1. Work and Energy 2/4/2019 SHOW WORK DONE BY A COMPRESSED SPRING
SIMPLE MACHINES. WHY DOES A KNIFE WORK?
PENDULUM WITH A MOVEABLE PEG
2/4/2019

2. Assignment 2 Due Monday 9:00 AM
Chapter 5 : 5-18, 5-40, 5-100
Chapter 7: 7-11, 7-63, 7-86
Chapter 8: 8-20, 8-42, 8-103
2/4/2019

3. What is Kinetic Energy
Energy is a quantity or characteristic of an object that is conserved or constant in a closed system. Kinetic energy is a particular form of energy that is related to an objects motion or speed.
For a particle, the kinetic energy is defined as
The SI unit of energy is the joule.
1 joule = 1 kg m2/s2 .
2/4/2019

4. Kinetic Energy
The 0.2 kg ball with speed v=10 m/s has kinetic energy given by
Kinetic energy is a scalar. It is not a vector. It has
no components, even though you can write
dot product
2/4/2019

5. Kinetic Energy -Work Theorem
Recall we found the following relationship for a particle of mass
m moving in a constant gravitational field.
If we multiply both sides by we get
where Fx is called W, the work done by the force acting on the particle.
If the particle speed increases, the force does positive work.
If the particle speed decreases, the force does negative work.
2/4/2019

6. WORK
SI unit of work = Newton-meter = Joule
Net work done on the particle by the forces F in moving a distance x is the change in kinetic energy of the particle. This is true for any case with constant acceleration- not just gravity,
which is force at a distance. It is also true for contact forces.
The concept of work is very useful in physics. It will allow us to solve a number of problems more easily than if we were to use F = ma alone.
If I drag a weight across the room through a displacement x, using a force F, then the work done is Fx. All that counts is the component of F in the direction x.
SHOW: Lifting up a weight, lowering it down, carrying it across the room at constant height.
SHOW pushing on the wall. No work is done.
So our everyday idea of doing physical work differs from the definition we use in physics. I could carry bags of fertilizer across the room, put them back down, and I have done no work at all according to our definition, but I would be exhausted and have sore muscles the next day.
Work does not have a direction associated with it like force and displacement. Technically this means that work is a scalar quantity. The same amount of work is done whether a weight is dragged North, South, East, or West. Scalars are easier to work with than vectors, such as force and displacement, so any time we can approach a problem using work rather than forces, it is probably a good idea to do so.
It is also true for a rigid body and not just a particle. By a rigid
body we mean one that does not change shapes. Consider a man
pushing a rigid cart along the floor.
2/4/2019

7. Work - Kinetic Energy Theorem for any case with constant a
The concept of work is very useful in physics. It will allow us to solve a number of problems more easily than if we were to use F = ma alone.
If I drag a weight across the room through a displacement x, using a force F, then the work done is Fx. All that counts is the component of F in the direction x.
SHOW: Lifting up a weight, lowering it down, carrying it across the room at constant height.
SHOW pushing on the wall. No work is done.
So our everyday idea of doing physical work differs from the definition we use in physics. I could carry bags of fertilizer across the room, put them back down, and I have done no work at all according to our definition, but I would be exhausted and have sore muscles the next day.
Work does not have a direction associated with it like force and displacement. Technically this means that work is a scalar quantity. The same amount of work is done whether a weight is dragged North, South, East, or West. Scalars are easier to work with than vectors, such as force and displacement, so any time we can approach a problem using work rather than forces, it is probably a good idea to do so.
is true for any constant acceleration. Also true for objects, if they are rigid and behave like particles.
Key: Point of application of the force is what is displaced
2/4/2019

8. Work done by a TRUCK on a CRATE (Force comes from the truck
EXAMPLE
What is the work done on a 150 kg orange crate by a blue truck whose acceleration is 2 m/s2 over a distance of 50 m. Draw the free body diagram of the crate first. Then find the force. Then find the work. F
50 m
Work done by a TRUCK on a CRATE (Force comes from the truck
and acts on the crate)
In the previous example, the person dragging the box was doing work against an opposing force, namely friction. So the work was being done against friction.
Here is an example where static friction plays a role, but we ignore air friction, so work is being done, but not against an opposing force. The crate is being accelerated. Its velocity is increasing.
The friction force needed to accelerate the crate is given by Newton’s second law. Here we assume the static friction force is large enough that the crate does not slip.
In this example, work is done on the crate and the result is its velocity has been increased.
Key: Identify the two systems : Work done by a ….. on a ……
Key: Point of application of the force is what is displaced
2/4/2019

9. Draw the free body diagram of the crate.
In the previous example, the person dragging the box was doing work against an opposing force, namely friction. So the work was being done against friction.
Here is an example where static friction plays a role, but we ignore air friction, so work is being done, but not against an opposing force. The crate is being accelerated. Its velocity is increasing.
The friction force needed to accelerate the crate is given by Newton’s second law. Here we assume the static friction force is large enough that the crate does not slip.
In this example, work is done on the crate and the result is its velocity has been increased.
2/4/2019

10. How much work is done by a worker’s force to pull a 50 kg crate a distance of 3 m across a frictionless horizontal floor with a force of 210 N 20 degrees above the horizontal? x i f
2/4/2019

11. We want the component of the force along x
What work is done by the gravitational force on the crate?
What work is done by the normal force on the crate?
2/4/2019

12. Computing Work using unit vector notation
In 2 dimensions you would just have x and y components : Example x y
2/4/2019

13. What is the work done by gravity on a 1000 kg mass falling10 m starting from rest? What is the final kinetic energy? x
Falling
mass
We say the force of gravity did positive work
on the falling block
SHOW demo lowering and raising a mass.
Lowering it, the gravity force and the displacement are in the same direction, so gravity is doing work on the mass.
If I raise it, gravity is doing negative work on the mass.
The work done is just the force times the change in vertical height.
What if we don’t move vertically, but at some other angle? If I carry the mass horizontally its gravitational energy does not change. Its gravitational energy remains the same.
SHOW the pile driver pounding the nail into wood.
2/4/2019

14. There is also negative work.
What is the work done by gravity when an applied force lifts a 1000 kg mass at constant speed? (Be careful here) x
Lifting
mass Fa
SHOW demo lowering and raising a mass.
Lowering it, the gravity force and the displacement are in the same direction, so gravity is doing work on the mass.
If I raise it, gravity is doing negative work on the mass.
The work done is just the force times the change in vertical height.
What if we don’t move vertically, but at some other angle? If I carry the mass horizontally its gravitational energy does not change. Its gravitational energy remains the same.
SHOW the pile driver pounding the nail into wood.
The work done by the applied force in lifting the mass
is Wa. If the mass is at rest initially and finally or the speed is the the same then, Wa+Wg=ΔK=0 or Wa=-Wg
2/4/2019

15. Problem: A worker pushes parallel to the incline plane on a block of ice just hard enough so that it slides down at constant speed. The mass of the block is m = 45 kg. The height of the support is h=0.91 m. Answer the following questions. θ
h=0.91 m
1.5 m F
2/4/2019

16. Draw the free body diagram for the blockθ N mg +x +y h
2/4/2019

17. (b) How much work is done on the block by the worker’s
force F when it slides 1.5 m down the inclined plane?
Sum of the x-components of the forces = 0 Why? θ N mg +x +y
1.5 m
0.91 m
2/4/2019

18. (c) How much work is done on the block by the
gravitational force when it slides down the inclined plane?
Take the gravitational force as mg sinθ θ N mg +x +y h
2/4/2019

19. (d) How much work is done on the block by the
Normal force N when it slides down the inclined plane? θ N mg +x +y h
2/4/2019

20. (e) How much work is done on the block by the
net force when it slides down the inclined plane? θ N mg +x +y h
2/4/2019

21. How much work is done on the block if I raise the ice block straight up without accelerating to the height h=0.91 m? θ N mg +x +y h
2/4/2019

22. Now look at a force that is not constant
Now look at a force that is not constant. Does a spring force do any work on a mass?
Consider the force F= - kx. known as Hook’s Law.
2/4/2019

23. Find the Work when an applied force elongates or compresses the spring by acting on the mass
SHOW DEMO OF MASS ON A SPRING
2/4/2019

24. Or between any two arbitrary positions.
Where x is measured from equilibrium position
SHOW DEMO OF MASS ON A SPRING
2/4/2019

25. Figure 7.8

26. Figure 7.9

27. What is work done by me on the spring
= work done by me pushing in the block
2/4/2019

28. Work-Energy Theorem for a Variable Force
2/4/2019

29. Problem 7-62 T h m a m=265 kg h=23 m a=0.150g
What is the tension in the cable in lifting the load while accelerating
a=0.150 g ?
b) What is net work done on the load?
c) What is work done by cable on load?
d) What is work done by gravity on the load?
e) What is final speed of load assuming it started from rest?
2/4/2019

30. Problem 7-86
What is the work done by the applied force F in moving the
bob from θ=0 to θ0?

31. Problem A 5.0 kg block moves in a straight line
along a frictionless surface under the
influence of the force shown in the figure. How much work is done by the force on the block in moving from the origin to x =8.0 m? If it starts from rest, what is its speed at x=8 m?
2/4/2019

32. Power Power is the rate at which work is done. Average Power =
Instantaneous Power =
The SI unit of power is joule/s or watt
Consider a particle moving along the x axis acted upon by a constant force at an angle φ with respect to the x axis x F φ
2/4/2019

33. A 1400 kg car accelerates from rest to 95 km/hr in 7.4 secs.
Problem :
A 1400 kg car accelerates from rest to 95 km/hr in 7.4 secs.
What is the average power delivered by the engine?
2/4/2019