Section 4.1 - The Nature of Energy

WEDNESDAY, 11/27• Warm-Up:

• Examine your lab data from yesterday. What relationship do you notice between momentum and kinetic energy? Can you think about why this might be true?

• Learning Goal(s): Compare and contrast kinetic energy and momentum.

• Agenda: - 1. Warm-Up2. Work on Analysis and Conclusions for the Kinetic Energy

Lab3. Complete the Kinetic Energy and Potential Energy Problem

Set

•HW: Kinetic Energy and Potential Energy Problem Set (1st side only) due on Tuesday, 12/3

Have a Happy Thanksgiving!

Kinetic Energy vs. Momentum

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MomentumKinetic Energy

Students

Mom

entu

m (k

gxm

/s)

Kine

tic E

nerg

y (J)

What Is Energy?

• Changes are constantly occurring in the world.– Examples: light bulbs heating the air, sunlight on

window, you breathing

• Every change requires energy.

• Energy – ability to cause change

Energy is Found in Many Forms• Electromagnetic energy – energy in waves

– Examples: UV, Infrared, Radio • Mechanical – energy in physical movement

– Examples: moving car, flying baseball• Electrical energy – energy carried by moving electrons• Heat – thermal energy

– Example: cup of hot tea• Chemical – stored in molecules

– Example: food, batteries• Nuclear – energy stored in the nucleus of an atom

– Example: Uranium used in nuclear reactors• Kinetic – energy an object in motion has

– Example: person running, roller coaster going down a hill• Potential – stored energy due to an object’s position

– Example: roller coaster at the top of a hill

Units of Energy – the Joule

• Units of energy• kg x m2/s2

• Abbreviated J

James Prescott Joule

Kinetic Energy

• Energy a moving object has because of its motion.

• Depends on an object’s mass and its speed.

KE = ½ mv2

KE – kinetic energy in Joules (J)m – mass in kg

v – velocity (m/s)

Examples of Solving for Kinetic Energy

• A jogger with a mass of 60 kg is moving at a speed of 3 m/s. What is the kinetic energy?

• A bowling ball is moving with 300 J of energy. The ball weighs 2 kg. How fast is the ball moving?

Potential Energy

• Stored energy due to an object’s position

Three types:– Elastic potential energy – energy stored by something that

can stretch or compress

– Chemical potential energy – energy stored in chemical bonds

– Gravitational potential energy – energy stored in objects due to their position above the Earth’s surface

Solving for Potential Energy

GPE = mgh

GPE – gravitational potential energy (in J)m – mass (in kg)

g – 9.8 m/s2

h – height (in m)

Examples of Solving for Potential Energy

• What is the gravitational potential energy of a ceiling fan that has a mass of 7 kg and is 4 m above the ground?

• How high above the ground is a baseball with a mass of 0.15 kg and a GPE of 73.5 J?

Changing GPE

• As an object begins to fall it has GPE and KE.• As the object gets closer to the ground it has

less GPE and more KE.

• Example – An 80 kg skydiver jumps from a 10 m platform. What is the GPE of the skydiver at the top of the platform? What is the GPE of skydiver after falling 5 m?

Changing GPE – Energy Diagram