HRW Physics 5.2 Energy

1. 5.2 Energy
Applied Physics

2. Objectives
• Identify several forms of energy.
• Calculate kinetic energy for an object.
• Distinguish between kinetic and potential energy.
• Classify different types of potential energy.
• Calculate the potential energy associated with an object's
position.

3. Kinetic Energy
• kinetic energy: energy of an object due to its motion
• KE = ½ mv2
• kinetic energy = ½ · mass · velocity2
• KE is a scalar quantity whose units are joules (J)

4. Sample 5B - p.173
• A 7.00 kg bowling ball moves at 3.00 m/s. How much KE does the
bowling ball have?
• How fast must a 2.45 g ping-pong ball move in order to have the same
KE as the bowling ball?

5. Practice 5B
p.173 #1-3

6. Potential Energy
• potential energy: energy associated with an object because of the position,
shape, or condition of the object
When an object falls, where does the KE come from?
- Gravity converts the energy of position aka gravitational potential energy
(GPE)

7. Potential Energy
• gravitational potential energy: PE stored due to the object's position
relative to a gravitational source
PEg = mgh
• gravitation potential energy = mass · accelerationg · height
*Only valid when the free-fall acceleration is constant over the entire height

8. Potential Energy
• PEg is a result of an object's position, so it must be measured relative to
some zero level.
• The zero level is where PEg is defined to be zero.
• The zero level is arbitrary, but is chosen to make a specific problem easier
to solve.
• Most problems will suggest what to use as a zero level.

10. Potential Energy
• elastic potential energy: PE that is stored when an elastic object is
deformed by stretching or compressing it
• Ex: rubber band, bungee cord, springs on car
• Length of a spring when no external forces are acting on it is called the
relaxed length of the spring
• Amount of PEelastic depends on the distance the spring is
compressed or stretched from its relaxed length

11. Elastic Potential Energy
PEelastic = ½ kx2
• PEe = ½ · spring constant · (distance compressed or stretched)2
• k is the spring constant, or force constant
• Flexible springs have a small spring constant while stiff springs have a
large spring constant
• Spring constant has a unit of N/m

12. • Picture on p.175 - Regarding Spring Distance
• If given two values, subtract to find the actual value for x

13. Sample 5C
• A 70.0 kg stuntman is attached to a bungee cord with an unstretched
length of 15.0 m. He jumps off a bridge spanning a river from a height
of 50.0 m. When he finally stops, the cord has a stretched length of
44.0 m.
• Assuming the spring constant of the bungee cord is 71.8 N/m, what is
the total potential energy relative to the water when the man stops
falling?

14. Practice 5C
p.177 #2, 3, 4c

15. Mechanical Energy
• Motion of many objects is a combination of KE and PE
• Ex:
• There are many other forms of energy, such as chemical PE,but we ignore
them because their influence is negligible or they're not relevant to what's
being analyzed
• mechanical energy: sum of KE and all forms of PE
• ME = KE + all different forms of the PE

16. Section Review
p.178 #1, 5-7

17. Homework
p.194 #12, 16, 17, 20