State your answers in terms of the given variables.
66) A box of canned goods slides down a ramp from street level into the basement of a grocery store with acceleration 0.63 m/s2 directed down the ramp. The ramp makes an angle of 41° with the horizontal. What is the coefficient of kinetic friction between the box and the ramp?
74) A high-speed railway car goes around a flat, horizontal circle of radius 430 m at a constant speed. The magnitudes of the horizontal and vertical components of the force of the car on a 45.0 kg passenger are 217 N and 441 N, respectively. (a) What is the magnitude of the net force (of all the forces) on the passenger? (b) What is the speed of the car?
78) A 115 g hockey puck sent sliding over ice is stopped in 12.4 m by the frictional force on it from the ice. (a) If its initial speed is 6.9 m/s, what is the magnitude of the frictional force? (b) What is the coefficient of friction between the puck and the ice?