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Chapter 5 Work, Energy, and Power.

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Presentation on theme: "Chapter 5 Work, Energy, and Power."— Presentation transcript:

1 Chapter 5 Work, Energy, and Power

2 What is work? W = fd W = N • m = Joule (J)
force and distance have to be in the same direction. distance distance Force Force Work Not Work

3 Mechanical Energy Energy of motion – Kinetic Energy (KE)
KE = ½*mass*(velocity)2 KE = ½ mv2 Depends directly on mass, on the square of the velocity Eureka

4 Work – Energy Relationship
Work-energy Theorem W=ΔE The net work done on an object is equal to its change in energy Objects with a lot of energy have had a lot of work done to them, to get rid of that energy, they must do a lot of work

5 Units Work and Energy have the same units – Joules which are kg*m2/s2
W=Fd KE= ½mv2 James Prescott Joule Showed KE and Thermal Energy were the same

6 Mechanical Energy Energy of Position – Potential Energy (PE)
Gravitational PE, position above ground PEg = mass*gravity*height PEg = mgh Eureka

7 Mechanical Energy Elastic Potential Energy (PE)
Elastic PE, position away from normal PEe = ½*(spring constant k)*(distance from normal x)2 PEe = ½kx2

8 Changes of Potential energy
Path doesn’t matter – only total gain or loss of height Rolling down a ramp vs. dropping Massive ball machine at Science center

9 Conservation of Energy
Law of Conservation of Energy - Within a closed, isolated system, energy can change form, but the total amount of energy is constant (energy can not be created or destroyed)

10 Energy form changes Work can be done to increase PE or KE
PE or KE can be used to do work PE can be changed to KE, KE can be changed to PE Examples of each?

11 Energy form changes Pendulums Pole-vault Bungee cord Coasters
Friction results in energy being ‘lost’ any calculation with friction must subtract out lost energy

12 Energy Problems What is the Potential Energy of a 65 kg Mr. Rassi at the top of a 4.0 meter ladder? How much work did he do climbing the ladder? How fast would he be moving when he hit the ground if he stepped off?

13 Energy Problems How much higher do you have to lift a barbell on the moon to give it the same PE on the moon as on the earth? On mars?

14 Energy Problems Why does a ball not bounce back up to the same height it is dropped from? Why does a ball bounce higher from a hard surface than from carpet? A 15.0 kg bowling ball on a 4.00 meter rope is pulled back until the angle it formed was 25.0° from the vertical. What is the velocity of the ball as it passes through the vertical?

15 Energy Problems Carl’s mass is 32.0 kg. He climbs a 4.80 meter ladder up a slide, then reaches a speed of 4.50 m/s at the bottom of the slide. How much work is done by friction on him? How much force is required to give a 45.0 kg vehicle a velocity of 6.50 m/s if it reaches that speed in just 12.0 meters?

16 Energy Problems A cantaloupe of unknown mass is dropped from rest 3.4 meters. What is its velocity when it hits the ground? An archer with a .30 kg arrow uses 201 N of force to pull a bowstring 1.3 meters back. If there is no loss of energy to friction, what is the velocity of the arrow when it leaves the string?

17 More related to work What is the difference between the tourist elevator and the freight elevator at the Sears Tower? Work and energy are not time dependent! Power is. Power = Work/time or Energy/time

18 Power Power = Energy/time Units – Joules/seconds = Watts
Electrical power – KiloWatts, MegaWatts lightbulbs

19 Practice Problems A box that weighs 575 N is lifted a distance of 20.0 m straight up by a rope. The job is done in 10.0 s. What power is delivered in watts and kilowatts? An electric motor delivers 65 kW of power as it lifts a loaded elevator 17.5 m in 35.0 s. How much force does the motor exert?

20 Practice Problems A rock climber wears a 7.50 kg knapsack while scaling a cliff. After 30.0 min, the climber is 8.2 m above the starting point. How much work does the climber do on the knapsack? If the climber weights 645 N, how much work does she do lifting herself and the knapsack? What is the average power delivered by the climber?

21 Practice Problems You are mowing the grass with an old-time push lawnmower. It requires 350 N of force to move the mower at a constant speed. If the mower makes an angle of 45° with the ground, what force must you push on the mower with? If the mower made an angle of 25°?

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