- Work and Power
- Work and Kinetic Energy Relationship
- Conservation of Energy
- Inclined Planes and Machines
The equation we use for work is:
Work = Force * Distance
One important thing to remember is that in order to calculate work, the force and distance MUST BE PARALLEL
For instance, if someone picks up a box, you can calculate the force because the force (weight of the box) is parallel with the distance they lift it.
However, if the same person then carries the box while walking, the work cannot be calculated because the distance is now forward, and not parallel with the force of the box.
One example we used in class was walking up the stairs. To see how to calculate the work for this example, watch this helpful video:
Power:
Power is how quickly work is done
The equation for power is
Power = Work / Time
The units for power are (Joules/ Second), which is the same as one WATT
Another unit commonly used when referring to power is horsepower.
1 horsepower = 746 Watts
A term new to us is "kinetic energy". Kinetic energy is the energy of movement, or motion.
How is kinetic energy connected to work? The change in Kinetic energy equals work, or:
Work = ∆ Kinetic Energy
To be able to use Kinetic Energy in problem solving, we must remember this equation:
KE = 1/2 m v
²Because work is a change in kinetic energy, use this set up to find work:
∆KE = KE final - KE initial
Potential Energy
Potential Energy is the energy an object has because of its position, how far it is from the ground, and how much gravity is acting upon it.
Potential Energy can apply to an object in motion or an abject at rest.
Kinetic Energy and Potential Energy are inversely related. You can see this in a swinging ball on a string:
Notice where the Potential energy is the greatest, the kinetic energy is the least
and where Kinetic Energy is greatest, Potential Energy is the least
Inclined Planes and Machines
Why do movers use ramps to move boxes into an truck?
It is because using an inclined plane decreases the force needed to move the box.
How is this so?
Think back to work. We know work = force * distance. Since force cannot change (because of conservation of energy), the only way to decrease the force exerted is to increase the distance:
Work = Force * Distance
Work = Force * Distance
The ramp above does not reduce the amount of work you must do to get the box to the top, but it does reduce the FORCE you must exert to get it there. It does this by INCREASING THE DISTANCE you push the box. So overall, you're really still doing the same amount of work, but your force is more spread out in more distance.
This is how machines work.
Work in = Work out
F d = F d
They increase the distance in which you exert your force, therefore decreasing your force needed. But overall, your work in will always equal your work out because of CONSERVATION OF ENERGY
Personal Review
Overall, I think this was the most challenging unit we have covered all year. I think the hardest concept to grasp was the relationship between potential energy and kinetic energy. I think the reason I didn't fully understand this was because I didn't have solid, straightforward notes to look back on. However, after going over some problems in the book, I was able to overcome this confusion and it's a lot more clear now.
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