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WORK / ENERGY concept WS
A string is used to pull a wooden block across the floor without accelerating the block. The string makes an
angle to the horizontal, as shown in the diagram.
a. Does the force applied via the string do work on the block? Explain.
b. Is the total force involved in doing work, or just a portion of the force? Explain.
Does the frictional force, which is obviously present, do work on the block? Explain.
d. Does the normal force of the floor pushing upward on the block do any work? Explain.
e. Does the block gain any energy during this interaction? What does that tell you about the work done
by the string and by friction?
A ball is being twirled in a circle at the end of a string; the string provides the centripetal force necessary to
keep the ball moving in the circle at constant speed. Does the force applied by the string do work on the ball
as it moves through one complete circular path? Explain.
Different net forces (let’s call them A and B) accelerate two blocks of the same mass. One block gains a
speed twice that of the other block in the process. Is the work done by the net force on the faster moving
block twice that done on the slower block? Explain.
A block is moved from the floor up to a tabletop, but gains no velocity in the process. Is there work done on
the block? If so, what has happened to the energy added to the system?
A pendulum is pulled back from its equilibrium position and then released.
a. What form of energy is added to the system prior to its release?
b. At what points in the motion of the pendulum is its kinetic energy the greatest?
Are there points along the bob's path at which its mechanical energy exists in both states at the same
time? If so, where?
d. In reality, is the mechanical energy of the pendulum conserved during its motion? If not, where does the
energy go?
A mass attached to a spring, which in turn is attached to a wall, is free to move upon a frictionless, horizontal
surface—like an air-hockey table. The mass is pulled back and then released.
a. What form of energy is added to the system prior to the release of the mass?
b. At what points in the motion of the mass is its potential energy the greatest?
At what points is the kinetic energy the greatest?
Give an example of an object with both gravitational potential and kinetic mechanical energy at the same
When a stone is dropped from a bridge, work is done on the stone by gravity. Yet the stone neither gains nor
loses any mechanical energy. Does this violate the idea of mechanical energy conservation? Explain.
Two objects are dropped onto level ground, with object A falling four times the distance of object B. How will
their speeds compare just before they hit the ground?
(back to the last question) Assuming they do not bounce, but simply hit and stop, where did the mechanical
energy go?
Below is a track for an old roller coaster ride. Assuming no external motors or propellants (or retarding
forces), where along the track should the car come to rest initially? Derive an equation that could be used to
calculate the minimum starting height, h, necessary to safely negotiate the loop of radius 'r'.