Work
In which of the following is work done?
a.Hold a book above your head
b.Lifting a book
c.Pushing on a desk, but the desk does not move
d.Pushing on a concrete wall
b. Lifting the book
What is the Unit of Power?
Watts (W)
The energy of motion is called ____________________________.
Kinetic Energy
What is the Unit for Energy?
Joules (J)
Remember, energy is the ability to do work, so same units
What is the most common system?
Open System
In most cases, energy AND matter can leave systems
Think: Open Cup of water
If you exert a force of 5 N to lift a box a distance of 0.5 m, how much work do you do?
Work = F x D
2.5 J
A turbine provides enough power to do 500,000 J of work in 60 s. What is the power output of the turbine?
8333.33 Watts
A 5kg cat is resting on top of a bookshelf that is 2.5 meters high. What is the cat’s gravitational potential energy relative to the floor if the acceleration due to gravity is 9.8m/s2? PE = mass x g x height
PE= 122.5 J
Hot pavement has what kind of energy?
Thermal Energy
What has a higher specific heat, Water or pavement?
Hint: Which one is harder to heat up??
Water
High specific heat= Needs more energy to raise temperature
Low SH= Needs less energy to raise temperature (gets hot easily)
If you perform 30 Joules (J) of work lifting a 15 N box from the floor to a shelf, how high is the shelf?
D = Work/Force
2m
A machine provides 2,000 Watts of power in 10 s. How much work is done by the machine?
Work = P x t
20,000 J
A small 25 kg canoe is floating downriver at a speed of 3 m/s. What is the canoe’s kinetic energy?
KE = ½ (mass)(velocity)2
KE= 112.5 J
What kind of energy does a battery have?
Chemical
Converts chemical into electrical
What happens to matter and energy in a closed system? Give a real example
Matter: can't leave
Energy: Can leave
Examples: Pot with lid on it, etc.
A family is moving to a new house. While lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 250 N for 2 s. How much work is required to do this?
Work = F x D; P = Work/time
375 J
How can you increase power?
a. Increase work, increase time
b. Increase work, decrease time
c. Decrease work decrease time
d. Decrease work, increase time
b. Increase work, decrease time
P=W/t
More work or do the work faster! (less time)
A girl puts a 10 kg box on a shelf. The potential Energy is 300J. How high is the shelf?
Height=PE/mg g=9.8 m/s2
Height= 3.06 m
Friction converts ____________ energy into ___________ energy
mechanical, thermal
Determine the amount of energy in joules required to raise the temperature of 50g of water from 30 C to 40 C.
q= mcΔT
c= specific heat (For water, it is 4.186)
2093 J
If 15000 J of work is done to fire the human cannonball through a 3 m barrel, then how much force is applied to the person to launch them out of the cannon? F = W/D
5000 N
If you exert a force of 500 N to walk 4 m up a flight of stairs in 10 s, how much power do you use?
200 Watts
Find Work First, then Power
Calculate the mass of an dog that is running at a speed of 7.5 m/s with 500 J of Energy.
Mass= 2*KE/v2
Mass= 17.8 kg
Light bulbs use ______________ energy and convert it into ____________ energy and _________ energy
Use: electrical
Converts to: electromagnetic and thermal
Determine the amount of energy in joules required to raise the temperature of 75g of water from 30 C to 75 C.
q= mcΔT
c= specific heat (For water, it is 4.186)
14127.75 J