HW#16 Ch 6: Force and Motion — II
Instructions: Show all work using Newton’s 2nd law and free body diagrams.
1. The coefficient of kinetic friction between a jeep’s tires and the flat road is µk = 0.5. If the net force on the jeep is the force of kinetic friction exerted by the road, (a) what is the maximum deceleration of the jeep when the brakes are applied? (b) What is the shortest distance in which the jeep can stop if it is traveling at 25 m/s before the application of the brakes?
2. Your pet horseshoe crab, Zippy, has grown to 4.9 kg and is feeling somewhat lethargic. Sensing that something is missing from his life, you decide to give him a thrill ride by attaching a rope to his exoskeleton with a bit of contact cement. (Your friend complains that this is animal cruelty, but you reply that he is about to molt.) The other end is attached to a hanging mass of 3.2 kg via a low-friction, low mass pulley, as shown in the accompanying figure. If the coefficient of kinetic friction between the crab and the table is 0.4, find the time that it takes for Zippy to scoot 1.2 m across the table, if he starts from rest.
3. A shipping crate of mass m1 = 28,200 kg is at rest on a ramp that makes an angle of θ = 10° with the horizontal. The coefficient of kinetic friction between the crate and the ramp is µk = 0.207. The crate is attached to a large lead counterweight of mass m2 = 12,000 kg that hangs vertically by means of a steel cable that passes over a low-friction, lightweight pulley. The lead counterweight is then released from rest. When it has fallen 70.0 cm, what is its speed?
4. It’s your friend’s birthday, and you want to impress her with your knowledge of physics, so you build a rocket car to deliver her birthday present. The front of the car has a vertical piece of plywood with a coefficient of static friction of µs = 0.64. You reason that if you accelerate fast enough, there will be enough friction between the plywood and the present so that it will not fall. a) If the present weighs 1.2 kg, what is the minimum acceleration that you need to prevent the present from falling? b) How great is the frictional force between the plywood and present? c) What is the force of friction on the present if you accelerate at twice the minimum acceleration in part a?
