Work, Energy, Power - Physics

When is work done on an object?

When a force causes a displacement of an object

Definition of work done on an object

the amount of energy transferred to an object through work (when you are exerting a force on an object that causes its position to change in the same direction as that of the force, you are doing work on this object)

Why doesn’t a student holding a chair do work?

The force does not move the object

Under what conditions does a force acting on an object which undergoes a displacement do work?

A force does work when it has a component parallel (or antiparallel) to displacement

What is the equation for the work done by a force applied at an angle to displacement?

W = |F| |d| cos (theta)

What is the SI unit for work and energy and what units is it a product of?

Joule (J) ; 1J = 1N x 1m

Why is the sign of work important?

If the net work is positive, the object speeds up and if the net work done is negative, the object slows down

When is work negative?

For angles greater than 90 _degrees and less than 270 _degrees

What two quantities affect an object’s KE?

Mass and speed

Work-Energy Theorem

The net work done to an object equals its change in kinetic energy

How does the work energy theorem allow us to think about KE?

It allows us to think about KE as the work done an object can do as it changes speed

Potential energy is associated with what aspect of an object?

its position

Give the equation for gravitational PE

Gravitational PE = mgh

What does the spring constant tell you about a spring?

the stiffness of a spring

What two quantities does elastic PE depend on?

the spring constant and the distance compressed or stretched

Give the equation for elastic PE

Elastic PE = (1/2)kx²

What is ME?

ME is the sum of all forms of KE and PE

Explain what is meant by conservation of ME

It means that the amount of ME is constant

Explain why total energy is conserved even if ME is not

It is because ME can be converted to (or from) non mechanical forms

What is power?

the rate at which work is done or energy is transformed

Give two equations for power

P = W/t

P = Fv

Law of conservation of energy

energy cannot be created or destroyed, only transformed

what does ME include

KE and PE

What do non conservative forces include

friction, air resistance, forces from people and animals, forces from engines and machines

what type of force is friction

a non conservative force

What do conservative forces include

the force of gravity, the force of static electricity, and elastic forces (the force of an ideal spring)

Non conservative forces can increase…

ME

what is the angle between the force of gravity and the displacement of a freely falling object

180 degrees

How does the impact of positive work done to an object differ from the impact of negative work done to an object

Positive work increases KE while negative work decreases KE

Why isn’t work from gravity included in the equation for the law of conservation of energy

It is already accounted for by the initial and final potential energy

Energy

the ability to do work

How can an object with KE do work

An object in motion can collide with another object and exert a force on it as the object is moved over a distance

What does the statement that ME is conserved mean

It means that the sum of all potential and kinetic energies before and after a process is that same

Under what conditions is ME conserved

when no work is done by non conservative forces

Is energy still conserved when ME is not conserved?

Yes, because the net amount of ME gained or lost is exactly equal to the amount of other forms of energy that are lost or gained

What is meant by dynamic equilibrium

It occurs when an object is in constant velocity motion under the influence of balanced forces

What is the rate at which work is done or energy is transformed called

power

what kind of energy is energy of position

potential energy

what two quantities does KE depend on

mass and the magnitude of velocity (speed)

According to the Work-Energy Theorem, what’s is equal to the change in an object’s KE

the total work done to an object

what two things must happen for work to be done to an object

the object must move and a force with a component in the direction of motion must be applied to the object

under what conditions does a force do negative work

when the force has a component opposite to the direction of motion

what are two reasons why a force might not do any work

either the force is perpendicular to displacement or the object does not move

what is the work done by gravity to move an object to zero height called

Gravitational PE

what does the spring constant of a spring tell you about the spring

The spring constant tells you how stiff the spring is with stiffer springs having larger spring constants