Work and Energy

Work and Energy

By Mr Leavings

Chapter 5

What is WorkIn science Work has a specific meaning. If you push a box with a force of one newton for a distance of one meter, you have done exactly one joule of work.

Units of Work: joules. One joule = One newton-meter

What is Work

W = F x d

What is Work

What is WorkWork is force times distance moved in the direction of the force

Force A

Force B Force C

Force A: Does 1 joule

Force B: Does < 1 joule

Force C: No Work 1 Meter

WorkWhen we apply Force in machines we are doing Work. For example , when a Block and Tackle machine lifts weight, force is applied to do this.

As a result of the force, the weight moves a distance.

Work done by MachinesFor a machine to do work it must have an input force.

The work output of a simple machine CAN NEVER exceed the work input.

Work done by MachinesNature does not give something for nothing.The work input of the block and tackle is the same as the work output. You get Mechanical Advantage by trading force for distance

Work and EfficiencyIn a very Efficient machine, all (or most) of the input work becomes output work. In the block and tackle example before the 10 joules of input work was transformed to 10 joules of output work. This machine was 100% efficient.

Work and EfficiencyIs 100% efficiency possible?

In a very efficient machine, all (or most) of the work input becomes output.

In REAL machines, work output is always less than work input. Other forces such as friction use up some of the input work.

Efficiency =______________Work OutputWork Input

Work and Efficiency

Input work = Output workThen it is 100% efficient

4 Joules 3 Joules

1 Joule 75 % Efficiency

Input Work does not equal output work

4 Joules 4 Joules

Work and Efficiency65% of the energy in gasoline is converted to heat. As far as moving the car goes, this heat energy is “lost”. The energy doesn’t vanish, it just does not appear as useful output work.

Work and EfficiencyThe most efficient machine designed?

The bicycle is the most efficient machine ever invented for turning the work of humans muscles into motion. Its efficiency is more than 95%!

Work and PowerWhat is power?

You and a friend are pulling boxes across a floor. You drag one box with a force of 100 Newtons for 10 meters, and it takes you 10 seconds. Your friend then drags a similar box for the same distance but it takes him 60 seconds. Have you done the same amount of work?

Work and Power

Power: The RATE at which work is done.

Energy The different Kinds of Energy:

6 kinds that we will talk about

1. Kinetic Energy (more on that Later)2. Potential Energy (more on that Later)3. Radiant Energy4. Electrical Energy5. Chemical Energy6. Nuclear Energy

Energy

Mechanical energy is the energy possessed by an object due to its motion or its position. Potential energy and kinetic energy are both forms of mechanical energy.

Energy

Radiant energy is energy that is carried by electromagnetic waves. Light is one form of radiant energy.

Energy

Electrical energy comes from electric charge, which is one of the fundamental properties of all matter.

Energy

Chemical energy is a form of energy stored in molecules. Batteries are storage devices for chemical energy.

Energy

Nuclear energy is a form of energy stored in the nuclei of atoms.In the Sun, nuclear energy is transformed to heat that eventually escapes the sun as radiant energy.

Potential EnergyPotential Energy comes from the position of an object relative to earth.

Consider a marble that is lifted off the table. Since the Earth’s gravity pulls the marble down we must apply a force to lift it up. Remember the concept of WORK, we are applying a force over a distance, which is stored as the potential energy of the marble.

Potential EnergyFrom Newton’s second law we know that the weight (Force due to weight) is equal to the mass of the marble (m, in kilograms) times the acceleration of gravity (g, equal to 9.8 m/sec2). We also know that the work is equal to force times distance.

Since the force is the weight of the marble (mg) and the distance is how far up we lift the marble (h), the work done equals weight times height.

EP = mgh

mass of object (kg)

height object raised (m)

gravity (9.8 m/sec2)

PE (joules)

Potential Energy

Potential Energy

Systems or objects with potential energy are able to exert forces (exchange energy) as they change.Potential energy is energy due to position.

Potential Energy

Kinetic EnergyObjects also store energy in motion. A moving mass can exert forces, as you would quickly observe if someone ran into you in the hall.

Kinetic energy increases as the square of the speed. This means the faster you are going the harder it is to stop!

EK = ½ mv2

Kinetic Energy

KE (joules) mass of object (kg)

velocity (m/sec)Going 60 mph a car has four times as much kinetic energy as it does at 30mph! At 90 mph you have 9 times the energy!

Solving Problems

A 2 kg rock is at the edge of a cliff 20 meters above a lake.

It becomes loose and falls toward the water below.

Calculate its potential and kinetic energy when it is at the top and when it is halfway down.

Its speed is 14 m/s at the halfway point.

Solving Problems

1. Looking for: …initial EK, EP and EK, EP half way down.

2. Given: mass = 2.0 kg; h = 20 m v = 14 m/s (half way)

3. Relationships: EP =mgh EK = ½ mv2

Assume rock starts from rest.

Conservation of Energy

Law of Conservation of Energy: Nature NEVER creates or destroys energy; energy only gets converted from one form to another.