Introduction to Work and Power
pliers
(Are we working hard or hardly working?)
Name
Class Period
Date
Mechanical work is done when a force or torque moves an object. Mechanical systems use force and torque
to cause movement, and thus, to do useful work. The effect of work is to increase the energy of an object.
The increase in energy happens because the object is moved to a higher position, the speed of an object is
increased, or both.
w
The definition of work done by a force in a mechanical system is:
work = force applied X distance the object moves
w = F X d
F•d
When calculating work, the force must be in Newtons. The distance MUST be in meters. The resulting unit for
work will be in Newton•meters or Joules.
force - Newtons (N)
distance - m
work - N•m or J
One important fact about work is that if the object doesn’t move, no work is done. If you push against a wall,
and the wall doesn’t move, no work is done. According to the formula, if the distance equals zero, work equals
zero.
Another fact is that work is done only while the force is applied. When you throw a ball, your arm no longer
exerts a force on the ball after it leaves your hand. Although the ball may go 10 meters after it leaves your
hand, your arm does no work on the ball once you release the ball.
We can calculate the work done on an object by a person. If a weight lifter uses a force of 200 N
to raise a barbell a distance of 2 meters, how much work does he do?
w = F X d
w = 200 N X 2 m = 400 N•m (or 400 J)
Easy, right? Okay, so how much work do you do if a force of 8 Newtons is needed to raise your
backpack 1.5 meters from the floor?
How much work is done when you hold your backpack as you wait for the bus?
Solve the following problems:
1.
How much work is done if a force of 294 N is needed to pull a block with a weight of 1470 N over a
distance of 6 meters?
2.
A suitcase weighing 60 N is lifted 0.5 meters. How much work is done in lifting the suitcase?
3.
A force of 14 N is used to push the 60 N suitcase a distance of 0.5 meters across the floor. How
much work is done?
4.
A force of 0.5 N is needed to press the key of a computer keyboard. If the key moves .5 cm, how
much work is done?
Work and Power
Power is the rate at which work is done. The faster you do work, the more power is generated. The faster
work is done, the more power is required.
power =
work
time
w
p•t
When calculating power, the work must be in Newton•meters or Joules. The time MUST be in seconds. The
resulting unit for power will be in Watts.
work - Nm or J
time - s
power - W
Consider a weight lifter doing bench presses. The barbell weighs 890 N. The weight
lifter raises the barbell 1 meter with each repetition. If he does 10 reps, how much
work has he done?
The weight lifter does his 10 reps in 12 seconds. How much work did he do? How much power was required?
If he took 22 seconds to do the 10 reps, how much work did he do? How much power was required?
So as the time increases, power ____________________.
As the time decreases, power ____________________.
This is an ___________________________ relationship.
Introduction to Work and Power (continued)
pliers
We’ve learned that the total amount of energy never changes as a system evolves through time. The energy
can change form, say from potential to kinetic or from kinetic to thermal, but the total energy has to stay the
same. If you have 100 J of energy at the beginning, you need to have 100 J of energy at the end.
But this isn’t the whole story. Think about our skate park lab. The skater started off with almost all potential
energy at the top of the ramp. As the skater rolled down the ramp, that potential energy changed into kinetic
energy. But where did that original potential energy come from? It came from us. We picked up the skater
and put her on top of the ramp. We changed the energy of the skater. In other words, we added energy to the
system.
•
Whenever we add or subtract energy from a system, it’s called doing WORK.
Let’s look at an example.
You can find this simulation at https://phet.colorado.edu/en/simulation/legacy/energy-skate-park
I do a certain amount of work to the bulldog to put her up on the ramp. Let’s find out how much work I do.
The bulldog has a mass of 20 kg, and I lift her to a height of 7 meters. The energy that the bulldog has is all
potential energy.
w = f X d
w = 196 N X 7 m = 1392 J
Her GPE at that point on the ramp is:
GPE = mgh
GPE = 20 kg X 9.8 m/s2 X 7 m = 1392 J
So I added the energy to the system that allows it to work as we’d like. Most mechanical systems require
some sort of initial energy input. Think about a car engine. What has to happen for the engine to begin
to run?
Introduction to Work and Power
pliers
(Are we working hard or hardly working?)
Name
Class Period
Date
Mechanical work is done when a force or torque moves an object. Mechanical systems use force and torque
to cause movement, and thus, to do useful work. The effect of work is to increase the energy of an object.
The increase in energy happens because the object is moved to a higher position, the speed of an object is
increased, or both.
w
The definition of work done by a force in a mechanical system is:
work = force applied X distance the object moves
w = F X d
F•d
When calculating work, the force must be in Newtons. The distance MUST be in meters. The resulting unit for
work will be in Newton•meters or Joules.
force - Newtons (N)
distance - m
work - N•m or J
One important fact about work is that if the object doesn’t move, no work is done. If you push against a wall,
and the wall doesn’t move, no work is done. According to the formula, if the distance equals zero, work equals
zero.
Another fact is that work is done only while the force is applied. When you throw a ball, your arm no longer
exerts a force on the ball after it leaves your hand. Although the ball may go 10 meters after it leaves your
hand, your arm does no work on the ball once you release the ball.
We can calculate the work done on an object by a person. If a weight lifter uses a force of 200 N
to raise a barbell a distance of 2 meters, how much work does he do?
w = F X d
w = 200 N X 2 m = 400 N•m (or 400 J)
Easy, right? Okay, so how much work do you do if a force of 8 Newtons is needed to raise your
backpack 1.5 meters from the floor?
w = F X d
w = 8 N X 1.5 m = 12 N•m (or 12 J)
How much work is done when you hold your backpack as you wait for the bus?
Zero work because you are not moving the backpack, just holding it.
Solve the following problems:
1.
How much work is done if a force of 294 N is needed to pull a block with a weight of 1470 N over a
distance of 6 meters?
w = F X d
2.
w = 294 N X 6 m = 1764 N•m (or 1764 J)
A suitcase weighing 60 N is lifted 0.5 meters. How much work is done in lifting the suitcase?
w = F X d
w = 60 N X 0.5 m = 30 N•m (or 30 J)
3.
A force of 14 N is used to push the 60 N suitcase a distance of 0.5 meters across the floor. How
much work is done?
w = F X d
4.
w = 14 N X 0.5 m = 7 N•m (or 7 J)
A force of 0.5 N is needed to press the key of a computer keyboard. If the key moves .5 cm, how
much work is done?
w = F X d
w = 0.5 N X .005 m = 0.0025 N•m (or 0.0025 J)
(You must convert the cm to m.)
Work and Power
Power is the rate at which work is done. The faster you do work, the more power is generated. The faster
work is done, the more power is required.
power =
work
time
w
p•t
When calculating power, the work must be in Newton•meters or Joules. The time MUST be in seconds. The
resulting unit for power will be in Watts.
work - Nm or J
time - s
power - W
Consider a weight lifter doing bench presses. The barbell weighs 890 N. The weight
lifter raises the barbell 1 meter with each repetition. If he does 10 reps, how much
work has he done?
w = F X d
w = 890 N X 1 m = 890 J for each rep.
Work for 10 reps: 890 J X 10 = 8900 J
The weight lifter does his 10 reps in 12 seconds. How much work did he do? How much power was required?
Work for 10 reps: 890 J X 10 = 8900 J
Power = work / time = 8900 J / 12 s = 741.67 W
If he took 22 seconds to do the 10 reps, how much work did he do? How much power was required?
Work for 10 reps: 890 J X 10 = 8900 J
Power = work / time = 8900 J / 22 s = 404.55 W
decreases
So as the time increases, power ____________________.
increases
As the time decreases, power ____________________.
inverse
This is an ___________________________
relationship.