Download Chapter 5 – Work and Energy Study Guide

Chapter 5 – Work and Energy
Study Guide
WORK:
1. For a force to do work:
a. There must be a displacement, and
b. The force must cause the displacement
2. Work is only done by forces (or components of forces) that are parallel to the displacement
3. No work is done by forces (or components of forces) that are perpendicular to the displacement
4. Work = force X displacement
5. W = F dcos θ
6. Wnet = Fnet  d  cos
7. Units of work: N m = J
8. PRACTICE PROBLEMS: #1 pg 156; #2, 3 pg 157
KINETIC ENERGY:
1.
2.
3.
4.
5.
Kinetic energy is the energy an object has due to its motion
KE = ½ mv2
Units for KE: J
Kinetic energy depends on the mass and velocity of an object
PRACTICE PROBLEMS: #4 pg 160;
WORK-KINETIC ENERGY THEOREM:
1. The net work done by all the forces acting on an object is equal to the change in the object’s kinetic energy
2. In other words, when you apply a force to an object over a distance, you change the object’s speed.
3.
Wnet = ΔKE
Wnet = KEf – KEi
Wnet = ½ mvf2 - ½ mvi2
Wnet = Fnet X d = ½ mvf2 - ½ mvi2
4. PRACTICE PROBLEMS: #3 pg 162
POTENTIAL ENERGY:
1. Potential energy is the stored energy of an object due to the object’s position or condition
a. Gravitational potential energy: the stored energy of an object due to its position
PEg = mgh
b. Elastic potential energy: the stored energy in any compressed or stretched object
PEelastic = ½ kx2
(k = spring constant; x = distance compressed or stretched)
2. Units for PE: J
3. PRACTICE PROBLEMS: #1, 3 pg 166
CONSERVATION OF ENERGY:
1. Mechanical energy (ME): the sum of kinetic energy and all forms of potential energy associated with an
object
ME = KE + ΣPE
2. Conservation of mechanical energy: in the absence of friction, the total mechanical energy remains the
same
MEi = MEf
½ mvi2 + mghi + ½kx2 = ½ mvf2 + mghf + ½kx2
3. In the absence of friction: if the potential energy decreases, then the kinetic energy increases; if the kinetic
energy decreases, then the potential energy increases.
4. PRACTICE PROBLEMS: #1 pg 171; #33 pg 182
POWER:
1. Power: the rate at which work is done; the rate of energy transfer
2. Units for Power: J/s; aka Watt (W). A kilowatt is 1000 W.
3. Power is based on time.
a. Two forces might do the same amount of work (F  d), but the force that does the work in the
shortest amount of time delivers more power.
4. PRACTICE PROBLEMS: #3, 5 pg 175