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Problems for forces and friction

  1. Your shoes have rubber soles. As you stand on an asphalt walkway, the coefficient of static friction between your shoes and the ground is mu = 0.95. A giant mutant gopher, hidden in his tunnel directly below you, begins to dig up towards the surface. The ground tilts slowly, so you are able to keep your balance and stand on your two feet ... but the ground becomes more and more vertical. At what angle will your feet start to slide along the asphalt?

  2. Big block A, with mass mA = 89 kg, sits on a frictionless floor. Little block B, with mass m_B = 15 kg, is held against the right side of block A. The coefficient of static friction between the two blocks is 0.72.

    At time t=0, a force F starts to push the big block A. At the same time, the little block B is released. How large must the force F be in order to keep the little block B from sliding down to the floor? If the force is exactly this minimum required magnitude, how long will it take the blocks to slide 50 meters across the floor?

  3. A hockey puck of mass m = 0.5 kg slides without friction in a circle of radius R = 80 cm on a table. The puck is attached to a string which hangs down through a hole in the middle of the table; from the other end of the string hangs a weight of mass M = 4 kg.

    How fast must the puck be moving in order to keep the weight motionless?

    Actually, the puck is moving at a different speed: it makes two revolutions every second. What is the puck's actual speed?

    If the puck moves at this speed, is the weight M moving up or down?

    What is the acceleration of the weight M?

  4. A 2.0 kg block and a 1.0 kg block are connected by a string and are pushed across a horizontal surface by a force applied to the 1.0 kg block. The coefficient of friction between the blocks and the horizontal surface is 0.20. If the magnitude of the force F is 20 N, what is the tension in the string that connects the two blocks?

  5. A roller-coaster has a mass of 1200 kg when fully loaded with passengers. As the car passes over the top of a circular hill of radius 18 m, its speed is not changing. What are the magnitude and direction of the force of the track on the car at the top of the hill if the car's speed is (a) 11 m/s, and (b) 14 m/s?

  6. A horse pulls a barge down a canal. Suppose the horse pulls the rope with a force of 7900 N at an angle of 18 degrees to the direction of motion of the barge, which is headed straight along the canal. The mass of the barge is 9500 kg, and its acceleration is 0.12 m/s2. What are the (a) magnitude and (b) the direction of the force on the barge from the water?

  7. Riddick is 85 kg of pure muscle. As he grabs onto a rope to climb up out of the underground prison, his hands squeeze the rope with a grip of 1000 Newtons.

    What is the minimum coefficient of static friction between Riddick's hands and the rope which allow him to hang (by one hand) on the rope?

    Riddick climbs the rope by holding on with one hand at a time and pulling himself upward, so that he grabs the rope 1 m higher up with the other hand. During each hand-over-hand manuever, he accelerates upwards at a uniform rate for half the distance, and then decelerates uniformly for the other half, coming to a momentary rest while he shifts his grip to the other hand.

    What is the minimum coefficient of static friction which will allow Riddick to climb up a height 12 meters in 10 seconds?

    Optional: Extra Credit ...

  8. Sample Problem 6-8 describes "The Rotor", an amusement park ride also known as "The Cyclone" or "The Twister". Review the problem first.

    Joe Midway, a travelling carnival worker, is assigned the job of setting up "The Rotor" when the show arrives in Rochester. Just as in the sample problem, the ride has a radius of R = 2.1 meters. In this case, however, the coefficient of static friction between the walls and clothing is mu_s = 0.3.

    Unfortunately, Joe has a bad hangover, and he makes two mistakes. First, he tilts the walls of the cylinder so that they tilt outwards at theta = 30 degrees from vertical. Second, he forgets to engage the device which limits the speed of the ride.

    As a result, when the first batch of RIT students get on the ride, there is trouble. The cylinder starts to spin, getting faster and faster and FASTER. The ride is supposed to stop accelerating after the speed reaches 8 meters/second ... but this time it just keeps accelerating.

    At what speed will the RIT students start to slide UP the walls?


Sorry, we could not find this page | RIT CIS - Center for Imaging Science

Sorry, we could not find this page

We apologize, but the page you were looking for is not available. Most of our material is available from the menus above.

Last Modified: 2:01pm 10 Aug 11