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Chapter 7

PHY 101 Chapter Notes - Chapter 7: Horsepower, Kinetic Energy, Dot Product


Department
Physics
Course Code
PHY 101
Professor
Dr.sharon Zane
Chapter
7

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7-1 Work Done by a Constant Force
Force in the Direction of Displacement
The greater the force, the greater the distance, the greater the work
Work: W= Fd, when a constant force is in the direction of displacement
(N*m, J)
Scalar quantity; a large force over a small distance is the same as small
force over a large distance
1 J= 1 N*m= 1 (kg*m/s2)*m= 1 kg *m2/s2
Work: W= F/d when the distance is vertical
1 J of work lifts a gallon of milk about an inch
W=0 if d=0 regardless of the size of the force
Force at an Angle to the Displacement
When F is at angle to the horizontal that is in the direction of motion, work is
the component of force in the direction of the displacement times the
magnitude of the displacement
Work= (Fcos )d= Fdcos ; unit= J
When the angle is 90 (when force and displacement are at right angles) the
work done by force F = 0
The displacement equals dcos + dsin
Work is the component of force in the direction of the displacement
times the magnitude of displacement
Work is the component of displacement in the direction of the force
times the magnitude of the force
Mean the same thing; is an angle b/t the force vector and the
displacement vector placed tail-to-tail
Dot product- the magnitude of one vector (in this case the force vector)
times the magnitude of the second vector (in this case the displacement
vector) times the cosine of the angle between them
Negative Work and Total Work
Work depends on the angle b/t the force, F, and the displacement (or
direction of motion), d which gives rise to three possibilities
Work is positive if the force has a component in the direction of
motion (-90 < < 90)
Work is zero if the force has no component in the direction of
motion ( + 90)
Work is negative if the force has a component opposite to the
direction of motion (90 < <270)
When more than one force acts on an object the total work is the sum of the
work done by each separately
Total work can also be calculated by first performing a vector sum of all the
forces acting on an object and then using the definition of work W= Ftotal
dcos
7-2 Kinetic Energy and the Work- Energy Theorem
In general whenever the total work done on an object is positive, its speed
increases; when the total work done on an object is negative, its speed
decreases
Wtotal= 1/2mvf2- 1/2mvi2
Kinetic energy- the energy an object has due to its motion; KE= 1/2mv2;
unit= J
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