Energy

Physical Science10/18 – 10/19

Energyhttp://www.youtube.com/watch?v=XoNhZtPO5S4

Bell Ringer10/17

• Serena Williams serves a tennis ball at 120 miles per hour (54 m/s). The ball is at its highest point as the racket hits it.

• Where does the ball have the most potential energy?

• Where does the ball have the most kinetic energy?

• BONUS: What kind of energy does this kinetic energy convert to when the tennis ball hits the court?

BR 10/18:Section 4-2 CYU (on slip of paper) – HAND IN!

Bell Ringer10/18

1. Give two examples where work done on an object produces kinetic energy.

2. A 15 kg model airplane flies horizontally at 2.5 m/s. Calculate its kinetic energy.

3. You pick up a beanbag from the table and lift it over your head, then drop it to the floor. Discuss the work done and changes in energy that take place during these actions (3-4 complete sentences).

Conservation of Energy TOTAL

MECHANICALENERGY

• Total Energy = Kinetic Energy + Potential.

• Think about the tennis BR. The ball starts with potential energy because it is high above the ground, but also starts with kinetic energy because it is moving.

Conservation of Energy Tennis

Example• Finding total energy…• Tennis ball has mass =

0.057 kg, is served at a velocity of 54 m/s, and starts at a height of 2 m.

• (1) FIND the kinetic energy of the tennis ball when it is hit.

• (2) FIND the potential energy of the tennis ball when it is hit.

Conservation of Energy Tennis

Example• m = 0.057 kg, v = 54 m/s,

h = 2 m, g = 9.8 m/s2.• Kinetic Energy = ½ mv2

• Kinetic Energy = ½ x(0.057)x(54)2

= ½ x(0.057)x(2916)= 83 Joules

Conservation of Energy Tennis

Example• m = 0.057 kg, v = 54 m/s,

h = 2 m, g = 9.8 m/s2.• Potential Energy = mgh• Potential Energy

= (0.057)x(9.8)x(2)= 1.1 Joules

Conservation of Energy Tennis

Example• m = 0.057 kg, v = 54 m/s,

h = 2 m, g = 9.8 m/s2.• Total Energy = Kinetic

Energy + Potential Energy = 83 Joules + 1.1 Joules = 84.1 Joules

Conservation of Energy TOTAL

MECHANICALENERGY

• Total Energy = Kinetic Energy + Potential.

• The tennis ball starts with both potential and kinetic energy. What kind of energy does it have right before it hits the tennis court?

Conservation of Energy TOTAL

MECHANICALENERGY

• Energy is NEVER LOST, it just changes form. For example, the potential energy of the tennis ball is converted into kinetic energy.

• When the ball hits the court, its kinetic energy is converted into thermal energy (heat) and sound energy.

Conservation of Energy Bell Ringer

10/19You and your friend (who

weighs the same amount as you) go for a bike ride. You get to the bottom of a steep hill and decide to ride up. You bike all the way to the top, but your friend has to stop and walk his bike partway. You reach the top of the hill first.

Did you both do the same work (remember, work = force x distance)?

Who used more power?

Conservation of Energy Types of

Energy Mechanical Energy

Non-Mechanical Energy

Conservation of Energy Types of

Energy Mechanical Energy

Kinetic Energy

Gravitational Potential Energy

Elastic Potential Energy

Conservation of Energy Types of

Energy Non-Mechanical Energy

Thermal Energy

Sound Energy Nuclear Energy

Electrical Energy

Conservation of Energy How is

energytransferred?

• Friction and other forces can contribute to energy transfer.

• The tennis ball going through the air feels friction with the air and gives some of its kinetic energy to the air molecules, which speed up.

Conservation of Energy How is

energytransferred?

• As an ice skater moves across the ice, some of the skater’s kinetic energy is lost to friction between the skate blades and the ice.

• Thermal energy produced as a result of friction is part of the reason that ice melts.

Conservation of Energy How do I

analyzeenergytransfers?

• Look at each situation as a “Before” picture and an “After picture.

• Figure out what kinds of energy you have “Before” and “After” a process occurs.

Conservation of Energy What kinds

of energyare present?

• A pencil falling from a desk– “Before” = pencil

resting on the desk– “After” = pencil falling

mid-air

• What kind of energy is there “Before”?

• What kind of energy is there “After”?

Conservation of Energy What kinds

of energyare present?

• Alizah jumping up to spike a volleyball over the net.– “Before” = Alizah mid-air

going up to the net– “After” = Alizah spiking the

ball

• What kind of energy is there “Before”?

• What kind of energy is there “After”?

Conservation of Energy What is

conservationof energy?

• Law of Conservation of Energy: the total amount of energy in a system remains the same, though it may change from one form to another.

• “Conserved” means to stay the same.

Conservation of Energy How is

energy likemoney?

• You have $20 in your pocket. This money has potential to buy, just like potential energy has potential to do work.

• You could…• Change $5 of your money

into quarters• Buy $10 worth of candy• Give $5 to a friend

• No matter what you do, though, it’s still worth the same amount. Your money may have transferred, but the original amount stays the same.