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WORK
WORK:
Title: Work Lab
Objective & Procedures: To determine the relationship between Force and Distance
Design: Pull a cart up a ramp using four different path lengths.
DC:
Record the weight of the car (N) DATA TABLE: Distance up path (m) | Force (N)
Record the height of the chair (m)
DA:
We are looking for patterns here. Make three new data columns in your table.
D+F
D/F
D*F
Fill in each new column.
Now take each column and divide the smallest value by the largest value to get the % match.
Which column had the best % match? That column is the one we care about.
Take the average of that column.
Conclusion: As the distance up the path INCREASED the force required __________________.
This means that force and distance are ________________ proportional to each other.
When you ___________ (add or divide or multiply) the force and the distance you get almost the same
value every time. We call this value the “work” to get up the hill.
How much work did it take to get your cart up the hill? (This is the average from your chosen column.)
Now calculate the potential gravity energy of the car at the top of the hill. (U g = Weight * height )
How does the work to get up the hill compare to the gravity energy? (They should be close.)
Evaluation: Do you trust your results? Why wasn’t your chosen column a perfect match? What is making our data
imperfect?
Work Equation:
Units:
If you take a short steep path then:
If you take a long gradual path then:
The total work (if gravity is involved) is always equal to:
PRACTICE:
Bob pushes a crate with 50 N of force for 6 meters. How much work did he do?
Bob did 240 J of work yesterday when he pushed a heavy box across the floor. If he pushed it 3 meters, then how
much did friction resist?
Climbing in Tarzan’s tree house is hard work! Literally. It takes 1300 J of work to get up there. If he weighs 650
N, then how high is the tree house?
RAMPS:
Mechanical Advantage:
Bob is trying to get his 210 kg four wheeler up into the back of his pick up truck. The bed of his truck is 1.5 meters
above the ground.
a. How much energy will it take to get the wheeler into the truck?
b. If Bob just lifts it straight up like superman, how much force will it require?
c. Bob isn’t this strong. He decides to use a ramp. It is twelve meters long. How much force will it take this time?
d. What is the mechanical advantage of the ramp?
e. What if Bob used a ramp that was 20 meters long?
Betty (65 kg) and Babby (65 kg) decide to race each other to the top of a 240 meter tall building. Betty is going to
climb straight up on a rope, and Babby is going to run up the stairs. The total length of the stairs is 960 meters.
a. How much energy will it take for either girl to get to the top of the building?
b. How much force will Betty’s arms need to provide?
c. How much force will Babby’s legs need to provide?
d. Even though Babby is taking a much longer route, she is likely to win the race. Why?
It turns out that friction can do work too. We call it negative work because it is NOT helping! We use the same
equation. If the force of friction on a box is 35 Newtons, and you slide the box 13 meters across the floor, how
much work did friction do?
TOTAL WORK
Bob pushes a box straight forward with 210 N of force for 12 meters. Meanwhile, friction resists with 45 N of force.
1. How much work did Bob do?
2. How much work did friction do?
3. How much work was done total?
EFFICIENCY SKILLS TEST 6
NAME: ____________________________ PERIOD: _______
For this skills test you will be tested on the following. Student can:
1 – Determine potential and kinetic energy
2 – Calculate work in and work out
3 – Calculate efficiency
4 – Explain Energy Loss
INSTRUCTIONS:
1) Make sure the ramp is set on the longest path (ramp will have BOTTOM
on the floor and the last marker will be lined up with the table).
2) Record the distance/length of the ramp in meters.
3) Record the height of the table in meters.
4) Record the weight of the cart in Newtons.
5) Pull a cart up a ramp at a constant speed. Record the force in Newtons.
6) Pull the cart again up a ramp at a constant speed. Time how long it
takes the cart to get from the bottom of the ramp to the top of the ramp.
Record the time in seconds.
7) Calculate the potential energy the cart had at the top of the ramp.
8) Calculate the kinetic energy the cart had at the top of the ramp right
before it stopped.
9) Calculate the work in.
10) Calculate the work out.
11) Calculate the efficiency.
12) There isn’t a machine that will be 100% efficient. So if efficiency
compares energy, and if energy is conserved, then where does the
energy go?
v = velocity
x = length of ramp
t = time
Helpful Equations:
Potential Energy = mgh = Wh
m = mass
g= gravity (9.8 m/s2)
h= height of table
W = mg = mass*gravity
Kinetic Energy = ½ mv2
m = mass
v = velocity
Length of ramp:
Height of table:
Weight of cart:
Pulling Force:
____________ m
____________ m
____________ N
____________ N
Time:
_____________ s
Potential Energy: ____________ J
Kinetic Energy: _____________ J
Work in:
__________ Nm
Work out:
__________ Nm
Efficiency:
____________ %
Where does energy go? _________
____________________________
____________________________
Work in= Fd
F = pulling force
d = distance= length of the ramp
Work out = mgh = potential
energy
Efficiency =
x 100 %
Efficiency:
How do we calculate the efficiency of a machine?
Formula:
Work in =
Work out =
PRACTICE:
Bob pushes a 4 kg crate up a ramp that is 2 meters long. He pushes with 40 N of force to a height of 1.9 meters.
Calculate the:
Work In:
Work Out:
Efficiency:
Gertrude pulls a bucket of 2.3 kg water up out of a well 6 meters. She must apply 25N of force on the rope to
accomplish the task. Calculate the following:
Work In:
Work Out:
Efficiency: