Download 15.13 Energy Conservation Problems

15.13
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Energy Conversion and Conservation
Using the Law of Conservation of Energy
Energy Conversion and Conservation: energy can be changed from one form to another. The
process of changing energy from one form to another is call energy conversion. For example, a
light bulb changes electrical energy into thermal energy (heat) and electromagnetic energy
(light). The law of conservation of energy states that energy cannot be created or destroyed.
Thus, when energy changes from one form to another, the amount of energy remains the same.
Examples of Energy Conversion: One of the most common energy conversions is between
potential energy and kinetic energy. Whenever an object falls, the gravitational potential energy
of the object is converted to the kinetic energy of motion. When a spring is released, it also
results in the conversion of potential energy to kinetic energy.
The swinging of a pendulum and the movement of a pole vaulter are other examples of kinetic
and potential energy conversions. When a pendulum swings from side to side, it has kinetic
energy at the high point of each swing, the pendulum momentarily stops. At that point it has
gravitational potential energy but no kinetic energy. Then it swings in the opposite direction, and
its potential energy is converted to kinetic energy again. A pole vaulter runs and uses her pole to
gain kinetic energy that helps propel her into the air. In the air, she has gravitational potential
energy. As she falls back to the ground, her potential energy is converted back to kinetic energy.
Conservation of Mechanical energy: Recall that mechanical energy is the total kinetic and
potential energy of an object.
Mechanical energy = KE + PE
Because of the law of conservation of energy, Total mechanical energy remains constant. This
can be written as:
(KE + PE) beginning = (KE + PE) end
Name
_________________________
Block ________________ PSA
West
Date Due _______________
1. A watermelon with 67 joules of gravitational potential energy fell from a shelf and landed on
the floor. What was its kinetic energy when it landed on the floor?
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Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
2. A 0.2 kg object is lifted into the air and then dropped to the floor. If it is moving at a speed
of 3 m/s when it hits the floor, what was its gravitational potential energy before it was
dropped?
Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
3. Harry threw a ball straight up into the air. The ball’s gravitational potential energy at its
peak height was 0.4 joules, and its speed when it hit the ground was 4 m/s. What was the mass
of the ball?
Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
4. Alfredo, who has a mass 60 kg, jumped off a retaining wall. His kinetic energy when he
landed on the ground was 882 joules. How high was the wall?
Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
5. A water feature in a garden recycles water with a pump. Water is pumped from a stone
basin up through a pipe 1 meter high. At that height, the water flows out through a tap and
falls down through the air to the basin below, where the cycle begins again. What is the
gravitational potential energy of 2.5 kilograms of water at the top of the pipe? How fast is the
falling water moving by the time it reaches the basin?
Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
Givens & unknown
Formula used
Plug #’s into formula
Answer (No naked #’s)
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