work, energy, and power physics 1
TRANSCRIPT
Work
Work is the product of the magnitude of the displacement and the component of the force acting in the direction of the displacement
Work
Most of the time F is in the direction of d so θ = 0° and cos 0° = 1 so…
Work is done by a force acting on a body!
FdW max
Work
Symbol: W Unit : J, joule 1 J = 1 Nm
If force and displacement are in the… Same directions, + W Opposite directions, - W Perpendicular directions, W = 0
Work
Work generally falls into 2 categories:
1. When you force something to move against the influence of an opposing force
– (push-ups & gravitational force, walking & friction, bow & elastic force)
2. When you change the speed of something
– A net force is present
Work - graphing
Work - graphing
Work and Energy
Energy is the ability to do work
OR
Work causes changes in energy
OR
Work is a transfer of energy
Work and Energy
EW
Power
Power is the rate at which work is done or the rate at which energy is transformed.
aveave Fvt
Fd
t
E
t
WP
Power
Symbol: P Unit: W, Watt 1W = 1J/s
Power and Work
Example:One liter of gas can do a specific amount of total work.
However, the POWER produced when we burn the gas can be any amount depending on the rate of burn
If…
1L is burned over 30 min or 1800 sec, like a car ~50 units of power is produced
1L is burned in 1 sec, like a supersonic jet ~90,000 units of power is produced
Energy - Types
Mechanical Energy: Energy due to position in a field force or energy due to movement
Non-mechanical Energy: Energy that does not fall into the above category
Energy – Flow Chart
Energy
Mechanical Non-mechanical
Light Sound HeatKinetic Potential
Linear
Rotational
Gravitational
Elastic
Electric
Magnetic
Mechanical Energy - Types
1. Kinetic Energy, KE: Energy of a moving object
Linear, KE → center of mass moving (this chapter)
Rotational, KR → object rotating around center of mass (not covered)
Mechanical Energy - Types
2. Potential Energy, PE: Energy due to position in a field force
Gravitational, PEg (this chapter)
Elastic, PEs (this chapter)
Electric, PEE (2nd Semester)
Magnetic, PEB (Not covered)
*You must choose a zero point for these*
Kinetic Energy Equation
2
2
1mvKE
Potential Energy Equations
mghPEg
2
2
1kxPEs
Work – Energy Principle or work done by a net force or net work done on an object
KEdFW netnet
for why board theSee
Energy Conservation
The total energy is neither increased nor decreased in any process.
Energy can, however, be transformed from one type to another AND transferred from one body to another, BUT, the total amount of energy in the process remains CONSTANT!
Conservative and Nonconservative Forces Conservative Force: A force such that the work
done on an object by the force does not depend on the path taken, rather it depends only on the initial and final positions (gravitational, elastic, electric)
Nonconservative Force: A force such that the work done on the object by the force does depend on the path taken (friction, air resistance, rocket propulsion). A lot of times these forces generate heat or sound which are non-mechanical energies.
Work – Energy Principle Redefined
So if energy is conserved we can write it this way using mechanical and non-mechanical energies
NCWPEKE
Work – Energy Principle & Mechanical Energy Conservation If we ignore nonconservative forces (friction and
the such), the implication is that no non-mechanical energies are present (heat, sound, light, etc) therefore…
0 PEKE
Mechanical Energy Conservation
21 EE
Mechanical Energy Conservation with energy lost
friction
lost
WEE
EEE
21
21
Homework and Reading
Chapter 5:
Work, Energy, and Power
Read:
p 159 - 191
Problems:
p 162, 1 - 4; p 163, 1 - 6; p 166, 1 - 5;
p 168, 1 - 4; p 172, 1 - 3; p 177, 1 - 5;
p 178 1 - 3; p 181, 1 - 5, 1 - 2; p 185, 19 - 25; p 186, 26 - 36; p 187 42, 44, 48; p 189, 52