Work & Energy

Explaining the Causes of Motion in a Different Way

Which of the following is a measure of the ability to do work?

1. Power2. Energy3. Photosynthesis4. Joules

Power

Ener

gy

Photo

synth

esis

Joule

s

4%

24%

0%

72%

What the heck is energy anyway?

Energy- the ability to do work If an object or organism does work (exerts a force over a distance

to move an object) the object or organism uses energy.

Work = force x distance

ex. When stretching a sling shot, you do work, and energy is transferred to the sling shot

Measured in the unit joules (honors Mr. Joule) b/c work is a force x distance it is measured in Newtons x meters. These units are called joules. Work = force x distance

1 N x m = 1 J= 1 kg x m2/s2

Energy and Work are closely related

Work is a transfer of Energy VERY simple: Work has been done when one object

transfers energy to another. (actually more complicated than that, but its OK for now)

SI unit for work is Joules (J)

Energy is the capacity to do Work We recognize it by the changes it causes

We use energy to walk upstairs.

(work)

Calculating WORK

The change in Energy of an object SO WORK DONE = CHANGE IN KINETIC

ENERGYInvolves an object moving in the

direction of the FORCE applied

Work = Force x Distance

Try it…

Involves an object moving in the direction of the FORCE applied

A Force in physics- is a “push or a pull”

Ex- Let’s try it- 3 volunteers: 1- Hold 2 book out to your sides for 2 minutes 2- Carry 10 books around the room for 2 minutes 3-Constantly lift up a box of tissues for 2 minutes

Who did the most work?

WorkThe product of force and the amount

of displacement along the line of action of that force.

Units: ft . lbs (horsepower) Newton•meter (Joule)

ntdisplacemeForceWork

Work = F x d

To calculate work done on an object, we need:

The ForceThe average magnitude of the force The direction of the forceThe DisplacementThe magnitude of the change of positionThe direction of the change of position

Calculate WorkDuring the ascent phase of a rep of

the bench press, the lifter exerts an average vertical force of 1000 N against a barbell while the barbell moves 0.8 m upward

How much work did the lifter do to the barbell?

Calculate Work

Table of Variables:Force = +1000 NDisplacement = +0.8 m

Force is positive due to pushing upwardDisplacement is positive due to moving

upward

Calculate Work

Table of Variables:Force = +1000 NDisplacement = +0.8 mSelect the equation and solve:

JJouleNmWork

mNWork

ntdisplacemeForceWork

800800800

8.01000

Work performed climbing stairs

Work = Fd Force

Subject weightFrom mass, ie 65 kg

Displacement Height of each step

Typical 8 inches (20cm)

Work per step 650N x 0.2 m = 130 Nm (Joules)

Multiply by the number of steps

Work on a stair stepper

Work = FdForce

Push on the step????

Displacement Step Height

8 inches

“Work” per step???N x .2m = ???Nm (Joules)

Energy Energy (E) is defined as the capacity to do

work (scalar) Many forms

No more created, only convertedchemical, sound, heat, nuclear, mechanical

Mechanical Energy Kinetic Energy (KE):

energy due to motion Potential Energy (PE):

energy due to position

Kinetic EnergyEnergy due to motion reflects

the mass the velocity

of the object

KE = 1/2 mv2

Calculate Kinetic Energy

How much KE in a 5 ounce baseball (0.145 kg) thrown at 80 miles/hr (35.8 m/s)?

Calculate Kinetic Energy

Table of VariablesMass = 0.145 kgVelocity = 35.8 m/s

Calculate Kinetic EnergyTable of VariablesMass = 0.145 kgVelocity = 35.8 m/sSelect the equation and solve:KE = ½ m v2

KE = ½ (0.145 kg)(35.8 m/s)2

KE = ½ (0.145 kg)(1281.54 m/s2)KE = ½ (185.8 kg m/s2)KE = 92.9 kg m/s2, or 92.9 Nm, or 92.9J

Work - Energy Relationship

Work is the change in the mechanical energy of the object

Fd = KE + PE

Fd KineticEnergy

Fd KE

1. You are asked for kinetic energy and stopping distance

2. You are given mass, speed and force of brakes.3. Use Ek = 1/2mv2 and W= fd4. Solve for Ek = ½ (1,300 kg) ( 30 m/s)2 = 585,000

J To stop the car, work done by brakes = Ek of car, so W =

Ek Solve for distance = W ÷ f = 585,000J ÷ 9,500 N = 62 m

Calculating kinetic energy

A car with a mass of 1,300 kg is going straight ahead at a speed of 30 m/s (67 mph). The brakes can supply a force of 9,500 N.Calculate:

a) The kinetic energy of the car.b) The distance it takes to stop.

Work - Energy Relationship

If more work is done, greater energy greater average forcegreater displacement

Total Mechanical Energy

The mechanical energy of an object can be the result of its motion (i.e. kinetic energy) and/or the result of its stored energy of position ( i.e. potential energy).

The total amount of mechanical energy is merely the sum of the potential & kinetic energy (total mechanical energy

Mechanical energy

TME = PE + KE