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What is energy?

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What is energy?

- Energy measures the ability for things to change themselves or to cause change in other things.

- A joule (J) is the S.I. unit of measurement for energy.

Pushing a 1-kilogram object with a force of one newton for a distance of one meter uses one joule of energy

1. Kinetic Energy

An object that moves has kinetic energy (when you walk, run, jump -- you have K.E.)

How much Kinetic Energy an object has depends on 2 things:

a. The mass of the object

b. The speed its moving at

The more speed an object has the more KE it has.

Ex: a fastball has more KE than a slow curve ball.

The more massan object has the more KE it has.

Ex: Think of a tennis ball rolling across your foot …compared to a bowling ball rolling across your foot

The formula used to find kinetic energy is:

Ek = mass x velocity (speed)2

2

** Notice the velocity (speed) is squared.

This means if you go twice as fast, your energy increases by four times (22=4)

SAMPLE PROBLEM: Find the kinetic energy of a 50 kg swimmer, swimming at a speed of 4 m/sec. (Use 5 steps to solve) YES -- 5 STEPS!!

1. Ek = mass x speed 2

2

2. Ek = 50 kg x (4m/sec) 2 ÷ 2

3. Ek= 50kg x 16 m2/sec2÷2

4. Ek = 800kg/m2/sec2 ÷2

5. Ek = 400 joules

- 2. Potential Energy - is "stored" energy or energy an object has due to its position.
- Ex: An archer gives potential energy to a
- bow by pulling it back. The stored energy can be
- used to send the arrow flying through the air.
- Potential energy associated with objects that can be stretched (bow) or compressed (spring) is called elastic potential energy.
- Potential energy that depends on height is called gravitational potential energy.
- Ex: A person standing on the edge of
- an 3-meter cliff.

The formula for finding gravitational potential energy (when the mass is in kilograms) is:

Ep= mass x acceleration due to gravity x height

SAMPLE PROBLEM: Find the potential energy of a 50 kg diver on a 15 meter cliff.

Ep = mass x acceleration due to gravity x height

Ep = 50 kg x 9.8 m/sec2 x 15 meters

Ep = 7350 joules

When the mass (weight) is given in newtons you would use the formula Ep = mass x height