12. W = ΔE
W = Work
Δ = “change in” or “the difference”
E = Energy (Potential and Kinetic)
Work equals the change in energy of an object.
13. Scientifically Speaking
Work is done on an object only if the object moves slightly
in the direction of the force applied on it.
Negative work is done to an object if the object moves
exactly opposite the direction of the applied force.
Remember force is calculated as the mass of an object
multiplied by it’s acceleration.
14. Negative work
Leonard Fournette versus Ole Miss.
https://youtu.be/yyOIe205Wp4
Which player applied negative work?
16. My Planet Diary Page 184
Read “Feats of Strength” article and answer the following
questions.
What factors might have affected how much work these
competitors did?
Name an everyday task that you would consider to be a
lot of work?
18. Carrying a book demo
Why isn’t the carrier doing work to the book?
Would dragging the book make a difference in work being
done?
19. How is work calculated?
The amount of work done on an object can be determined
by multiplying force times distance. pp.186-187
Work = Force x Distance
Force is measured in Newtons.
Distance is measured in meters.
20. Practice
How much work do you do when you push a shopping cart
with a force of 50 N for a distance of 5 m?
21. Practice
How much work do you do when you push a shopping cart
with a force of 50 N for a distance of 5 m?
22. Power
Power equals the amount of work done on an
object in a unit of time.
Power = Work .
Time
Power is measured in watts.
23. Power Units
When work is measured in joules and time in seconds, the
SI unit of power is the joule per seconds (J/s).
One joule of work done in one second is one watt.
24. Practice
When a tractor pulls a disc harrow in a field, it applies
force over a distance. Work is done in a horizontal
direction. How much power is the tractor applying to the
disc harrow if the tractor 10,000 J of work for 5 seconds?
Power = work/time
30. Machines
A machine makes work easier by changing at least
one factor:
1. the amount of force you exert
2. the distance over which you exert the force
3. the direction in which you exert your force
pp. 191
31. Input vs. Output
You exert the input force.
The machine exerts the output force over the output
distance.
32. How much work is done?
If lifting a large box from the floor and placing it on a
desk without the use of a machine, your input force is
equal to your output force jut as the input distance is
equal to output force.
Without a machine: input force = output force
input distance = output distance
Work is work with no advantages.
With a machine: input force is less than output force
33. How much work is done?
If lifting a large box from the floor and placing it on a desk without
the use of a machine, your input force is equal to your output force
jut as the input distance is equal to output force.
With a machine: input force is less than output force of the machine
input distance can vary depending on the machine
Input work is always less than output work
34. Machines Change Work
Changing Force
If work stays the same, the machine changes output force and
distance.
Work = Force x Distance
The machine changes output force and output distance to
keep work the same.
A decrease in output force causes the machine to increase
the output distance to keep work the same.
36 joules = 12 N x 3 m
36 joules = 8 N x 8 m
35.
36. Machines Change Work
Changing distance
To keep output work the same, machines can
change the distance over which the output
force is being applies. P. 193
37.
38. Machines Change Work
Changing direction
Some machines don’t change force or distance, they
change the direction the load was moving.
Pulleys – The flagpole rigging is an example
39.
40. Mechanical Advantage
The ratio of output force to input force is the mechanical advantage
of the machine.
Mechanical advantage = Output force /Input force
Whenever the output force is greater than the input force, the
mechanical advantage is greater than 1.
When a machine increases distance, the output force is less than 1.
If a machine only changes the direction of the force, the input and
output forces are the same. Mechanical advantage remains 1. P. 194
41. Efficiency
A comparison of a machine’s input and output work.
Expressed as a percentage
The higher the percentage, the more efficient a machine
is.
42. Calculating Efficiency
Efficiency = output work X 100%
Input Work
To calculate the efficiency of a machine, divide the
output work by the input work and multiply the result by
100.
