Work and Energy Relation - General Physics I - Lecture Slides, Slides of Physics

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Chapter 5

Work and Energy

Units of Chapter 5

Work Done by a Constant ForceWork Done by a Variable ForceThe Work–Energy Theorem: Kinetic EnergyPotential EnergyConservation of EnergyPower

5.1 Work Done by a Constant Force

In (a), there is a force but no displacement: nowork is done. In (b), the force is parallel to thedisplacement, and in (c) the force is at an angleto the displacement.

5.1 Work Done by a Constant Force If the force is at an angle to the displacement,as in (c), a more general form for the workmust be used: Unit of work: newton • meter (N • m) 1 N • m is called 1 joule.

5.1 Work Done by a Constant Force^ If there is more than one force acting on anobject, it is useful to define the net work:^ The total, or net, work is defined as the work doneby all the forces acting on the object, or the scalarsum of all those quantities of work.

5.2 Work Done by a Variable Force

The force exerted by aspring varies linearlywith the displacement:

5.3 The Work–Energy Theorem:

Kinetic Energy

The net force acting on an object causes theobject to accelerate, changing its velocity:

5.3 The Work–Energy Theorem:

Kinetic Energy

We can use this relation to calculate thework done:

5.3 The Work–Energy Theorem:

Kinetic Energy

This relationship is called the work–energy theorem.

5.4 Potential Energy

Potential energy may be thought of as storedwork, such as in a compressed spring or anobject at some height above the ground. Work done also changes the potential energy(

U

) of an object.

5.4 Potential Energy

Gravitationalpotential energy:

5.4 Potential Energy

Only changes in potential energy arephysically significant; therefore, the pointwhere

U

= 0 may be chosen for convenience.

5.5 Conservation of Energy

So, what types of forces are conservative?Gravity is one; the work done by gravitydepends only on the difference between theinitial and final height, and not on the pathbetween them. Similarly, a nonconservative force: A force is said to be nonconservative if the workdone by it in moving an object does depend on theobject’s path. The quintessential nonconservative force isfriction.

5.5 Conservation of Energy

Another way of describing a conservativeforce: A force is conservative if the work done by it inmoving an object through a round trip is zero. We define the total mechanical energy: