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## Fundamentals of Physics

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### Work Done by a Gravitational Force. Work Done by a Spring Force ... English Units: horsepower. 1 hp = 550 ft lbs / s = 746 Watts. Work = Power x Time ... – PowerPoint PPT presentation

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Title: Fundamentals of Physics

1
Fundamentals of Physics
• Chapter 7 Kinetic Energy Work
• Energy
• Work
• Work Kinetic Energy
• Work Done by a Gravitational Force
• Work Done by a Spring Force
• Work Done by a General Variable Force
• Power
• Review Summary
• Questions
• Exercises Problems

2
Energy Work
• Energy comes in many forms
• Thermal, Chemical, Atomic, Nuclear Energies
• Energy is conserved.
• A new tool to solve problems.
• Descriptions of Energy
• Potential Energy (Chapter 7)
• Kinetic Energy - motion of a massive object

3
WOW!
August 10, 1972, a large meteorite skipped across
the atmosphere.
4
Work
• Work (W) is energy transferred to or from an
object by means of a force acting on the object.
• Energy transferred to the object is positive
work.
• Stepping on the gas causes positive work to be
done on a car.
• Energy transferred from the object is negative
work.
• Stepping on the brakes causes negative work to be
done on a moving car.

5
Work Kinetic Energy
Consider a force acting on an object constrained
to move in the x-direction
The force causes a change in kinetic energy the
force does work
Units 1 Joule 1 kg (m / s)2 (1 kg m /
s2) m 1 N m
6
A force acting on a particle moving along a curve
7
Work
• Work (W) is energy transferred to or from an
object by means of a force acting on the object.
• A force does positive work when it has a vector
component in the same direction as the
displacement.
• A force does negative work when it has a vector
component in the opposite direction as the
displacement.
• A force does zero work when it is perpendicular
to the displacement.

8
Work Kinetic Energy Theorem
chain rule for derivatives
Net Work Done Change in Kinetic Energy
9
Scalar (Dot) Product of Two Vectors
Commutative Law
10
Example
Tipler p 157
11
Example
vf ?
Cutnell p 160
12
Work done by the forces on the box?
13
Object sliding on a frictionless floor
Work ?
14
Work Done by the Gravitational Force
• Going up

The work done by the gravity force reduced the
kinetic energy of the object.
15
Work Done Lifting Lowering an Object
To raise an object to a new location, an applied
force, F, must act to overcome the gravity force,
Fg. Consider a case where an object is at rest
before and after the move.
Work- Energy Theorem Kf - Ki Wa Wg But
Kf Ki 0 0 Wa Wg Wa - Wg
The work done by the applied force is the
negative of the work done by the gravitational
force.
Raising
Lowering
The work done by the lifter.
16
Pull with constant force F.
Is less energy expended or less work done when
using a pulley (or other such device)?
17
Pull with constant force F.
d
2d
Same amount of work!
18
An object accelerated downwards
a g/4 drops d Work by cord ? Work by
gravity? Final velocity
T
a
Mg
Work Energy Theorem
19
Work Done by a Spring Force
Relaxed
Hookes Law where k spring
constant (N/m)
Stretching
The force varies linearly with displacement.
Compressing
20
Work Done by a Spring Force
Hookes Law
Relaxed
Work done by the spring force
Stretching
Compressing
21
A block is dropped on a relaxed spring
m 250 g k 2.5 N/cm d 12 cm Wg ? Ws? v ?
22
Work Kinetic Energy Theorem with a Variable
Force
23
Problem
A 10 kg brick moves with the following variable
acceleration how much work is done on the brick
in the first 8 seconds.
24
Power
Power time rate at which Work is done due to
a Force.
SI Units 1 Watt 1 Joule / s 1 N
m / s English Units horsepower 1 hp
550 ft lbs / s 746 Watts
For a constant force
Work Power x Time
1 kW hr 3.6 x 106 J