2.  How can I solve problems using the law
of conservation of energy?

3.  Law of Conservation of Energy: in a
closed, isolated system, energy cannot
be created or destroyed
 Energy can be changed from one form
to another, but the total amount of
energy stays the same
 E1 = E2

4.  We will consider only mechanical energy
 Potential and Kinetic
 E = PE + KE
 Ignore other forms (air resistance, etc…)

5.  E1 = E2
 KEi + PEi = KEf + PEf

6.  Suppose you a at the top of a ski slope
 What kind of energy do you have?
 Once you start skiing, what happens to
your energy?
 What kind of energy do you have at the
bottom of the slope?

8.  You are holding a ball 2.0 m above the
ground. What is the PE of the ball?
 You drop the ball, what is the PE after it falls 1.0
m?
 What is KE after it falls 1.0 m?
 What is PE when it reaches the ground?
 What is KE just before it reaches the ground?

9.  Look for the word rest, that means v = 0
 When an object is at the peak of its
trajectory, v = 0

10. During a hurricane, a large tree limb, with
a mass of 22.0 kg and a height of 13.3 m
above the ground, falls on a roof that is 6.0
m above the ground.
a. Ignoring air resistance, find the kinetic
energy of the limb when it reaches the
roof.
b. What is the speed of the limb when it
reaches the roof?

11.  Page 297, # 15 - 18
 Page 308, # 73 - 77

12. The largest apple ever grown was 1.47 kg.
Suppose you hold such an apple in your
hand 1.50 m above the ground. You
accidentally drop the apple, then
manage to catch it when it is 0.5 m above
ground.
a. What was the apples KE at this point?
b. What was its velocity?

13.  In the real world, there is friction
 How do we account for it in our
conservation of energy equation?
 E1 + Wf = E2

14. A 36.0 kg child slides down a slide that is
2.5 m high. At the bottom of the slide, she
is moving at 3.0 m/s. How much energy
was lost as she slid down the slide?

15.  Worksheet

16.  How can we analyze collisions to find the
change in kinetic energy?

17.  We will analyze collisions just before and
just after the actual collision
 If the system is isolated, then momentum
and energy are conserved
 Energy can be converted to other forms
› Potential, thermal and sound energy
› Can increase, stay the same or decrease

18.  Cannot predict whether kinetic energy is
conserved
 Strategy:
› Conservation of momentum first
› Conservation of kinetic energy second

20.  KE increases: superelastic or explosive
› A compressed spring was released
 KE remains the same: elastic collision
› Hard elastic items such as marbles
 KE decreases: Inelastic
› Energy is converted to other forms
› Soft items, car crashes, when items stick
together after the collision

21.  Momentum and energy are different
 Momentum is (almost) always conserved
 Energy is only conserved in elastic
collisions
 Momentum is what makes objects stop

22. In an accident on a slippery road, a compact
car with a mass of 575 kg moving at 15.0 m/s
smashes into the rear end of a car with mass
1575 kg moving at 5.00 m/s in the same
direction.
a. What is the final velocity if the cars stick
together?
b. How much kinetic energy was lost in the
collision?
c. What fraction of the original kinetic energy
was lost?

23.  Page 300, # 19 – 21
 Page 309, # 78 – 82, 85