work, energy and power - physicsmsbrown.weebly.com · work, energy and power test: tuesday 11/13....
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Example Problem #1
A box slides down an inclined plane that is at an angle of 37° to the
horizontal. The mass of the block, m, is 35kg, the coefficient of kinetic
friction between the block and the ramp, μk, is 0.3 and the length of the ramp, d, is 8m.
a) How much work is done by gravity?
b) How much work is done by the normal force?
c) How much work is done by friction?
d) What is the total work done?
Example Problem #1
A box slides down an inclined plane that is at an angle of 37° to the
horizontal. The mass of the block, m, is 35kg, the coefficient of kinetic
friction between the block and the ramp, μk, is 0.3 and the length of the ramp, d, is 8m.
a) How much work is done by gravity?
b) How much work is done by the normal force?
c) How much work is done by friction?
d) What is the total work done?
Example Problem #1
A box slides down an inclined plane that is at an angle of 37° to the
horizontal. The mass of the block, m, is 35kg, the coefficient of kinetic
friction between the block and the ramp, μk, is 0.3 and the length of the ramp, d, is 8m.
a) How much work is done by gravity?
b) How much work is done by the normal force?
c) How much work is done by friction?
d) What is the total work done?
Example Problem #1
A box slides down an inclined plane that is at an angle of 37° to the
horizontal. The mass of the block, m, is 35kg, the coefficient of kinetic
friction between the block and the ramp, μk, is 0.3 and the length of the ramp, d, is 8m.
a) How much work is done by gravity?
b) How much work is done by the normal force?
c) How much work is done by friction?
d) What is the total work done?
Example Problem #2
What is the work done by a spring that pushes and
object out from x=-x2 to x=-x1 (where x2>x1)?
Example Problem #3
A force F(x) acts on a particle. The force is related to the
position of the particle by the formula F(x) = Cx3, where C
is a constant. Find the work done by this force on the
particle when the particle moves from x = 1.5m to x = 3m.
Graphical Analysis
Work is the area under the curve of F vs d graph.
Force might be constant or might be variable.
Example Problem #4
A tennis ball, m=0.06kg, is hit straight upward with an initial
speed of 50m/s. How high would it go if air resistance were
negligible?
Types of Force
Conservative
Obeys conservation
of energy
Examples
Spring force
Gravity
Non-conservative
Energy is transferred into
non-mechanical forms
Examples
Friction
Air drag
Example Problem #5
A box is projected up a long ramp with an incline of 37° to
the horizontal with an initial speed of 10m/s. If the surface of
the ramp is very smooth (frictionless), how high up the ramp
will the box go? What distance along the ramp will it slide?
Example Problem #6
A crash test dummy, m=40kg, falls off of a 50m high cliff. On
the way down, the force of air resistance has an average
strength of 100N. Find the speed with which he crashes into
the ground.
Net Work done by conservative & non-
conservative forces is equal to the change in
kinetic energy
But work done by conservative forces is also
equal to the negative of the change in
potential energy
Example Problem #7
The force acting on an object is given by F(x) = a/x2, where
a is a constant. Determine the potential energy of the
object as a function of x.