Energy
Chapter 3
pp. 53-75
Mechanical Energy• Potential energy –
– energy stored in an object due to its position
• Kinetic energy– energy in motion
Potential Energy• Equation– PE = mgh
– m - mass
– g - acceleration. of
gravity
– h - height
Kinetic energy• Equation
– KE = ½ mv2
• m – mass
• v - velocity
Conservation of Energy-
TE=PE+KE
The total amount of energy
always stays the same it
just changes forms!
As they fall
potential
energy
decreases
and kinetic
energy
increases!
Momentum• the product of the mass of an
object and its velocity
Equationm = mv (momentum =
mass x velocity)
m is called mew. We use it so
there is not two m’s in the
equation
1.) Does a parked car have
momentum?
• No, there is
no velocity.
2.) What has greater momentum a
truck at 30 mph or a car’s at 30
mph?
• A truck because it
has more mass
Apply to egg drop!
• What is the potential energy of a 0.05
kg egg at a height of 4 meters?
No calculation required!
• What is the potential energy of a 0.05
kg egg at a height of 4 meters?
• Based on conservation of energy what
is the kinetic energy of the egg just
before it hits the ground (height=0)?
Tough algebra, you can do it!
• What is the potential energy of a 0.05
kg egg at a height of 4 meters?
• Based on conservation of energy what
is the kinetic energy of the egg just
before it hits the ground (height=0)?
• What is the speed of the egg just
before it hits the ground?
Finally momentum!
• What is the potential energy of a 0.05
kg egg at a height of 4 meters?
• Based on conservation of energy what
is the kinetic energy of the egg just
before it hits the ground (height=0)?
• What is the speed of the egg just
before it hits the ground?
• What is the momentum of the egg just
before it hits the ground?
Impulse• The product of force acting
on an object and the time during which
it acts.
Equations•
I = Ft =
mv
– (Impulse = Force x time)
– (Impulse = change in mass x velocity)
•
I=
µ
– Impulse = change in momentum
Impulse- (cont.)
• Impulse =
µ and is therefore constant
• The force is dependant on the time applied
• Short time means large force
–I=
Ft
• Long time means small force
–I=F
t
Review
(you should have all these equations)
•
Work• Equation– W = fd
– (Work = force x
distance)
– More weight or more
movement means
more work was done
1.) How much work did a mover do if
he lifted a 500 N box to a shelf that
was 4 m high?
• W = fd
= 500 N X 4 m
– = 2000 N-m
– = 2000 J (Joule)
–
• A joule is a measurement of energy
just like calories. (the amount of
energy in food)
Power- The time rate of work
• Equation– P = w/t
– Power = work ÷ time
Lets compare some athletes.
ALWAYS USE A SPOTTER WHEN LIFTING!
At nearly 7 feet 9 inches
tall, with size 19 feet,
Chinese basketball player
Sun Mingming dreams of
being in the NBA.
If 2 people do the same job (the same
amount of work), and one finishes faster,
the person who did the job faster has
more power.
8.) Mr. Foster and a small student
run a 5K. Mr. Foster slows up so
they cross the finish line in a tie.
Who has more power?
Mr. Foster, Of course!
9.) Why?
Mr. Foster has more junk in the
trunk and therefore did more work.
4.) Which of the
following car
crashes have a
greater force?
A.
B.
Answer: Collision A
5.) Why?
• The car was stopped faster in crash A
because a pole is less forgiving than
another car.
• The force in B was carried out over a
longer period of time.
Conservation of momentumNet momentum before collision = Net
momentum after collision
mV
Momentum before collision
Mv
=
Momentum after collision
The velocity changes because the mass changes making it hard to
see that momentum is conserved
Collisions• Elastic Collision– colliding objects rebound without lasting
deformation or generation of heat.
• Inelastic Collision
– deformation and generation of heat or
both during a collision
6.) Are these collisions elastic or
inelastic?
7.) Are these
collisions
elastic
or inelastic?
3.) What is the momentum of a 80 kg
jogger that is running 1 m/s?
• Answer:
m = mv
•
•
•
•
m = 80 kg
v = 1 m/s
80 x 1 = 80
Kg-m/s
4.) m = 80_________?