WORK, POWER, MACHINES & ENERGY. Work is the product of the component of the force exerted on an...
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Transcript of WORK, POWER, MACHINES & ENERGY. Work is the product of the component of the force exerted on an...
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WORK, POWER,WORK, POWER,MACHINES & ENERGYMACHINES & ENERGY
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Work is the product of the component of the force exerted on an object in the direction of the displacement and the magnitude of the displacement.
W = FW = FΔΔdd
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WORK
In order for work to be done, three things are necessary:•There must be an applied force.•The force must act through a certain distance, called the displacement.•The force must have a component along the displacement.
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Read the following statements and determine whether or not they represent examples of work.
A teacher applies a force to a wall and becomes exhausted.
A book falls off a table and free falls to the ground.
NO, displacement doesn’t occur
Yes, displacement in the direction of force
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A truck carries a box in it’s bed 100 m.
NO, This is not an example of work. The force is upward on the box but the displacement is along the ground.
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You pull your luggage on a cart that makes an angle of 30º for 5 m
Yes, but only in direction of the displacement
5 m
So the force that does the work is the component of the force along the ground or Fx
So Work = Fd = (Fcos Θ)d
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W = F d
W = Nm
W = (Newtons )(meters)
The units of work are;
W = Joule (J)
In customary; W = foot pounds
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Matt lifts a 80 kg barbell upward for 1 meter at a constant speed, how much work does he do?
F = w = mg = (80 kg) (10m/s/s) = 800 N
What force must Matt provide ?
W= Fd = (800 N) (1 m) =
800 J
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What work is done by a 60 N force in dragging the bag a distance of 50 m when the force is transmitted by a handle making an angle of 30 with the horizontal?
F = FcosΘ d = 50m
W= FcosΘd
W= (60 N)(cos 30º)(50m)
W= 2598 J FcosΘ
F = 60 N
Θ
W= F∙d
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POWER is the rate at which work is done.
Power = (work)
(time)
P = W t
P = Joules sec
P = J/s = Watts = W
In customary; Power = horsepower= hp
760 W = 1 hp
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What is the man’s power in lifting a 3.0 kg object through a vertical distance of 1.6 m in 10 sec?
F = w = mg d = 1.6 m t = 10 s
P = W = Fd t t
P = (3 kg) (10 m/s/s) (1.6 m)
10 sec
P = 4.8 W Ability to do work?
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Energy is the ability to do work or that which can be converted into work..
When something has energy, it is able to perform work or, in a general sense, to change some aspect of the physical world.
Pg. 66
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PE = m g h
In mechanics we are concerned with two kinds of energy: KINETIC ENERGY: KE, energy possessed by a body by virtue of its motion.
Units: Joules (J)
POTENTIAL ENERGY: PE, energy possessed by a system by virtue of position or condition.
KE = ½ mv2
Units: Joules (J)
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Example: Find the kinetic energy of a 3200 N automobile traveling at 20.8 m/s?
Fg = 3200 Nv = 20.8 m/sm = W/g = 320 kg
KE = ½ mv2 = ½ (320 kg) (20.8m/s)2
= 6.92 x104 J
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Example: A 250 g object is held 200 mm above a workbench that is 1 m above the floor. Find the potential energy relative to
a. the bench top
m = 0.25 kgh = 0.2 m
PE = mgh = 0.25 kg (10 m/s2) (0.2m) = 0.50 J
b. the floorh = 1.2 m PE = mgh
= 0.25 kg (10 m/s2) (1.2m) = 3.00 J
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Today:Notes: Work and Power
Tuesday:Energy Quiz
Tomorrow:Notes: Conservation of Energy
Monday:Roller Coaster Lab