Today we will…. Define work and power Explain how work relates to force Explain how power and work are related

Chapter 8 Energy

What is work? Force × distance = work We do work when we lift against Earth’s gravity.

Two factors that determine the amount of work done: application of a force the movement of something by that force --simple case when the force is constant and the motion takes place in a straight line in the direction of the force.

Work = force × distance Quantity Abbreviation Units Force F N, Newton Distance d m, meters Work W J, Joule (N*m)

Example of Work walking up a flight of stairs; the first trip you carry a load of groceries; the second trip you decide to carry two loads at the same time Compare the work done?

Example of Work When is the work done? A weight lifter holds a barbell over his head…no work is being done. When is the work done? When he lifts the barbell from the floor

Work falls into two categories work done against another force Example: an archer stretches bowstring; work against the elastic forces of the bowstring; push-ups – you do work against your own weight; pushing on a table (friction)

2 cat of work con’t 2. work done to change the speed of an object; This kind of work is done in bringing an automobile up to speed or in slowing it down

8.2 Power If you carry the same load up the stairs when just walking and then again when running up the stairs. How does the amount of work compare? you have done the same amount of work.

Power – is how fast work is done (the rate at which work is done) 𝑃𝑜𝑤𝑒𝑟= 𝑤𝑜𝑟𝑘 𝑡𝑖𝑚𝑒 Unit for power is watt (W)

Concept Check Work is done lifting a barbell. How much more work is done lifting a twice-as-heavy barbell the same distance? Twice as much How much more work is done lifting a twice-as heavy barbell twice as far? Four times as much

Concept Check If a forklift is replaced with a new forklift that has twice the power, how much greater a load can it lift in the same amount of time? If it lifts the same load, how much faster can it operate? forklift that delivers twice the power will lift twice the load in the same time or the same load in half the time

Today we will: Define mechanical energy, potential and kinetic energy Determine how to find both potential and kinetic energy Explain the variables that are involved in PE and KE

8.3 Mechanical Energy something has been acquired that enables the object to do work Example: bowstring, spring

This something may be in the form of: A compression of atoms in the material of an object A physical separation of attracting bodies A rearrangement of electric charges in the molecules of a substance This “something” that enables an object to do work is energy Note: available energy enables an object to do work not all energy can be transformed into work

Mechanical Energy the energy due to the position of something, or the movement of something can be in two forms either potential or kinetic or the sum of the two Energy is also measured in Joules (just like work)

8.4 Potential Energy (PE)– the energy that is “stored” and held in readiness energy in the “stored” state has the potential for doing work Examples: compressed spring, stretched rubber band, roller coaster on top of a hill

PE con’t Chemical energy in fuels is also PE Any substance that can do work through a chemical action possesses potential energy. Examples: PE is found in fossil fuels, electric batteries the food we eat.

PE con’t Work is required to elevate objects against Earth’s gravity. The potential energy due to elevated positions is called gravitational potential energy.

GPE Gravitational potential energy = weight x height PE = mgh Note: W = Fd Work done is: mgh Therefore, work = potential energy

Concept check: How much work is done on a 100N boulder that you carry horizontally across a 10m room?

How much work is done on a 100N boulder when you lift it 1m? F=100N d=1m W= Fd = 100N * 1m = 100J

What power is expended if you lift the boulder a distance of 1m in a time of 1s? Since the boulder has the same force (100N) and the same distance (1m); we have already found the work in the last problem W=Fd so work = 100J time = 1s Power = work 100J time 1s Power = 100W

8.5 Kinetic Energy Kinetic Energy – energy of motion Depends on the mass and speed of an object If an object is moving it is capable of doing work KE = ½ mass X velocity2 KE = ½ mv2

When work is done…energy changes Work-Energy Theorem: Work = Δ energy