1. Physics: Work and Energy
9/9/2019
Objective: Describe the relationships between work and energy.

2. Work and Energy
As we saw in the video, there are two related properties of matter: work and energy
Energy is defined as the ability of an object to produce a change in itself or the world around it.
Energy and work are directly related.
If an object has energy, it can do work.
In fact, we sometimes define energy as “the ability to do work”.

3. Work Work is done when we apply a force over a distance: W = Fd
Before
Work is done when we apply a force over a distance: W = Fd
Work equals the force exerted on an object in the direction of motion, times the object’s displacement
F = 10 N

4. Work
In this example, we have a force of 10 N pushing a mass over a distance of 5 m
The work done is:
W = (10 N)(5 m) = 50 J
Work is measured in Joules: 1 J = (1 N)(1 m)
After
F = 10 N
d= 5 m

5. Work F
W=Fd d
With work, it’s important that we just use the force in the direction of the displacement
If they are perpendicular, no work is done
If they’re at an angle, we find the component of the force in the direction of d. d
W=0 F d θ
W=Fdcosθ F

6. Work worksheet
Use what we’ve just learned about work to complete the blue worksheet

7. Mechanical Energy
We said before that energy is the ability of an object to do work.
Most of the energy that we deal with is mechanical energy – energy due to the position or motion of an object
Mechanical energy breaks down into two parts: kinetic energy and potential energy

8. Kinetic Energy
We’ve seen the term “kinetic” before with friction. What does it refer to?
Kinetic means moving. Kinetic energy is the energy of an object due to its motion.
The faster the object moves, the more kinetic energy it will have
20 kg
5 m/s

9. Potential Energy
Potential energy is energy due to the position of an object
We can think of it as “stored energy”
There are many different types of potential energy, but the main one we deal with in physics is gravitational potential energy
If we don’t specify otherwise, “potential energy” usually refers to gravitational PE.

10. Potential Energy and Work
Think about lifting a mass to a height “h”.
Since gravity pulls on the mass, you need to apply a force to balance out gravity. To lift an object, you’re doing work against gravity.
Because you’re doing work to the object, you’re giving it energy – in this case, potential energy.
The higher you lift, the more work you do, and the more potential energy it has m h

11. Work-Energy Theorem
We said before that energy could be defined as the ability to do work
When work is done on an object, it changes the energy of that object.
The work done is equal to the change in total mechanical energy. This is referred to as the Work-Energy Theorem
If I do 5 J of work on an object, I change its total mechanical energy by 5 J.

12. Conservation of Energy
We know that work is a way of transferring energy from one object to another
The Law of Conservation of Energy states that, in a closed, isolated system, energy can neither be created nor destroyed
The total amount of energy always stays the same, the energy just changes forms

13. Conservation of Energy
For example, lets say we drop a mass from some height above the ground
Initially, it’s not moving; what kind of energy does it have?
All of its energy is potential. The higher it’s lifted, the more potential energy it will have.
10 kg
5 m

14. Conservation of Energy
We drop the mass and let it fall to the ground.
The moment before it hits, its height is 0 m and it’s moving.
Its potential energy is now zero (because its height is zero). Where did the energy go?
It all got transferred to kinetic energy. As it fell, it started moving because gravity did work on it.
10 kg v

15. Energy “Loss”
The total amount of energy in a system must be conserved, but energy can change to forms other than kinetic and potential
Think about a book pushed across a table. You do work on it and give it kinetic energy, but eventually it comes to a stop.
Where did its energy go?

16. Energy “Loss” We know friction causes the book to stop
Friction also produces heat (thermal energy)
As the book slides, its kinetic energy gets turned into thermal energy
So, even though it looks like energy is being lost, our total amount of energy remains constant; the “lost” energy is just transferred to an unwanted form

17. Conservation of Energy Worksheet
Use today’s notes to complete the pink “conservation of energy” worksheet.

18. Kinetic Energy KE = ½mv2 Kinetic energy depends on mass and speed
A 20 kg mass moving at 5 m/s has a kinetic energy of:
KE = ½(20)(5)2 = 250 J
Kinetic energy, like work, is also measured in Joules
20 kg
5 m/s

19. Potential Energy and Work
Think about lifting a mass to a height “h”.
Since gravity pulls on the mass, you need to apply a force to balance out gravity: F = mg
Since work = force x distance
W = Fd = mgh m h

20. Potential Energy and Work
W = mgh
If mgh represents the amount of work that’s done on the mass, it must have gained the same amount of energy
This represents its gravitational potential energy:
PE = mgh m h

21. Potential Energy
With potential energy, it’s important to choose a reference point for our height (where potential energy is zero)
We can choose any point that seems convenient, as long as we’re consistent
The ground/floor is a common choice, but sometimes it makes more sense to choose a different point for certain problems

22. Conservation of Energy
For example, lets say we drop a 10 kg mass from 5 m above the ground
Initially, it’s not moving; what kind of energy does it have?
All of its energy is potential:
PE = mgh = (10)(9.8)(5) = 490 J
10 kg
5 m

23. Conservation of Energy
We started with 490 J of potential energy
All of that transformed to kinetic energy, so we can use that to solve for the speed as it hits the ground
490 = ½(10)v2
V = 9.9 m/s
10 kg v

24. Energy Problems For Friday P. 287, #1-3 P. 291, # 6-7
Reminder: Lab Reports are due Friday 11/11