Work & Energy Explaining the Causes of Motion in a Different Way
Dec 30, 2015
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