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Potential and Kinetic Energy

What is Energy

- Energy
- is the ability to do WORK!!!
- Two types of energy
- Potential Energy
- Kinetic Energy

Potential and Kinetic Energy

- Animation
- http//www.teachersdomain.org/resource/hew06.sci.p

hys.maf.rollercoaster/

Potential Energy

- 1. Energy at rest
- 2. Stored Energy
- 3. Higher the object - greater the potential

energy

Potential Energy

- Which boulder has the least amount of potential

energy and Why?

Types of Potential Energy

- Elastic potential energy
- Energy stored by things that stretch or compress

- Gravitational potential energy (GPE)
- The amount of GPE an object has depends on its

mass, the acceleration due to gravity, and its

height above ground.

Quick Write Elastic Potential Energy

- Explain how this rubber band has elastic

potential energy!

Quick Write Potential Energy

- When is the spring having potential energy?

Brain Pop Movie Potential Energy

- http//glencoe.mcgraw-hill.com/sites/0078802482/st

udent_view0/brainpop_movies.html

Calculating potential energy

- PEg mgh or PEg Fwh
- Units
- PEg gravitational potential energy J or Nm
- m mass kg
- g acceleration due to gravity m/sec2
- h height m
- FwForce of Weight N
- NOTE Units for kinetic and potential energy are

J (joules). 1 joule 1 kg m2/sec2 1 Nm

FLASH CARD

Potential Energy Problems

- A weight lifter lifts a 120 kg mass from the

floor to a position above his head, 2.5 meters

above the floor. What is the potential energy

expressed by the weight? - PE m g h

PE (120)(9.8)(2.5)

PE 2940 J The potential energy of the weight is

2940 J.

- 2. What is the potential energy of a 90 N boat

before it falls from the top of a 30 m waterfall? - PE ?
- Fw 90 N
- h 30 m
- PE Fwh (90 N) (30 m)
- PE 2700 N m 2700J

- If the acceleration on a new planet is 5.2 m/s2

and a meteor is sitting on a hilltop at 100 m

with 650 J of PE, then what is the mass of the

meteor?

Kinetic Energy

- 1. Energy of motion
- 2. The amount of kinetic energy an object

has depends on its mass and its velocity

Calculating Kinetic Energy

- KE ½ mv2
- Units
- KE kinetic energy J or Nm
- m mass kg
- v velocity m/sec

Example

- What is the kinetic energy of a 120 kg object

moving at 30 m/sec? - KE ½ mv2
- m 120 kg v 30 m/sec KE?
- KE ½ (120 kg)(30 m/sec)2
- KE (60 kg)(900 m2/sec2)
- 54000 kg m2/sec2 54000 J

Quick Writes Kinetic Energy

- Which one has the highest KE?
- Which one has the least KE?

FLASH CARD

Brain Pop Movie Kinetic Energy

- http//glencoe.mcgraw-hill.com/sites/0078802482/st

udent_view0/brainpop_movies.html

Potential and Kinetic Energy Practice Problems

- 1. A 100kg rock sets at the top of a 50 m cliff.

What is the rocks potential energy?

- 2. A 70 kg rock is falling at 30 m/sec. What is

its kinetic energy?

- 3. A 315 kg object sets at the top of a 50 m

building. What is its potential energy?

- 4. A car has 3600 J of kinetic energy and is

going 3 m/sec. What is its mass?

Work

- Transfer of energy through motion.
- 2. The force must act in the same direction as

the motion. - 3. Force exerted through a distance

- Example 1 Lifting a log is work.
- Log moves in the direction the force is applied
- Example 2 Carrying a log is not work.
- Log is held up, but moves forward, therefore no

work is done on the log.

Calculating Work

- W Fd
- Units
- W work J
- F force N
- d distance m

Example

- How much work is done when 87 kg mass is lifted

1.2 m? - W F d d 1.2 m FW mg
- m 87 kg g 9.8 m/sec2
- W mgd
- W (87 kg)(9.8 m/sec2)(1.2 m)
- 1023 J

FLASH CARD

Work Practice Problems

- 1.You lift 19.0 kg of books 1.3 m. How much work

did you do?

- 2.A carpenter lifts a 80 kg beam 1.4 m and

carries it 9 m. How much work is done during the

lift? How much work is done on the beam during

the carry?

- 3.If 19.6 J are used to move 16 N object, how far

was the object moved?

- 4.If 1040 J are used to move a 76 N object, how

far was it moved?

- 5.If 763 J are used to move an object 20 m, how

much force was used?

- 6.If 70043 J are needed to move an object 73 m,

how much force was needed?