Chapter 4 part II
Conservation of momentum
• Suppose two objects collide:
p-1f
p-1
1
2
p-2
forces the same but
opposite
1
2
p-2f
A compact car and a Mack truck have a head-on collision. Are the
following true or false?
•
The force of the car on the truck is equal and
opposite to the force of the truck on the car. T
•
The momentum transferred from the truck to
the car is equal and opposite to the momentum
transferred from the car to the truck. T
•
The change of velocity of the car is the same as
the change of velocity of the truck. F
A compact car and a Mack truck have a head-on collision. The force of
the car on the truck is equal and opposite to the force on the car.
1.True
2.False
A compact car and a Mack truck have a head-on collision. The
momentum transferred from the truck to the car is equal and opposite
the momentum transferred from the car to the truck.
1.True
2.False
A compact car and a Mack truck have a head-on collision. The change
of velocity of the car is the same as the change of velocity of the truck.
1.True
2.False
Angular Momentum
• Rotational equivalent of momentum
• Also conserved*
*for no external torque
r big, v small
r smaller, v bigger
Works for planets
big r, small v
small r, big v
* This agrees with Kepler’s Laws
Orbital Motion
• Moving in a circle - needs a force
• Gravity depends on distance
• Change in momentum depends on radius
and speed
20 mph
20 mph
30 mph
A car turns on a road while staying
at 30 mph. Is there a force acting
on the car?
1.Yes, force from the
engine
2.Yes, force from
friction
3.Yes, force of the
speed
4.Yes, centrifugal
5.No force needed
More Orbital
• If gravity is the force, (depends on r)
• force needed depends on r and v
• speed is set
The greater the orbital
distance the
1.Longer the period
2.Shorter the period
3.Period does not
depend on orbital
distance
Geosynchronous
• Orbital distance such that the period is 1
day
• Arthur C. Clarke
• Geostationary - above the equator
Energy
• What is it? (units of Joules)
• Two types:
• Particle
• mass - think nuclear
• motion (kinetic) - thermal energy
• Field - potential energy
• gravity
• electromagnetic
Gravitational Field Energy
at rest.
10 J of Grav
• Depends on:
• mass of BOTH objects
• Distance between
centers
moving
4 J of Grav
6 J of KE
Stopped
9 J of thermal
1 J of deformation
Suppose I drop a ball 3 meters.
After it hits the ground it will have
1.More kinetic energy
and less gravitational
energy
2.More gravitational
energy and less kinetic
energy
3.Less gravitational
energy, zero kinetic
energy and more
thermal energy
Thermal Energy
• Basically, more thermal energy is more
kinetic energy
Hot
Hotter
Temperature
• Not thermal energy
• Consider Pizza out of the oven
• Don’t touch the pizza! (hot)
• Touch the foil (hot)
http://www.instructables.com/files/deriv/FJA/GWQJ/FF78HWNQ/FJAGWQJFF78HWNQ.
MEDIUM.jpg
Temperature scales
• Absolute zero -
basically no motion
Fahrenheit Celsius
Kelvin
absolute
zero
-460
-273
0
water
freeze
32
0
273
water boils
212
100
373
Gravitational Energy
unbound
Distance from planet
rgy
e
n
E
l
a
tion
a
t
i
v
a
r
G
Energy
bound
Kinetic Energy
Escape velocity
•
•
•
The speed to “just” be unbound
3
Depends on the size of planet
depends on mass of planet
1
2