This unit, we studied Work and Power, the relationship between Work and Kinetic Energy, the Conservation of Energy, and Simple Machines.I feel really good about this unit, and my only difficulty with this unit is remembering what I have to show to get full credit on tests/quizzes, though I know how to get to the right answer. My effort has been good, though my organization needs to get better.
Real world application:
I can connect this unit with daily life very easily. While walking around campus, I notice when I either go up stairs or an inclined plain, knowing that, when going to the same place, the stairs and the inclined plain require the same amount of work, they just have different forces and distances.
Work and Power
Equation: Work=(force)(distance)
Units: Joules (J)
Equation: Power=work/time
Units: Watts
For work to happen, force and distance must be PARALLEL!!
Example problem:
If a 600N person walks up stairs that are 4 meters high, how much work is done?
Work=F*d
Work=600N*4m
Work=2400J
When is there more work? When a person goes up the stairs to third or when the elevator carries them to third?
The work required is the same. The upward force against gravity (or weight) is the same, and the distance that is parallel to that force (height) is the same, therefore work is the same. Horizontal distance does not matter because it is perpendicular to the upward force, and therefore contributes nothing to the work done.
When considering work, only the height matters.
Work happens to an object when force is exerted on that object over a distance. The equation for work is work=force*time, and it is measured in m/s or Joules. For there to be work, the force and distance must be parallel. In this example, the person's wright, or force, is vertical, and therefore for there to be work, the distance must be vertical as well. Work does not happen when the distance is perpendicular to the force, or when there is no distance.
If the force of the person is 4N, and the staircase is 15m tall, how much work will there be?
work=force*distance
= 4N*15m
work= 60 Joules
Power is how quickly work is done. The equation for power is Power=(work)/(time) and is measured in Watts.
If it takes the person 10 seconds to reach the top, how much power is exerted?
Power=(work)/(time)
=60J/10s
Power=6 Watts
Work and Kinetic Energy relationship
Formulas:
Kinetic Energy= (1/2)mv2
Change in KE=KEfinal-KEinitial
Change in KE=work
Work=fd
Why do airbags keep us safe?
KE=(1/2)mv2
Change in KE=KEfinal-KEinitial
You go from moving to not moving regardless of what you hit, therefore the change in Kinetic Energy is the same regardless of what you hit
Change in KE=work
Since the change in Kinetic Energy is the same, the work is the same regardless of what you hit.
work=fd
Airbag work=f*d
Dash work=f*d
Since the work is constant regardless of what you hit, the airbags increase the distance it takes to stop you, and thus decrease the force. Small force means less injury.
Conservation of Energy
Formulas:
Potential Energy=mgh
Change in PE= Change in KE
How does potential energy change if someone jumps off of a cliff?
As they fall, PE goes down and KE goes up by the exact same amount. The sum of PE and KE at any point will be equal to the PE they had when they were at rest at the top. Energy must be conserved.
Top: let PE=1000J, KE=0
¼ of the way down: PE=750J, KE=250J
½ of the way down: PE=500J, KE=500J
¾ of the way down: PE=250J, KE=750J
Just before impact: PE=0J, KE=1000J
When PE goes down KE goes up by the exact same amount.
Energy cannot be created or destroyed, but it can be transferred to other forms such as heat, light, and sound.
************PE is equal to the work required to raise an object to a certain height*************
PE=mgh
Work=F*d
Work=mg*h (mg is the amount of upward force required, h is the distance (height) it is lifted)
Machines
Purpose of a ramp: A ramp increases the distance over which a force is applied, thereby decreasing the amount of force required to do the same amount of work (lifting an object to a certain height).
Why do bolt cutters have such long handles but such short blades?
The work done on the handles equals the work done by the blades (Work in=Work out). The long handles allow for a large distance over which the force is applied. The short blades do the same work, but over a very short distance. This shorter distance means the force the blades apply is much greater than the force on the handles.
The machines in this picture are the pulleys. What a machine does is reduce the force it takes to do a job. Notice that it reduces force and NOT work. A machine does this by increasing the distance it takes for complete the job, making the force smaller. We know that this happens because workin=workout. this means that force-in*distance-in=force-out*distance-out.
Since the left mouse's rope is longer, that mouse can pull with less force to get the same sized cheese (it is not the same sized in the picture) up than the mouse on the left has to. The big block of cheese signifies that since there is more distance to cover, the left mouse will be able to pull up a larger piece of cheese than the right mouse.
******FOR MORE MACHINE NOTES... WATCH THE VIDEO AT THE TOP*************
Efficiency=Work out/Work in
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