November 16th 2009

Objectives SWBAT Relate the concepts of energy, power, and

time SWBAT Calculate power in two different ways

Catalyst Can the KE of an object be negative? Why? Vu walks up the stairs to the 3rd floor on Friday then he

takes the elevator to the 3rd floor on Saturday. On which day was more work done on Vu’s body?

Agenda

HW Review Power Practice!

HW Answers

Pg 176

1. 7.8m

2. 21m

3. 5.1m

4. 300N

POWER!

5-3

Power…

Power is the rate at which work is done So how fast work is being done

P = W∆t

Unit of power is the watt (W), which is one Joule per second

Horsepower, hp, is equal to 746 Watts This equation can take other forms…

Power Forms!

Conceptual Challenge…

Many mountain roads are built so that they zigzag up the mountain rather than go straight up toward the peak. How is this advantageous from the viewpoint of energy conservation and power?Hint look at the

displacement…

d

Conceptual Challenge dos…

If you have to use an engine to lift up the curtain at your theater show, and you have the choice of 3 engines; 1kW, 2.5kW, and 5.5kW. If all of the engines lift the curtain the same distance what is the advantage of using a more powerful engine?

The more powerful engine will do the same amount of work in less time.

A 193 kg curtain needs to be raised 7.5 m, at a constant speed in as close to 5.0s as possible. The power ratings for the three motors listed are 1.0 kW, 3.5kW, and 5.5 kW. Which motor is best for the job?

3.5kW motor

Group Whiteboarding! Two horses pull a cart. Each exerts a force of 250N at a speed of

2.0m/s for 10min. a. Calculate the power delivered by the horses.

b. How much work is done by the two horses?

a. 1000 Wb. 6.0x105 J

Practice

For Now: pg 189; 2 For HW: pg 189; 1-5 For now

Mini Lab

Tell me something I don’t know

Mechanical Energy is conserved…Your initial ME equals your final ME

Conservation of mechanical Energy: MEi=MEf

*****This is ONLY when we neglect friction (which we often do)

Misconception Alert!!! While ME is conserved, TOTAL Energy is also conserved (all the different kinds of Energy…we just won’t analyze it here)

Let’s see if it works…

I’m going to drop this 1kg object to the floor from a height of 1.5m… What is the maximum PE?

At the top when KE = 0

What is the maximum KE? (hint ME is conserved) At the bottom when PE = 0

Let’s prove it…with our handy dandy kinematic equations!

So…sum it up

We can use the distance the egg has fallen and a kinematic equation to find the speed at any time (KE)

Or…We can use the height of the egg to find the PEg at any time.

The great part… KE+ΣPE = 14.7J EVERY TIME in this case

Data Table…

Height above ground (m)

Displacement (m) ie. How far the object has dropped Vf KE PE ME = KE + PE

1.5 0 0 0 14.715 14.715

1.2 0.3 2.426108 2.943 11.772 14.715

0.9 0.6 3.431035 5.886 8.829 14.715

0.6 0.9 4.202142 8.829 5.886 14.715

0.3 1.2 4.852216 11.772 2.943 14.715

0 1.5 5.424942 14.715 0 14.715

Problem… Starting from rest, a child zooms down a frictionless slide from an initial

height of 3.00m. What is her speed at the bottom of the slide? Assume she has a mass of 25kg.

7.67m/s

Last thing

Energy is conserved even if acceleration varies. So if the slide in the last

problem had varying angles on the way down, we wouldn’t be able to calculate the acceleration easily, meaning no Kinematic formulas

BUT, b/c we know the ME we can skip acceleration and find the final speed without all the extra work!

Group Whiteboard it!!!!! A small 10.0g ball is held to a slingshot that is stretched 6cm.

The spring constant is 2.0x102 N/m. a. What is the elastic potential energy of the slingshot before it is

released?b. What is the KE of the ball just after the slingshot is released?c. What is the ball’s speed at that instant?d. How high does the ball rise if it is shot directly upward?

Questions…

Pg185; 1, 2, 4, 5 Pg186; 1, 2, 3,