Energy and Its Conservation

Energy• Energy is defined as the ability of a body or

system of bodies to perform work.• Energy can be subdivided into other categories

such as:– Mechanical energy– Heat energy– Electrical energy– Chemical energy– Atomic energy

Work

• Work (W) is done in displacing a body a distance x by a force F in the direction of the motion.

• Work is the product of the force acting on the body, in the direction of displacement, times the displacement x of the body.

FxW

Work

• According to the physics definition of work, work is not done on an object unless the object is moved.

• If you push against a wall for a period of time, you may feel tired, but if the wall did not move, you did not do any mechanical work.

• Your body may have used chemical energy.

Work• If the force acting on the body is not parallel to

the displacement, then the work done is the product of the force in the direction of the displacement, times the displacement.

F

Fx=Fcos

cosFxW x

Work

• The units of work are the based on the product of force and displacement which would be the newton-meter (N-m) or the foot-pound (ft-lb).

• 1 newton-meter = 1 joule (J)• The joule unit is named after James Prescott

Joule, a British physicist (and brewer.

Work

• What is the minimum amount of work necessary to lift a 1.00 lb box to a height of 5.00 ft?

• Given:– F=mg=1.00 lb– X=h=5.00 ft

• Answer: 5.00 ft-lb

Power• Power is defined as the time rate of doing

work.

tWP or

timedonework Power

• Doing work rapidly expends more power than when you go slowly.

Power

• The unit of power is the Joule/second or foot-pound/second.

• 1 J/s = 1 watt (W)• The watt unit is named after James Watt

(1736-1819), a Scottish engineer who perfected the steam engine.

• 1 horsepower (hp) = 745.7 W or 550 ft-lb/s

Power

• Power can also be calculated if you know the applied force and the velocity.

vtxand

tFx

tWP

so FvP

Gravitational Potential Energy

• Gravitational Potential Energy is defined as the energy that a body possesses due to its position above a reference level.

• As an object is lifted to a higher position, it takes more work and therefore gains more energy.

• Potential energy is inactive or stored energy.

Gravitational Potential Energy

• A dam is constructed to hold back water and create potential energy based upon the height of the dam.

• The energy comes from the work that is done to raise move an object to a new position.

mghPE

Gravitational Potential Energy

• Any change in potential energy is due to a change in position of the object with respect to the reference level.

• Any change in PE energy is due to work being done on the system.

)( of hhmghmgPE

Gravitational Potential Energy

• In addition to gravitational potential energy, a body can also have elastic potential energy and electrical potential energy.

• A compressed spring has potential due to the work done in compressing it.

221 kxPE

Kinetic Energy

• Kinetic Energy (KE)is the amount of energy that an object possesses due to its motion.

• The mass (m) of the object and its velocity (v) both factor into its kinetic energy.

221 mvKE

Kinetic Energy

• The amount of kinetic energy that an object has is equal to the work done getting the object into motion.

2)( :

2 ,2:since and

)(

2

22

mvaxmKEso

vaxaxv

xmaFxWKE

Kinetic Energy

• Any change in velocity will cause a change in the kinetic energy.

• Any change in energy is due to work being done on the system.

of KEKEKE

Conservation of Energy

• In any closed system (isolated), the total amount of energy remains constant.

• There may be a transfer of energy from one form to another, but the total remains constant.

• A ball that is thrown up in the air loses KE on the way up to its maximum height, but it gains PE as it rises.

Conservation of Energy

• Eventually, when the ball stops, all of its initial KE will be transformed into PE.

• At any point in between, there will be a combination of PE and KE, always adding up to the same total amount of energy.

• The same transformation occurs for an object that is falling. KEPE

Conservation of Energy

• Another example of transformation of energy occurs with a pendulum swinging or a person on a swing.

topbottom EE topbottom PEKE

mghmv 221

ghv 2