Energy
Work
Forms of Energy
Conservation of Energy
Gravitational & Elastic Potential Energy
Work - Energy Theorem
Conservation of Momentum & Energy
Power
Simple Machines
Mechanical Advantage
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Energy - Physics - Lecture Slides, Slides of Physics
In these Physics Lecture Slides, following major aspects of physics have been discussed : Energy, Forms of Energy, Work, Conservation of Energy, Gravitational, Elastic Potential Energy, Energy Theorem, Conservation of Momentum, Power, Simple Machines
Typology: Slides
2012/2013
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Energy
Work
Forms of Energy
Conservation of Energy
Gravitational & Elastic Potential Energy
Work - Energy Theorem
Conservation of Momentum & Energy
Power
Simple Machines
Mechanical Advantage
Work
The simplest definition for the amount of work a force does on an object is magnitude of the force times the distance over which it‟s applied:
W = F x
This formula applies when:
- the force is constant
- the force is in the same direction as the displacement of the object
F
x Docsity.com
Negative Work
f k = 20 N 7 m
A force that acts opposite to the direction of motion of an object does negative work. Suppose the crate of granola bars skids across the floor until friction brings it to a stop. The displacement is to the right, but the force of friction is to the left. Therefore, the amount of work friction does is -140 J.
Friction doesn‟t always do negative work. When you walk, for example, the friction force is in the same direction as your motion, so it does positive work in this case.
v
Tofu Almond Crunch
When zero work is done
7 m
N
mg
As the crate slides horizontally, the normal force and weight do no work at all, because they are perpendicular to the displacement. If the granola bar were moving vertically, such as in an elevator, then they each force would be doing work. Moving up in an elevator the normal force would do positive work, and the weight would do negative work.
Another case when zero work is done is when the displacement is zero. Think about a weight lifter holding a 200 lb barbell over her head. Even though the force applied is 200 lb, and work was done in getting over her head, no work is done just holding it over her head.
Tofu Almond Crunch
Tofu Almond Crunch
When the force is at an angle
x
F
When a force acts in a direction that is not in line with the displacement, only part of the force does work. The component of F that is parallel to the displacement does work, but the perpendicular component of F does zero work. So, a more general formula for work is
W = F x cos
F cos
F sin This formula assumes that F is constant.
Work: Incline Example
(^) k
F
A box of tiddlywinks is being dragged across a ramp at a
toy store. The dragging force, F , is applied at an angle
to the horizontal. The angle of inclination of the ramp is ,
and its length is d. The coefficient of kinetic friction
between the box and ramp is (^) k. Find the net work done
on the tiddlywinks as they are dragged down the ramp.
continued on next slide
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Work: Circular Motion Example
A „69 Thunderbird is cruising around a circular track. Since it‟s turning a centripetal force is required. What type of force supplies this centripetal force?
answer:
friction
How much work does this force do?
None, since the centripetal force is always to the car‟s motion.
answer:
r
v
Forms of Energy
When work is done on an object the amount of energy the object has as well as the types of energy it possesses could change. Here are some types of energy you should know:
- Kinetic energy
- Rotational Kinetic Energy
- Gravitational Potential Energy
- Elastic Potential Energy
- Chemical Potential Energy
- Mass itself
- Electrical energy
- Light
- Sound
- Other waves
- Thermal energy
Energy Units
The formula for kinetic energy, K = ½ m v 2 , shows that its units are:
kg · (m/s)^2 = kg · m 2 / s 2 = (kg · m / s 2 ) m = N · m = J
So the SI unit for kinetic energy is the Joule, just as it is for work. The Joule is the SI unit for all types of energy. One common non-SI unit for energy is the calorie. 1 cal = 4.186 J. A calorie is the amount of energy needed to raise the temperature of 1 gram of water 1 C. A food calorie is really a kilocalorie. 1 Cal = 1000 cal = 4186 J. Another common energy unit is the British thermal unit, BTU, which the energy needed to raise a pound of water 1 F. 1 BTU = 1055 J.
Kinetic Energy Example
A 55 kg toy sailboat is cruising at
3 m/s. What is its kinetic
energy?
This is a simple plug and chug
problem:
K = 0.5 (55) (3) 2 = 247.5 J
Note: Kinetic energy (along with
every other type of energy) is a
scalar, not a vector!
Work - Energy Sample 1 Schmedrick takes his 1800 kg pet rhinoceros, Gertrude, ice skating on a frozen pond. While Gertrude is coasting past Schmedrick at 4 m/s, Schmedrick grabs on to her tail to hitch a ride. He holds on for 25 m. Because of friction between the ice and Schmedrick, Gertrude is slowed down. The force of friction is 170 N. Ignore the friction between Gertrude‟s skates and the ice. How fast is she going when he lets go?
Friction, which does negative work here, is the net force, since weight and normal force cancel out. So, W net = -(170 N) (25 m) = -4250 J. By the work-energy theorem this is the change in her kinetic energy, meaning she loses this much energy. Thus,
-4250 J = K = ½ m v f^2 - ½ m v 02 = ½ m ( v f^2 - v 02 ) = ½ (1800 kg) [ v f^2 - (4 m/s)^2 ] v f = 3.358 m/s You should redo this problem using the 2nd^ Law & kinematics and show that the answer is the same. Docsity.com
Work-Energy Sample 2 A 62 pound upward force is applied to a 50 pound can of Spam. The Spam was originally at rest. How fast is it going if the upward force is applied for 20 feet?
Spam
62 lb
50 lb
W net = K F net x = K f - K 0 (12 lb) (20 ft) = ½ m v f^2 - 0
240 ft · lb = ½ ( mg ) v f^2 / g 240 ft · lb = ½ (50 lb) v f^2 / (32.2 ft / s^2 )
v f^2 = 309.12 ft^2 / s^2 v f = 17.58 ft / s
multiply & divide by g
mg is the
weight 9.8 m^ ^ 32.2 ft
continued on next slide
Gravitational Potential Energy
Objects high above the ground have energy by virtue of their height. This is potential energy (the gravitational type). If allowed to fall, the energy of such an object can be converted into other forms like kinetic energy, heat, and sound. Gravitational potential energy is given by:
U = m g h
The equation shows that...
... the more gravitational potential energy it‟s got. - the more mass a body has - or the stronger the gravitational field it‟s in - or the higher up it is
SI Potential Energy Units
From the equation U = m g h the units of gravitational
potential energy must be:
kg · (m/s^2 ) · m = (kg · m/s^2 ) · m = N · m = J
This shows the SI unit for potential energy is the Joule, as it is for work and all other types of energy.