Download Work or Not Work: Example 4 Lab Comments

6/3/2013
Work = Force X distance
W = Fd
W = Fdcosq
• Unit – Joules
• Force must be direction of motion
WNET = DKE
Mr. Fredericks pulls a 10 kg
box with 30 N of Force a
distance of 50 m, at an
angle of 50o with the
ground.
a. Calculate the work that
was done (964 J)
b. Calculate the normal force
on the suitcase. (75 N)
Work or Not
1. A teacher pushes against a wall until he is
exhausted.
2. A book falls off the table and falls freely to
the ground.
3. A waiter carried a full try of meals across the
room.
4. A rocket accelerates through space.
Work: Example 4
A 50-kg crate is pulled 40 m with a force of 100
N at an angle of 37o. The floor is rough and
exerts a frictional force of 50 N. Determine
the work done on the crate by each force and
the net work done on the crate. (1200 J)
Fp
Direction of motion
q = 50o
q
Ffr
FN
mg
Lab Comments
• First sentence should be summary/purpose
• Free body
• Force was constant in graph
A 150,000 kg rocket launches straight up with a
thrust of 4.0 X 106 N.
a. Calculate the work done by thrust at 500 m.
(2.0 X 109 J)
b. Calculate the work done by gravity. (-7.4 X
108 J)
c. Calculate the net work. (1.26 X 109 J)
d. Calculate the speed of the rocket. (130 m/s)
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A 500 g air hockey puck slides across an air table at
2.0 m/s. The player blows on the puck at an
angle of 30o to the horizontal with a force of 1.0
N for 50 cm. The player is trying to slow the puck.
a. Calculate the work done by the player. (-0.433 J)
b. Calculate the final speed of the puck (1.5 m/s)
Work: Variable Force
Work is really an area:
W =∫Fdx
(an integral tells you the
area)
WORK
The magnitude of a force on a spring varies
according to F(x) = 1500x2. Calculate the work
done stretching the spring 10 cm from its
equilibrium length.
A 1500 kg car accelerates from rest. The graph
below shows the force on the car.
a. Calculate the work done on the car. (1 X 106 J)
b. Calculate the speed after 200 m. (37 m/s)
(0.50 Joules)
A 100 g pinball is launched by pulling back a 20
N/m spring a distance of 20 cm. However, there
is friction and mk = 0.10.
a. Calculate the work done by the spring. (0.400 J)
b. Calculate the work done by friction. (-0.020 J)
c. Calculate the speed of the ball on release. (2.8
m/s)
Does the Earth Do Work on the Moon?
W = Fdcosq
W = Fd(cos 90o)
W = Fd(0)
W=0
v
FR
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English Unit of Work
•
•
•
•
Foot-pound – English unit of work.
Pound –Force
Foot –distance
W = Fd = (foot*pound)
Conservative and Nonconservative
Forces
Conservative Forces
– Work is independent of the path taken
– Gravity, electromagnetic forces
A 70 kg man on a sled is gliding at 2.0 m/s
when he starts down a slippery 10o slope.
He travels for 50 m.
a. Calculate the force parallel to the ground
pulling him down the hill. (120 N)
b. Calculate work done by gravity for the 50
m. (6000 J)
c. Calculate his speed at the bottom.
Remember that initially he was not at rest.
(13 m/s)
Nonconservative Forces
Will it take more work to push the box on path A
or path B? Or are they the same?
Nonconservative Forces
– Work depends on the path taken
– Friction (dissipative forces)
B
A
If nonconservative forces act, use:
KE1 + PE1 = KE2 + PE1 + Wfr
½ mv2 + mgy = ½ mv2 + mgy + Ffrd
Mr. Fredericks (100 kg)
slides down a 3.5 m tall
slide. If he leaves the
slide at the bottom at 6.3
m/s, what is the Force of
friction and the
coefficient of friction for
the slide? Assume the
slide is 6.0 m long. (0.31)
3.5 m
6.0 m
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A 70 kg skier starts at the top of the slope at
2.0 m/s. The slope is 50 m long and has an
elevation of 10o. There is a wind exerting a
50 N retarding force at the bottom.
a. Calculate the work done by gravity (6000 J)
b. Calculate the work done by the retarding
force. (-2500 J)
c. Calculate her speed at the bottom (10 m/s)
Force and Potential Energy
F = - dU
ds
• Force is the negative of the derivative of the
potential energy.
• Force is the negative slope.
Calculate the force being exerted on a particle
given the following potential energy curve:
A 5.0 kg box is attached to one end of a spring
(80 N/m). The other end is attached to the
wall. The spring is stretched 50 cm by a
constant force of 100 N. There is friction
and mk = 0.30.
a. Calculate the work done by the pull. (50 J)
b. Calculate the work done on the spring. (10 J)
c. Calculate the work lost to friction (thermal
energy). (7.4 J)
d. Calculate the speed of the box at 50 cm (3.6
m/s) [W = 1/2mv2 + 1/2kx2 + Etherm]
Example:
Calculate the gravitational force for gravitational
Potential energy (mgy)
Given the following potential energy graph,
sketch the force versus distance graph.
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A 500.0 gram box is pushed up a 30o incline with a force of
50.0 N as shown in the figure. The box moves 50.0 cm up
the incline.
– Calculate the work done by the force assuming no
friction.
– Calculate the work done against gravity and the net
work done by all the forces on the box.
– Suppose the box has a mass of 500 grams and mk =
0.20. Calculate the net work done by all the forces on
the box.
Power
Power = Work
time
P=W
t
• Metric Unit: Joules/s = Watt.
• Definition – rate at which work is done
– A powerful engine can do a lot of work quickly.
– Running and walking up the steps require the
same amount of work.
– Running up steps requires more Power
a. A donkey performs 15,000 J of work pulling a
wagon for 20 s. What is the donkey’s power?
b. What power motor is needed to lift a 2000 kg
elevator at a constant 3.0 m/s? (Hint: use 1
second in your calculations)
c. A motor and cable drags a 300 kg box across
a rough floor at 0.50 m/s. The coefficient of
kinetic friction is 0.60. Calculate the
necessary power.
1. How much horsepower is required to power
a 100 Watt lightbulb?
2. A 1500 kg car has a profile that is 1.6 m wide
and 1.4 m high. The coefficient of rolling
friction is 0.02.
1. Calculate the drag force if the car travels at a
steady 30 m/s (1/4Av2) (504 N)
2. Calculate the force the car must exert against
drag and friction. (798 N)
3. Calculate the power the engine must provide if
25% of the power is lost between the engine and
the wheels.
Horsepower
• The English Unit of power is horsepower
• Foot-lb = Horsepower (hp)
second
• 1 hp = 746 Watts
• 1 hp = ½ Columbus (who sailed in 1492)
Horsepower
Consider a 100 hp car engine that can go from 0
to 60 mi/hr in 20 seconds.
A 400 hp car could go from zero to 60 mi/hr in 5
seconds.
4 times as powerful means it can do the same
work in ¼ the time.
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Horsepower: Example 4
A crane lifts a 200 N box 5 meters in 3 seconds.
What is the crane’s power in Watts and in
horsepower?
P = (200)(5)/3 = 333 W
333W/746W = 0.45 hp
Power and Calculus
P=W
t
Work = Fd
P = FDd = F dx
Dt
dt
P = Fv
Power and Calculus: Ex 1
Find the power delivered by a net force at t=2 s
to a 0.5 kg mass that moves according to x(t) =
1/3t3
F = ma
F = (0.5 kg)(4 m/s2) = 2 N
P = Fv = (2N)(4 m/s)
v = dx/dt
v = t2
a = dv/dt
a = 2t
v = (2)2 = 4 m/s
a = 4 m/s2
Springs and Calculus
The force in a spring is variable (F = -kx)
Work = ∫0x F(x) dx
Work = ∫0x -kx dx
Work = - ½kx2
Work = -DPE
DPE = ½ kx2
A particle experiences a force F(x) = 2x.
a) Calculate the change in potential energy that
the particle undergoes from 0 to 3 m.
b) What kind of object might provide the force
shown above?
c) If the force is now F(x) = 2x2, calculate the
change in potential energy that the particle
undergoes from 0 to 3 m.
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Given the following force-distance graph, sketch
the potential energy-distance graph.
8. a) -12 J
b) 0 J
10. a) -29 J
b) 29 J
12. -1.95 kJ
1.69 kJ
1.07 kJ
14. -2.0 J
16. 5.1 m/s (at 2 m) 4.0 m/s (at 4m)
18. 100N, 0 N, -50 N
20. b) -12 N (y = 1 m)
-48 N (y = 2m)
30. a) 9.8 X 105 J b) 1.96 X 104 W
32. 41.7 m2
34.a) energy
b) 1.8 X 109 J
36. a, b, c) 50 J (Conservative)
38. a) 400 N
b) -2 J
c) 22.4 m/s
A horizontal force pulls a 20.0 kg carton across the floor at a
constant speed. The coefficient of kinetic friction, mk, is
0.60.
a. Calculate the work done moving the box 3.00 m.
b Suppose the force is now at an angle of 30.0o to the floor
as shown in the figure. Calculate the force needed to pull
the box across the floor at a constant speed. Be sure to
consider any effects the rope may have on the normal
force.
c. Calculate the work done pulling the box 3.00 m under the
conditions in (b).
Given the following potential energy-distance
graph, sketch the force-distance graph.
36.
38.
42.
44.
a, b, c) 50 J (Conservative)
a) 400 N b) -2 J
c) 22.4 m/s
a) 230 J
b) 230 N c) 6.8 kW
a) 15.7 m/s
b) 15.7 m/s
The following problems require calculus.
– The potential energy of a particle is defined as U =
ax3 – bx2. Determine the formula for the force
acting on the particle.
– The potential energy of a particle is defined as U =
Uosinbx. Determine the formula for the force acting
on the particle.
– Calculate the work done from 1.00 m to 3.00 m for a
particle that experiences a force Fx = 5.0x - 4.0
– Starting with the formula for the force of a spring,
Fspring = -ks, determine the formula for the potential
energy stored in a spring. Integrate from 0 to x.
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A 1.50 kg rocket from a craft store generates a
thrust of 40.0 N.
– Using energy-work calculations, determine the
speed at 10.0 m.
– Suppose the rockets only burns for the first 10.0
m. Calculate the total maximum height of the
rocket.
The following masses are attached to a pulley
and cord as shown. Mass M is heavier than
mass m.
– Using energy considerations, find an expression
for the speed of either mass just before mass M
hits the floor.
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