Work & Power

Work & Power

How de we use work in physics? What is power?

Work

First we will review what we've learned in class so far...

Multiple Choice

What is the unit of work?

• Newtons, N

• Joules, J

• Calories, Cal

• Watts, W

Multiple Select

Which of the following are correct?

(There are 3 correct answers)

• Work requires a force.

• Work cannot have acceleration.

• Work can only be positive.

• Work requires a distance.

• Work can be positive, negative, or zero.

Poll

If I ask a student to push against the wall for 2 hours, but the wall never budges-not even an inch, is that work?

• Yes, because a force has been applied.

• No, because the wall has not moved.

Great job!

Now that we've reviewed a bit, let's try one more. This one will use the equation for work.

 $W\ =\ F\ \times\ d$ 

(I'd write it down if I were you)

Multiple Choice

A cat tries to climb up onto a ledge In the process, she has to fight her own weight, 98.1 N. If the ledge is 3 m off the ground, how much work doe she do?

• 294.3 J

• 32.7 J

• 95.1 J

• 962.4 J

Let's solve that last problem...

• A cat tries to climb up onto a ledge In the process, she has to fight her own weight, 98.1 N. If the ledge is 3 m off the ground, how much work doe she do?

• Her weight is the force she has to overcome, so F = 98.1 N

• She climbs a distance of 3 m, so d = 3 m

• Our equation is W = F x d

• so... W = 98.1 x 3 = 294.3 J

Remember...

• Work is a form of USEFUL energy.

• If there is no force or no distance, there can be no work done.

• Also, don't forget it's always measured in Joules, J.

What is Power?

Power is a measure of how much work an object can do in a short amount of time.

 $P=\frac{W}{t}$  

Power is work over time.

• This means that if you can do MORE work in a small amount of time, you are MORE powerful.

• Power is not strength, it has nothing to do with muscle or anything, but rather speed.

• Power is measured in Watts, W.

Let's try some examples.

 We will use our equation for power.



 $P=\frac{W}{t}$ 

(Remember, the units of power are Watts, W)

Multiple Choice

How much power is used when 9000 J of work are done in 0.5 seconds?

• 4,500 W

• 18000 W

• 9453 W

• 917.4 W

Let's go over the problem...

• How much power is used when 9000 J of work are done in 0.5 seconds?

• So we have 9000 J of work, W = 9000 J

• We have a time of 0.5 seconds, so t = 0.5 s

• Our equation is  $P=\frac{W}{t}$  

• thus...   $P=\frac{9000}{0.5}$  

• P = 18,000 W      (the W stands for watts, not work)

Great job today!

Before you go, answer one more question, please.

Poll

How do you feel about this lesson?

• It was easy to follow and helpful.

• It was too busy, I had a hard time finishing it.

• I really didn't like it, and I don't feel as though I learned anything.

See you soon!

Galán