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Unformatted text preview: Physics 2070
Quiz 2
Read the entire problem carefully. Show all work and explain your reasoning when asked. Be
organized with solutions and circle your answer.
1. Today, the Johnson family is moving and the movers just arrived to load the truck. Boxes of
the same mass must be moved up a ramp from the basement and also up a ramp into the
moving truck. Both ramps have negligible friction and the vertical displacement of both boxes
is the same. The mover exerts forces of the same magnitude up both ramps, in each case
parallel to the ramp. a. Ms. Johnson assumes that the mover will do more work trying to get the boxes into the
truck than out of the basement because the truck ramp is steeper. She offers assistance
with the truck ramp. Mr. Johnson assures her that since the ramps have equal height
increases the mover will do the same amount of work moving the boxes over both
ramps and that she should make herself useful somewhere else.
Do you agree with Ms. Johnson, Mr. Johnson, or neither? Explain your reasoning.
Neither is correct. Since the workers exert the same force in both cases,
parallel to the ramp, more work is done on box 1 than on box 2. This is
because, for box 1, the force is applied over a longer distance.
b. Is the potential energy gained by box 1 greater than, less than, or equal to the potential
energy gained by box 2? Explain.
Since both boxes end up at the same final height and have equal mass,
then both boxes will have the same gain in gravitational potential energy.
c. Assuming both boxes start from rest, rank the kinetic energies of box 1 and box 2 just
as they reach the tops of their respective ramps. Show all work and explain your
reasoning.
Box 1 will have more kinetic energy at the top of its ramp than box 2.
This is because box 1 has more work done on it than box 2 (so it gains
more total energy), but gains the same amount of potential energy as box
2, therefore it will have a larger kinetic energy at the top. 2. A 1.50 kg block rests on a
rough, horizontal surface with a
coefficient of kinetic friction of
0.115, as shown in the figure.
The block is pressed back
against a spring with a spring
constant of 1.00 x 103 N/m,
compressing the spring by 10.0
cm. Then the block is released. The block moves a distance of 1.20 m along the horizontal
surface until it comes to the base of a frictionless ramp.
a. What is the kinetic energy of the block just as it leaves the spring?
As the block leaves the spring, all the elastic potential energy in the
spring is converted to kinetic energy of the block just as it leaves the
spring b. How much work is done by friction as the block moves to the base of the ramp?
The work done by friction is given by Wf = fd, and the force of friction
on the block is needed. Remember that f = µN and for an object on a
horizontal surface with no other vertical forces except gravity and the
normal force that the magnitude of the normal force is N = mg. Then: c. What is the kinetic energy of the block at the base of the ramp?
The kinetic energy at the base of the ramp is given by : d. What is the maximum height, h, the block will rise up the frictionless ramp?
At the maximum height, all the energy at the bottom of the hill has been
converted to gravitational potential energy at the top of the hill: e. What is the kinetic energy of the block when it reaches this maximum height?
At maximum height, all the kinetic energy at the base of the ramp has
been converted to gravitational potential energy, so the kinetic energy of
the block at the maximum height is 0J. ...
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This note was uploaded on 12/21/2011 for the course PHYS 2070 taught by Professor Thompson during the Spring '08 term at Toledo.
 Spring '08
 THOMPSON
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