Warm Up 9/24

1 of 8 © Boardworks Ltd 2008


Warm Up 9/24

Directions: Write which of Newton’s Lawsthese sentences describe:1. I can move the small snowball I made for

the snowman’s head, but I will need help moving his belly.

2. When I push back with my ski poles, I move forward.

3. The harder I pull, the faster my dog runs.


Potential Energy

• Potential Energy is stored energy.• Ex. A stretched rubber band has the

potential or ability to fly across the room.


Types of Potential Energy

1. Gravitational Potential Energy2. Chemical: This type of energy is stored

when you eat food or burn an object3. Electrical: Used to power houses (also

found in batteries)4. Elastic: Stored when you stretch a rubber

band (earthquakes too) 5. Nuclear: Produced by splitting the nucleus

of an atom.


A long way down…

How would you describe a high diver? As someone who is:

full of gravitational potential energy (GPE).

brave

insane

full of beans

GPE is the amount of energy an object has because of its position above the ground, i.e. its height.


When a bungee jumper starts to fall, he starts to lose GPE.

As the elastic cord pulls the bungee jumper back up, he gains GPE.

The gravitational potential energy (GPE) of an object on Earth depends on its mass and its height above the Earth’s surface.

What is gravitational potential energy?


How is GPE calculated?

The GPE of an object can be calculated using this equation:

GPE = mass x gravitational field strength x height

Height is measured in meters (m).

Mass is measured in kilograms (kg).

GPE is measured in joules (j).

Gravitational field strength is measured in newtons per kilogram (N/kg), usually taken as 10 N/kg (9.8) on Earth.


Factors affecting GPE


An eagle with a mass of2 kg flies at a height of200 m above the ground.

Calculating GPE question 1

How much gravitational potential energy does the eagle have?

= 2 x 10 x 200

GPE = mass x gravitational field strength x height

= 4,000 J


An apple with a mass of 200 g falls 3 m from its branch to the ground.

Calculating GPE question 2

How much GPE will the apple have lost when it reaches the ground?

= 0.2 x 10 x 3

= 6 J

= mass x gravitational field strength xGPElost

changein height


GPE, mass and height calculations


Kinetic energy is the energy an object has because it is moving.

What is kinetic energy?

The word ‘kinetic’ comes from the Greek word ‘kinesis’, meaning motion.

All moving things have kinetic energy, but the amount of energy they have is not just dependent on how fast they are moving.

What other factors affect the kinetic energy of a moving object?


Kinetic Energy


How is kinetic energy calculated?

The kinetic energy (KE) of an object can be calculated using this equation:

Velocity is measured in meters per second (m/s).

Mass is measured in kilograms (kg).

KE is measured in joules (j).

KE = ½ x mass x velocity2

= ½mv2


kinetic energy = ½ x mass x velocity2

A truck with a mass of 1,500 kg travels at a velocity of 20 m/s.

Calculating kinetic energy question

What is the kinetic energy of the truck?

= 300,000 J = 300 kJ

= ½ x 1,500 x 202


Sometimes it is necessary to rearrange the kinetic energy equation in order to calculate the mass or the velocity of a moving object.

KE = ½mv2

What are the rearranged versions of this equation for calculating mass and velocity?

m = 2KE v2

v = Ö 2KE m

Rearranging the KE equation


KE, mass and velocity calculations


Doubling the mass of a moving object doubles its kinetic energy, but doubling the velocity quadruples its kinetic energy.

If the velocity of a car is slightly above the speed limit, its kinetic energy is much greater than it would be at the speed limit. This means that:

Too much kinetic energy

It is more difficult to stop the car and there is more chance of an accident.

It the car does collide with something, more energy will be transferred, causing more damage.


The kinetic energy of cars


Use the KE = ½mv2 equation to fill in the kinetic energy values in the table below for two cars each traveling at two different velocities.

What factor – mass or velocity – has the greatest effect on the kinetic energy of a moving object?

Dangerous speeding?

KE = 160 kJ KE = 320 kJ

1,000 kg 2,000 kg

20 mph

40 mph

KE = 40 kJ KE = 80 kJ


Who has the most kinetic energy: Rita or the cat?

Feeling energetic?


A truck has a mass of 20,000 kg. If its kinetic energy is 2.25 mJ, at what velocity is it travelling?

Calculating velocity question

KE = ½ x mass x velocity2

= 15 m/s

velocity = Ö 2KE mass

= Ö

2 x 2,250,000 20,000

Warm Up 9/24

Documents

Transcript of Warm Up 9/24