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Conceptual Physics--Chapter 7: Energy
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Gravity
Conceptual Physics 10th e. by Paul G. Hewitt Summary of Terms, Summary of Formulas, and Terms Within the Textbook
Terms in this set (27)
Work
The product of the force and the distance moved by the force:
W = Fd
(More generally, work is the component of force in the direction of motion times the distance moved.)
Power
The time rate of work:
Power = work/time
(More generally, power is the rate at which energy is expended.) Power = work done/time interval
Energy
The property of a system that enables it to do work.
Mechanical energy
Energy due to the position of something or the movement of something.
Potential energy (PE)
The energy that something possesses because of its position.
Kinetic energy (KE)
Energy of motion, quantified by the relationship:
Kinetic energy = ½ mv²
Work-energy theorem
The work done on an object equals the change in kinetic energy of the object.
Work = ∆KE
(Work can also transfer other forms of energy to a system.)
Conservation of energy
Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes.
Machine
A device, such as a lever or pulley, that increases (or decreases) a force or simply changes the direction of a force.
Conservation of energy for machines
The work output of any machine cannot exceed the work input. In an ideal machine, where no energy is transformed into thermal energy,
work input =work output
;
(Fd) input = (Fd) output
.
Lever
Simple machine consisting of a rigid rod pivoted at a fixed point called the fulcrum.
Efficiency
The percentage of the work put into a machine that is converted into useful work output.
(More generally, useful energy output divided by total energy input.)
Note-on-"Work":
The word
work
, in common usage, means physical or mental exertion. Don't confuse the physics definition of work with the everyday notion of work.
"How long" means time, but also it can mean...
...distance also.
Two things enter the picture whenever work is done:
1. application of a force
2. the movement of something by that force.
joule (J):
SI unit of work and of all other forms of energy. One joule of work is done when a force of 1 newton is exerted on an objected moved 1 meter in the direction of the force (N⋅m).
watt (W):
SI unit of power. One watt is expended when one joule of work is done in one second. 1W = 1J/s
gravitational potential energy:
Energy that a body possesses because of its position in a gravitational field. On Earth, potential energy (PE) equals mass (m) times the acceleration due to gravity (g) times height (h) from a reference level such as the Earth's surface.
PE = mgh
*kinetic energy of an object*...
...depends on the mass of the object as well as its speed. It is equal to the mass multiplied by the square of the speed, multiplied by the constant ½.
*Work = ∆KE*
This can be derived as follows:
If we multiply both sides of
F = ma
(Newton's 2nd law) by
d
, we get
Fd = mad
. Recall from Chapter 3 that, for constant acceleration,
d = ½ at²
, so we can say
Fd = ma(½ at²) = ½ maat² = ½ m(at)²
; and substituting
∆v = at
, we get
Fd = ∆½mv²
. That is,
Work = ∆KE
, *Work = ∆KE*.
Work is not a form of energy, but a way of transferring energy from one place to another or one form to another.*⁴*
⁴
Work = ∆E + Q
, where
Q
Q* is the energy transfer due to a temperature difference.
thermonuclear fusion:
Nuclear fusion brought about by extremely high temperatures; in other words, welding together of atomic nuclei by high temperature.
The lever:
f
d
=
F
d
fulcrum:
Pivot point of a lever.
*Efficiency can be expressed by the ratio*:
Efficiency = useful energy output/total energy input
Two properties of motion:
Kinetic energy and momentum.
Differences in kinetic energy and momentum:
Momentum, like velocity, is a vector quantity.
Energy, on the other hand, like mass, is a scalar quantity.
Another difference is the velocity dependence of the two. Whereas momentum is proportional to velocity (mv), kinetic energy is proportional to the square of velocity (½mv²).
;