Transcript Mechanical Energy

The Law of Conservation of
Energy
Follow Me
Suppose a 0.500kg rock falls from a height of
78.4m and that its Eg at the bottom is zero (h=0).
1. It’s speed is:
2. Distance fallen is:
At the Top
3. Height is:
4. It’s gravitational potential energy at the top will
be:
5. It’s kinetic energy at the top will be 0.
At the Bottom
6. At the bottom, Eg=0 (h=0) and kinetic energy
is:
In The Middle
7. In the middle:
Eg+Ek=192+192=384J
Law of Conservation of Energy
• Energy can not be created or destroyed, only
changed from one form to another
• In any transfer or transformation of energy, the
total amount of energy remains constant.
• Energy is not necessarily changed into the form
you want (ex. Thermal energy produced by
friction)
Conservation of Energy Video
Wrecking Ball with Teacher (involves
conservation of energy)
http://www.youtube.com/watch?v=mhIOylZMg6Q
Example
As the water in a river approaches a 12.3 m vertical
drop, its average speed is 6.7 m/s. For each kilogram
of water in the river, determine the following:
a) the kinetic E at the top of the waterfall
b) the gravitational potential energy at the top of the
falls relative to the bottom
c) the total mechanical E at the bottom of the falls, not
considering friction (use the law of conservation of E)
d) the speed at the bottom of the falls (use the law of
conservation of E)
Challenge
Take a piece of gum, candy, anything with
nutritional information
Using the energy content, how high could you
throw a 10kg metal ball if your body converted
100% of this energy to the ball?
NOTE: You will need to use a conversion website
to choose the appropriate conversion factor (be
careful to note if calories is capitalized or not)
Practice Problems
1.
The toy car below continues to move and rolls off the table. Find the
car’s speed just before it hits the floor. [6.49m/s]
2. The speed of an acrobat swinging on a trapeze is 5.64 m/s at the lowest
point of her motion. Assume her mass is 53.7 kg.
(a) How high above the lowest point can she swing? [1.62m]
(b) Do you need to know her mass to answer part (a)? Explain.
Practice Problems
3. (a) When you flip a penny (2.35 g), it leaves your hand and moves upward at
2.85 m/s. Use energy to find how high the penny goes above your hand before
stopping. [0.414m]
(b) The penny then falls to the floor, 1.26 m below your hand. Use energy to
find its speed just before it hits the floor. [5.73m/s]
(c) Explain your choice of reference level for parts (a) and (b).
(d) Choose a different reference level and repeat part (b).
4. A roller-coaster train and its passengers have a combined mass of 1250 kg.
The train comes over the top of the first hill, 53.2 m above the ground, with a
speed of 1.17 m/s.
(a) The train goes down the first hill and through a loop. Ignoring friction,
calculate the speed of the train at the top of the loop, 21.3 m above the ground.
[25.0m/s]
(b) Before applying its brakes at the end of the ride, the train moves along a
level stretch of track 2.71 m above the ground. The train’s speed is 24.3 m/s.
How much mechanical energy has been lost during the ride? Where did this
energy go? [2.51 x 105 J]
Research Applications
1. Research into a machine or device that uses the
law of conservation of mechanical energy to
perform its primary function. Explain how the law
is applied using specific, relevant terminology
from this unit.
2. Research into another machine or device that uses
the law of conservation of energy to perform its
primary function, but not transforming
mechanical energy. Explain how the law is applied
using specific, relevant terminology from this unit.
Extra Help
Everything you need to know from this lesson:
http://www.youtube.com/watch?v=iYEWIuQBVyg