1. ENERGY Part I

2. Learning Goals for Section 1 The Nature of Energy
Distinguish between kinetic and potential energy.
Calculate kinetic energy.
Describe different forms of potential energy.
Calculate gravitational potential energy.

3. What is Energy?
the ability to do work
the ability to cause a change

4. Work, Power, Efficiency Work = (Force) (distance) Power = Work / time
Work measured in Nm
1 Nm = 1 Joule
Power = Work / time
Power measured in J/s
1 J/s = 1 Watt
1 HP = 764 Watts
Efficiency = (Wout / Win) x 100
Efficiency is a percentage (%)

5. Work Power Efficiency Problems
Amy uses 20N of force to push a lawn mower 10 meters. How much work does she do?
Frank does 4.8 E4 J of work in climbing a set of stairs. If he climbs the stairs for 2 minutes, what is his power output?
A block and tackle does 1.25 E5 J of useful work, but friction limits the block and tackle to an efficiency of 45%. What is the amount of input on work block and tackle?

6. Kinetic Energy Kinetic Energy Equation energy of motion
depends on mass and speed
depends on speed more than it depends on mass – LOOK AT THE EQUATION!!!
Kinetic Energy Equation
kinetic energy (joules) = ½ mass (kg) × [speed (m/s)]2
KE = ½ mv2

7. Kinetic Energy Equations
KE = ½ mv (Joules)
m = 2 KE (kilograms) v2
v = √2KE (m/s) m

8. KE Problems What is the KE of an 8 kg mass at 5 m/s?
What is the mass of an object that has 100 J of KE when moving at 5 m/s?
Determine the velocity of a 6 kg mass has 75 J of KE.
A rocket with a mass of 1.5 E4 kg accelerates at 220 m/s2 for 29s from an initial speed of 5200 m/s. Determine the kinetic energy of the rocket before and after the acceleration.

9. stored energy Potential Energy stored energy in an object
means the object has
potential to cause change

10. Gravitational Potential Energy
stored energy due to position
depends on both mass and height
Gravitational Potential Energy Equation
GPE (J) = mass (kg) × acceleration due to gravity (m/s2) × height (m)
GPE = mgh
remember… mass x acceleration due to gravity = weight
acceleration due to gravity is 9.8 m/s2

11. GPE Equations GPE = mgh (Joules) m = GPE (kg) gh h = = GPE (m) mg
g = GPE (m/s2) mh

12. GPE Problems
What is the GPE of a 3 kg ball that is 2 m above the floor?
How high do you have to lift a 5 kg box to give it 98 J of GPE?
What is the weight of an stone that has 1200 J of GPE when setting on a 275 m cliff

13. Elastic Potential Energy
energy stored by something that can stretch or compress – rubber band, spring
PEE = ½ kd2
K is the spring constant for the material
Measured in N/m
d is the compression/expansion distance of the material
Measured in meters

14. PEE Equations
PEE = ½ kd2 (Joules)
k = 2PEE (N/m) d2
d = √2PEE (m) k

15. PEEProblems
The elastic force constant of a spring in a toy is 550 N/m.  If the spring is compressed 1.2 cm, compute the potential energy stored in the spring.
The spring constant (k) of a car’s front coil is 1800 N/cm. When the front tire rolls over a rock, the spring is compressed 15 cm from its equilibrium position. How much PEE is stored in the spring?

16. PEEProblems continued
A 100.0 g arrow is pulled back 30.0 cm against a bow, which has an elastic force constant of 125 N/m.  If the entire potential energy stored in the bow is converted to the kinetic energy of the arrow, compute the speed of the arrow.