Transcript Lecture – Work & Energy

Work & Energy
Work
 Work
is the result of applying a force
over a distance.
 When you lift a book 1 m into the air
from the table, you have done work.
 The amount of work depends on the
distance traveled and the magnitude
of the force
 You
do more work lifting 5 books than just
1 book
Energy
 Energy
is the ability to do work.
 You eat a bowl of Wheaties to get
energy to accomplish work.
 The energy we will focus on will have
two variations – Potential and Kinetic
Potential Energy (P.E.)
A
student with a high IQ has the
potential to do well in school – does he
always do so? No! He has the potential
 Potential energy is similar – it is stored
and has the potential to make
something happen
 A rollercoaster at the top of the ride has
the potential to roll down
 A cup of gas has the potential to burn
Potential = stored, in place
 Potential
= Stored, in place
 Chemical, Gravitational, and Elastic
are three common variations of potential
energy
Calculation P.E.

The potential of any object due to gravity will
depend on the mass of the object and the
height of the object



A heavy book has the potential to hurt more when
it falls on your hand
A book high up has the potential to hurt more than
a book just above your hand
PE = mgh


M = mass (kilograms), g = gravity(m/s2), h =
height (meters)
PE is measured in Joules (J)
Lets Practice
 What
is the potential energy of a 700 kg
car at the top of a 20 m hill?
 PE
= mgh
 PE = (700 kg)(9.8m/s2)(20m)
 PE = 137,200 J
 What
is the P.E. of a 2 kg book 1.5 m
above the floor?
Kinetic Energy

Just as kinetic friction dealt with friction in
motion, kinetic energy deals with objects in
motion
 Just like P.E., K.E. will depend on the mass –
more mass means more K.E.
 The second factor in K.E. is velocity
 The more velocity, the more energy


K.E. = ½mv2
m = mass (kilograms), v = velocity (m/s)
Lets Practice
 What
is the K.E. of a 700 kg car moving
4 m/s?
= ½ mv2
 KE = .5 (700 kg) (4 m/s)2
 KE = 5600 J
 KE
 What
is the K.E. of a 300 g baseball
thrown at 30 m/s?
Identify the type of energy
(PE or KE)
A
falling skydiver
 A stretched rubberband
 A flying rubberband
 A book on the top shelf
 A person walking
 A waterfall
Which has more energy?
A
person running or walking?
 A child and an adult riding bikes at the
same speed?
 A football player on a diving board or a
gymnast on the same diving board?
 A baseball or a shot put?