Where have we been? Unbalanced forces cause acceleration Balanced forces = constant velocity We have...
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Where have we been? Unbalanced forces cause acceleration Balanced forces = constant velocity We have learned that forces are vectors too. We can work with forces on angles.
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Transcript of Where have we been? Unbalanced forces cause acceleration Balanced forces = constant velocity We have...
Where have we been?Unbalanced forces cause
accelerationBalanced forces = constant
velocityWe have learned that forces are
vectors too.We can work with forces on
angles.
Work and Power
What your forces do
– and how fast.
What is Work?
only done when a force causes motion in the same direction as the force acting on it.
Force
Motion
It’s Energy transferred by a force
What is Work?
If not moving,
no work is done.
Force
No Motion
It’s Energy transferred by a force
What is Work?
If moving in a direction different from force,
no work is done.
Fo
rce
Motion
It’s Energy transferred by a force
Formula:
W = F x d (cosθ)
Force distanceWork
Angle
So, when does the angle matter?
When 0º, W = Fd (cos 0º)
cos 0 = 1,
so, W = Fd
So, when does the angle matter?
When 90º, W = Fd (cos 90º)
cos 90 = 0,
so, W = 0
Formula:
W = F x d (cosθ)
Force (N)
distance (m)
Work (joules)
Angle (if not 0º, or 90º)
Remember . . .
Work done on an object increases the total energy of the object.
Work done by an object decreases the total energy of the object.
Work is Energy transferred by a force
Work is performed by . . .
Gravity (objects falling)
Friction (opposing motion)
Humans or machines (applying a push or pull)
Net Work isThe sum of work done by
all forces acting on an object
Use appropriate signs
Remember that work by 90º forces is
0J, so don’t waste time solving for it.
Quick quiz!Which of these are examples of work?
A book falls off a table and free falls to the ground.
A rocket accelerates through space. A boy pushes on a large boulder, but the boulder
is too heavy for him to move.
How about this one?
This waiter carries a tray of food across the room. Is the waiter doing work on the tray?
Calculate work
A crane lifts a steel beam with a mass of 1,500 kg.
Calculate how much work is done against gravity if the beam is lifted 50 meters in the air.
735750 JoulesHow many kilo-Joules is this?
Check your understanding!
500 J 433 J 735 J
One more…
Ben carries a 200-N suitcase up three flights of stairs (a height of 10.0 m) and then pushes it with a horizontal force of 50.0 N at a constant speed of 0.5 m/s for a horizontal distance of 35.0 meters. How much work does Ben do on his suitcase during this entire motion?
3750 J
Power
Power is the rate at which work is done.
Power = Work
time
W
P t
Power
Power is the rate at which work is done.
Power = Work
time
W
P t
Power
Units of power: 1 Joule/sec = 1 Watt 1000 Watts = 1 kilowatt
Question 1
A woman performed 278 J of work in 40 seconds. Calculate her power.
Question 2
A pulley is used to lift a 620 N box of construction equipment 24 m straight up into the air. The pulley system accomplishes this in 18 seconds. Calculate the power of the pulley system
Question 3
Consider a box that is slid 12 m across the room. Two people apply a force of 19 N each. It takes them a total of 11 seconds to accomplish the feat. What is the combined power of the people?
Question 4
A certain generator performs 4506.3 kJ of work in 2.3
hours. How much power does the generator have? (Pay close attention to units!)
Question 5
A man pulled a sled at an angle of 32o. The force he exerted was 106 N until he had moved the sled 95 m. It took him 312 s to accomplish this feat. Calculate his power.
