Physics 2 - UCSB Campus Learning Assistance Servicesclas.sa.ucsb.edu/staff/vince/physics 2/Physics 2 Ch11 problems.pdf · Physics 2 Chapter 11 problems ... front wheel, or just realize

Physics 2

Chapter 11 problems

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

11.44 A pickup truck has a wheelbase of 3m. Ordinarily 10,780N rests on the front wheels and 8,820N on the rear wheels when parked on a level road.a) A box weighing 3600N is now placed on the tailgate, 1m behind the rear axle.

How much total weight now rests on the front wheels? On the rear wheels?b) How much weight would need to be placed on the tailgate to make the front

wheels come off the ground?

It will be useful to find the center of mass of the truck before the weight is added to the tailgate. Choosing x=0 at the rear axle:

m65.1107808820

)3)(10780()0)(8820(xcm

We have a couple of options to solve this problem.We could continue to use center of mass ideas, or we could use torques. Both methods give pretty much the same formulas.

Balancing torques (remember, clockwise is negative):

FfrontFrear

3600 N

19,600 N

Pivot Pointx=0 here

N9590F

0)0)(F()m65.1)(N19600()m1)(N3600(

front

front

We could use another torque equation with the pivot at the front wheel, or just realize that the forces must also balance, leaving a total of 13,610N for the rear force.

For the final part, we could make the center of mass come out at the rear wheel, or force the torques to balance (with Ffront=0).

N340,32W

0)m65.1)(N19600()m1)(W(

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

11.72 You are trying to raise a bicycle wheel of mass m and radius R up a curb of height h. To do this, you apply a horizontal force F. What is the least magnitude of the force F that will succeed in raising the wheel onto the curb when the force is applieda) at the center of the wheel?b) at the top of the wheel?c) In which case is less force required?

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

11.77 An engineer is designing a conveyor system for loading hay bales into a wagon. Each bale is 0.25m wide, 0.50m high, and 0.80m long (the dimension perpendicular to the plane of the figure), with mass 30kg. The center of gravity of each hay bale is at its geometrical center. The coefficient of static friction between a bale and the conveyor belt is 0.60, and the belt moves with constant speed.

a) The angle of the conveyor is slowly increased. At some critical angle a bale will tip (if it doesn’t slip first), and at some different critical angle it will slip (if it doesn’t tip first). Find the two critical angles and determine which happens at the smaller angle.

b) Would the outcome of part (a) be different if the coefficient of friction were 0.40?

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

11.87 A 1.05-m long rod of negligible weight is supported at its ends by wires A and B of equal length. The cross-sectional area of A is 2 mm2 and that of B is 4 mm2. Young’s Modulus for wire A is 1.8x1011 Pa; that for B is 1.2x1011 Pa. At what point along the rod should weight w be suspended to producea) equal stresses in A and B?b) b) equal strains in A and B?

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB

Prepared by Vince Zaccone

For Campus Learning Assistance Services at UCSB