Work and Energy Learning Guide
1. When does a force do work on an object? when the force causes a displacement
2. Give an example of a situation where a force is applied but does no work. Pushing on a solid object that
doesn’t move
3. If the force and the displacement are at an angle to each other, how is the work calculated? W =
Fdcos
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If the force is perpendicular to the displacement, is the force doing work? No
What is the unit of work in the SI system? joule (J)
What is this unit equal to in terms of fundamental metric units? kgm2/s2
Is work a vector or scalar quantity? scalar
Can work be negative? yes, when the force is in the opposite direction of the displacement.
What is energy? The ability to do work
What is the unit of energy? joule
Is energy a vector or scalar quantity? scalar
Kinetic energy is the energy due to an object’s ____________. motion
What is the formula for kinetic energy? K = ½ mv2
Can kinetic energy be negative? No
The net work done on an object by the net force on the object equals what? The change in the kinetic
energy
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Can the change in kinetic energy be negative? yes, if the speed is decreased
Gravitational potential energy is the energy due to an object’s _____________. position
What is the formula for gravitational potential energy? U or PE = mgh or mg y
Can gravitational potential energy be negative? Yes, it depends on where the zero point is set.
What is the equation for elastic potential energy of a spring? U or PE = ½ kx2
What does the k stand for in the equation? The spring constant (how stiff the spring is.)
What is meant by saying a quantity is conserved? It’s value doesn’t change
What is mechanical energy? The sum of the potential and kinetic energy.
What is meant by conservation of mechanical energy? In a closed system with no friction, the sum of all the
potential and kinetic energy of the system remains constant. Energy can change forms but is never created or destroyed in any
process.
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Is mechanical energy conserved if friction is present? No, mechanical energy is lost as heat.
What becomes of the energy lost due to friction? It is dissipated into the environment as heat
What is the rate of energy transfer? Power
What is the SI unit of power? watt (W)
What is this unit in terms of fundamental metric units? kgm2/s3
What is the equation for average power? P = W/t