Work
What is work?
force applied through a distance
What is energy?
-ability to cause change
-ability to do work
-when something is able to change itself or its surroundings
What is the law of conservation of energy?
energy cannot be created nor destroyed - only transferred
If you throw a 0.4-kg ball at a speed of 20 m/s, what is the kinetic energy from the ball’s motion?
KE =(1/2) x (0.4 kg) x (20 m/s)2 = 80 J
When moving a couch, you exert a force of 400 N and push it 4.0 m. How much work have you done on the couch?
W = Fd = 400 x 4 = 1600 J
When an object is moving ______ to the moving object, work is not being done.
perpendicular
Define system and give an example
-anything that can an imaginary boundary
-portion of the universe chosen for grouping and analysis
-EX. a solar system, system between a ball and a person
Define mechanical energy and state what types of energy are included
-sum of the kinetic energy and potential energy of the objects in the system
-only includes kinetic, elastic potential, and gravitational potential energy
A car moving at 30 m/s has 900 J of kinetic energy from its motion. What is the car’s mass?
KE = (1/2) mv2
m =2 x KE / v2 =2 x 900 /(302) = 2 kg
What power is used by a machine to perform 800 J of work in 25s?
P = E/t = 800 / 25 = 32 W
Machines make doing work easier by . . .
by reducing friction
increase speed
changes direction of force
increases force
Define the two main types of energy and give an example of each.
-Potential energy: energy that is stored due to the interactions of objects
EX. chem potential -food, wood, batteries; elastic potential - rubber band, anything stretchy, gravitational energy - gravity)
-Kinetic energy: energy during motion
EX. anything moving -ball being thrown, car travelling down a road, person doing jumping jacks
Define power and state the formula to calculate it.
rate at which energy is conserved – how fast energy changes
P = E/t
A 2.0-kg book is moved from a shelf that is 2.0 m off the ground to a shelf that is 1.5 m off the ground. What is the change in GPE?
GPE =mgh =2 kg x 9.8 m/s2 x 0.5 m = 9.8 J
What is the efficiency of a machine if your work on the machine is 1,200 J and the machine’s output work is 300 J?
Efficiency = 300 / 1200 = 0.25%
Efficiency means . . .
-more work can be made with a smaller amount of effort
-ratio of output of work to input of work
Explain the differences between the 3 main types of potential energy.
-Chem potential: energy stored in chemical bonds
-Elastic potential: energy stored by compressing or stretching an object
-Gravitational potential: energy due to the gravitational forces between objects
Explain why is mechanical energy not always conserved.
Mechanical energy is conserved when the amount of mechanical energy remains the same after transforming.
If your weight is 50 kg, and you are standing on a floor that is 20 m above the ground, what is the gravitational potential energy between you and Earth, relative to the ground?
GPE = 50 kg x 9.8 m/s2 x 20 m = 9800 J
A 50 kg ball is being lifted at an acceleration of 2 m/s2. How much work is needed to lift the ball up 3.0 m?
W = Fd = (ma)d = (50 x 2)3=300 J
Difference between simple and compound machines
simple machines does work w/ only one movement of the machine vs compound machines does work w/ a combination of 2 or more simple machines
In one of our activities, we stretched a rubber band and flung a coin. Explain what is happening to the energy of the rubber band.
-the rubber band has elastic potential energy
-when the band is stretched, elastic forces create potential energy.
-when the band is released and flung, the potential energy becomes kinetic energy
Explain how potential and kinetic energy changes when you are on a swing.
-At the top of the swing in the back, potential energy (PE) is very high (that's why it sometimes feels like you're paused/slowing down in that position) and kinetic energy (KE) is very low.
-While swinging down your speed increases, so PE is decreasing and KE is increasing
-At the lowest point in the middle as your speed is very fast, the PE is very low and KE is very high
-Moving forward as speed decreases, the PE is decreasing and KE is increasing
-At the top of the swing in the front, potential energy (PE) is very high and kinetic energy (KE) is low.
You have a mass of 80 kg standing on a platform above the ground, and the gravitational potential energy between you and Earth is 2352 J relative to the ground. How high is the platform you are standing on?
GPE = mgh
h=GPE/mg = 2352/(80*9.8)= 3 m
A person pushes a box up a ramp that is 3 m long and 1 m high. If the box has a mass of 20 kg and the person pushes with a force of 80 N, what is the efficiency of the ramp? (HINT: W=Fd, F=ma, and a=9.8m/s2)
Efficiency = Wout/Win = Fd(out)/Fd(in) =(ma)dout/(ma)din =