Transcript Work and Kinetic Energy

Physics 111 Practice Problem Statements 06
Work and Kinetic Energy
SJ 8th Ed.: Ch. 7.1 – 7.5
Contents:
7-7E*, 7-15E, 7-19P, 7-22P, 7-23P, 7-25E*, 7-31E*, 7-37P, 7-39P
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Energy Overview
Systems
Work
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Constant force 1D
Constant force in 3D
Dot Product (Scalar Product)
Units
Variable force 1D
General Vector 3D definition with variable
force
The Work-Kinetic Energy Theorem
Gravitational Force and Examples
Variable (Spring) Force and Examples.
Power - Read
Physics 111 Problem Set 6
Chapter 7: Kinetic Energy and Work
Contents:
7-7E*, 7-15E, 7-19P, 7-22P, 7-23P, 7-25E*,
7-31E*, 7-37P, 7-39P
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Lecture 6: Chapter 7
Kinetic Energy and Work
• Energy Overview
• Work
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A simple constant force 1D
3D, constant force - Dot Product
Units
Variable force 1D
General Vector 3D definition with variable force
• Work and Kinetic Energy
– Kinetic Energy
– Simple derivation of Work-KE theorem
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General Form of Work-KE Theorem
Gravitational Force and Examples
Variable (spring) force and examples.
Power
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Problem 7 –7*E: To pull a 50 kg crate across a horizontal frictionless floor, a worker applies a force
of 210 N, directed 20° above the horizontal. As the crate moves 3.0 m, what work is done on the
crate by (a) the worker's force, (b) the gravitational force on the crate, and (c) the normal force on
the crate from the floor? (d) What is the total work done on the crate?
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Problem 7 – 15E: In the figure, a cord runs around two massless, frictionless pulleys; a canister
with mass m = 20 kg hangs from one pulley; and you exert a force Fon the free end of the cord. (a)
What must be the magnitude of Fif you are to lift the canister at a constant speed? (b) To lift the
canister by 2.0 cm, how far must you pull the free end of the cord? During that lift, what is the work
done on the canister by (c) your force (via the cord) and (d) the gravitational force on the canister?
(Hint: When a cord loops around a pulley as shown, it pulls on the pulley with a net force that is twice
the tension in the cord.)
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Problem 7 – 19P: A cord is used to vertically lower an initially stationary block of mass M at a
constant downward acceleration of g/4. When the block has fallen a distance d, find (a) the workk
done by the cord's force on the block, (b) the work done by the gravitational force on the block, (c)
the kinetic energy of the block, and (d) the speed of the block.
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Problem 7 – 22P: A 250 g block is dropped onto a relaxed vertical spring that has a spring constant
of k = 2.5 N/cm (see figure). The block becomes attached to the spring and compresses the spring 12
cm before momentarily stopping. While the spring is being compressed, what work is done on the
block by (a) the gravitational force on it and (b) the spring force? (c) What is the speed of the block
just before it hits the spring? (Assume that friction is negligible.) (d) If the speed at impact is doubled,
what is the maximum compression of the spring?
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Problem 7 – 23P: The only force acting on a 2.0 kg body as it moves along the positive x axis has
an x component Fx = -6x N, where x is in meters. The velocity of the body at x = 3.0 m is 8.0 m/s.
(a) What is the velocity of the body at x = 4.0 m? (b) At what positive value of x will the body have a
velocity of 5.0 m/s?
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Problem 7 – 25E*: A 10 kg brick moves along an x axis. Its acceleration as a function of its position
is shown in the figure. How much net work is performed on the brick by the force causing the
acceleration as the brick moves from x = 0 to x = 8.0 m?
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Problem 7 – 31E*: A 100 kg block is pulled at a constant speed of 5.0 m/s across a horizontal floor
by an applied force of 122 N directed 37° above the horizontal. What is the rate at which the force
does work on the block?
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Problem 7 – 37P: The force (but not the power) required to tow a boat at constant velocity is
proportional to the speed. If a speed of 4.0 km/h requires 7.5 kW, how much power does a speed of
12 km/h require?
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Problem 7 – 39P: A horse pulls a cart with a force of 40 lb at an angle of 30° above the horizontal
and moves along at a speed of 6.0 mi/h. (a) How much work does the force do in 10 min? (b) What is
the average power (in horsepower) of the force?
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