physics 218: mechanics instructor: dr. tatiana erukhimova lectures 16, 17, 18
DESCRIPTION
The advantage of a pulley What minimum force F is needed to lift the piano of mass M?TRANSCRIPT
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Physics 218: Mechanics Instructor: Dr. Tatiana Erukhimova
Lectures 16, 17, 18
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F
Find F necessary to drag the box 1 at constant speed.
1
2
Friction everywhere.
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The advantage of a pulley
What minimum force F is needed to lift the piano of mass M?
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WORK (Chapter 7)
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2 or 3 D motion
22
21
22
2
1
mVmVrdFWr
rnet
rdFWL
Definition of work:
Kinetic energy: 2222
|| 2222zyx mVmVmVVm
Work-energy theorem:
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A person carries a bag of groceries of mass M with a constant velocity at the same height from the ground. Find the work done by a person on a bag.
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Problem 1 p. 122
A man pushes up on a 100 kg block with a force such that it drops from 2 meters to the ground at a constant velocity.
a)How much work is done by gravity?b)What is the magnitude of the force he exerts?
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A block of mass m starts at the top of an inclined plane. The coefficient of friction between the plane and the block is . Assuming the block slides down the plane calculate the work done by each force.
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Work Energy Theorem
22
2222
11
22
11
initialfinalyx
yx
totaly
yx
yx
totalx
mVmVdyFdxF
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A person is pulling a crate of mass M along the floor with a constant force F over a distance d. The coefficient of friction is .
(a) Find the work done by the force F on the crate.(b) Same if F changes as F0(1+x2/d2). (c) Find the work done by the force of friction on the crate (F is constant). (d) Find the net work done on the crate if the crate is pulled with a constant
velocity. (e) Find the final velocity of the crate if the crate is pulled with a non-zero
acceleration starting from the rest.
M
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ixxkFLawsHooke)(:' 0
x
positionunstrechedisx0
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Problem 2 p.122
A 3 slug mass is attached to a spring which is pulled out one foot. The spring constant k is 100 pounds/ft. How fast will the mass be moving when the spring is returned to its unstretched length? (Assume no friction.)
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A 5.00 kg block is moving at v0=6.00 m/s along a frictionless, horizontal surface toward a spring with constant k=500 N/m that is attached to a wall.
a) Find the maximum distance the spring will be compressed.
b)If the spring is to compress by no more than 0.150 m, what should be the maximum value of v0?
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Quiz Ch7
y
A block of mass m slides down on the larger block of mass M (see the figure below). There is non-zero coefficient of friction between the two blocks. Neglect the friction between the block of mass M and the table.
1) Plot the free body diagram for each of the blocks;
2) Find the work done by gravity on the block of mass m (the smaller one) in two coordinate systems shown above, if the height of the inclined plane is H.
x x
y
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✔
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Have a great day!
Reading: Chapter 7Hw: Chapter 7 problems and exercises