Nose Crusher
| Onsite |
St. Mary's U. Astronomy and Physics Dept.
D) Law of Conservation of Energy - unless work is done on or by a system, its total mechanical energy does not change.
1. Total Mechanical Energy
= PE + KE
Ex 1) Falling object(no friction)
Answer
As the object falls,
PE Lost = ....
...KE gained
Predict how far up the incline each ball will go.
(Assume no friction)
Why does each ball rise to the
same height
from which it began?
Answer:
PE depends only on height. Ball stops when all KE turns into PE
Ex 2)
IF KE at A = 0
PE = mgh, g = 10 m/s2
| PE | KE | |
| A | 90 J | 0 |
| B |
30
J |
60 J |
| C |
0
J |
90 J |
| D |
30 J |
60 J |
| E |
0 J |
90 J |
| F |
30 J |
60 J |
| G |
90 J |
0 J |
| H | Never gets there! Doesn't have the energy to go any higher than G | |
Physics of Designing Rollercoasters
| Onsite |
Ex 3) Pendulum
Click on a length to start pendulum
|
Full Size |
St. Mary's Physics
If PE at A = 10 J, KE = 0
| PE | KE | |
| A | 10 J | 0 J |
| B | 0 J |
10 J |
| D | 10 J | 0 J |
If PE at C = 3 J, KE = ?
KE = 7 J
All Pendulums
|
PE Max. at
Extremes
KE Max. at
Bottom
PE at Extremes =
KE at Bottom
|
|
Ex 4) 1.0 kg pendulum swings to
a height of .20 m above its lowest
point. |
Ex 4) 1.0 kg pendulum swings to height of .20 m above lowest point. KE of pendulum at lowest point?
| PE @ A = KE @ B |
PE = mgh
= (1.0 kg)(9.8 m/s2)(.20 m)
PE = 2.0 J
PE = KE @ B = 2.0 J
|
Ex 5) 5.00-kg cart at
foot of a hill 10.0 m high. For cart to
reach the top of the hill, what is minimum KE of cart in the position
shown? |
Ex 5)
5.00-kg cart at
foot of hill 10.0 m high.
For cart to reach the top of the hill,
what is minimum KE
of the cart in the position shown?
[neglect friction.]
m = 5.00 kg
h = 10.0 m
KE = ?
KE at bottom = PE at 10.0 m
KE = PE = mgh
= 5.00 kg(10.0 m/s2)10.0 m
KE = 500. J
Author H. Christiaen, K.U.Leuven, Belgium
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