1. Work & Friction H.Science9

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Warm-Up Question
• When does a force do work?
• Which of the following athletes are doing
work on the object in their hands?
What is work? When is work done?
• Work
– The product of force and distance
– Is done when a force acts on an object in the
SAME direction the object moves
– requires motion
**For a force to do work on an object, some of the
force must act in the same direction as the object
moves  No movement = No work done
Work continued…
• Work depends on direction
– any part of a force that does not act in the
direction of motion does NO work on an object
Calculating Work
• W=Fxd
• W = work
• F = force
• d = distance (**must always be in meters!)
**Force (Weight) = mk, so Work = mkd
Calculating Work
• Units?
W=Fd
W = Newton-meters
Newton-meters = Joules (J)
Work Calculations
1. How much work is done on a car that is
pushed 35 meters with a force of 200
Newtons?
2. A sled is pulled 10 m up a hill with a force of
15 N. How much work was done on the sled?
Work Calculations
3. What is the force applied to a punching bag that
moves 0.3 meters when 45 Joules of work are done
on the bag?
4. A soccer player kicked a ball with a force of 150
Newtons. If 750 Joules of work were performed on
the soccer ball in total, how many meters must it
have traveled?
Work Calculations
5. A 160 kg basketball player has a vertical leap
of 0.5 meters. If he performs his maximum
vertical leap on Earth, how much work does
he do? How much work would he do if he did
the same test on the Moon?
Power
• What is power?
– The rate at which work is done
– Doing work at a faster rate requires more power
– To increase power, you can…
• Increase the amount of work done in a given amount of
time
• Do a given amount of work in less time
Which has the most power?
Calculating Power
•
•
•
•
Power = Work/Time (**must always be in sec)
P = W/t
P= Fd/t
P= mkd/t
Calculating Power
• Units?
P = W/t
P = Watts (W)
Watts = Joules/second
Power Calculations
1. A student rows a boat across a pond doing
3600 Joules of work on the oars in 60
seconds. What is the student’s power
output?
Power Calculations
2. Your family is moving to a new apartment.
While lifting a box 1.5 m straight up to put it
on a truck, you exert an upward force of 200
N for 1 second. How much power did you
exert?
Power Calculations
3. You apply a horizontal force of 10 N to pull a
wheeled suitcase at a constant speed of 5
meters/second across flat ground. How
much power is used?
Power Calculations
4. A truck pulls a trailer at a constant velocity
for 100 meters while exerting a force of 480
Newtons for 1 minute. What is the truck’s
power output?
Homework
1. Bart runs up a 2910 cm high flight of stairs on
Earth in 2.5 minutes. If Bart's mass is 65.9 kg,
how much work did he do? What was his
power output?
2. An elevator motor lifts 715 kg of mass to the
height of the fourth floor of an office building
(11.0 m above ground level) at a constant
speed in 9.5 seconds. Determine the work
done by the motor and its power rating.
Warm-Up Questions
• Is work done… Yes or No? Why?
– Two men exert 800 N of force, but are unable
to budge a piano that has been bolted to the
floor.
– A child lifts a 2 gram feather 0.5 meters.
– A tow truck pulls a car behind it with 500 N of
force a distance of 35 km.
– A waitress carries a heavy tray of food 10
meters from the kitchen to a customer’s table.
Homework
1. Bart runs up a 2910 cm high flight of stairs on
Earth in 2.5 minutes. If Bart's mass is 65.9 kg,
how much work did he do? What was his
power output?
Homework
2. An elevator motor lifts 715 kg of mass to the
height of the fourth floor of an office building
(11.0 m above ground level) at a constant
speed in 9.5 seconds. Determine the work
done by the motor and its power rating.
Warm-Up Questions
1. A weight lifter lifts a 1600 Newton barbell 3
meters (from the ground to above his head).
How much work was done by the weight
lifter?
2. A football player applies a 200 Newton force
to a blocking sled while driving it 2.5 meters.
How much work was done by the football
player?
Warm-Up Questions
3. If a 20 N was pulled 100 meters, how much work was
done?
4. You exert a vertical force of 72 Newtons to lift a box
to a height of 1.0 meters in a time of 2.0 seconds.
How much power is used to lift the box?
5. You lift a book from the floor to a bookshelf 1.0 meter
above the ground. How much power is used if the
upward force is 15.0 Newtons and you do the work in
2 seconds?
Warm-Up Question
• In gym class at BPHS, you have to push your
friend across the gym floor on a old school
scooter-thingy. If you push her 100 cm in 5
seconds while exerting 150 Watts of power,
what is your friend’s mass in kg?
Warm-Up Questions
• Imagine what a frictionless world would be
like…
• How does it affect you in your everyday life?
• What is friction?
• Are there different types of friction that work
in different ways?
What is friction?
• The rubbing of one surface along another
surface, regardless of each surface’s texture
Types of friction
• Static Friction
– The friction force that acts on objects that are not
moving
– Always acts in the opposite direction of the
applied force
Types of friction
• Sliding Friction
– The friction force that opposes the direction of
motion of an object as it slides over a surface
– Sliding friction ___ Static Friction
***Less force is needed to keep an object moving than to
get an object to start moving
Types of Friction
• Rolling Friction
– The friction force that acts on rolling objects
• The rubbing of wheels along a surface
• Provides less contact area
– Rolling Friction ___ Sliding Friction
Types of Friction
• Fluid Friction
– The friction force that occurs between contact
with air or water
• Fluid friction in air = air resistance
What types of friction are occurring?
Friction Reduction
• Lubrication
– Oil
– Grease
– Etc.
General Rules -- Friction
• The rougher the surface…
– amount of friction ____
• The smoother the surface…
– amount of friction ____
• The larger the surface area in contact…
– Amount of friction ____
• The greater the amount of friction…
– Work ____
• With a lesser amount of friction…
– Work ____

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