#### Transcript Document

```Chapter 6 and 7
momentum = mass x velocity
p = mv
units: kgm/s or Ns
What is the value of the momentum
of a 10 kg ball rolling down a
bowling alley at a speed of 5 m/s?
p = mv
p = (10 kg)(5 m/s)
p = 50 kgm/s
Momentum is a
vector, it has a
direction
The ball bounces off the wall. What is
the change in momentum?
Dp = pfinal - pinitial = 2p
Newton’s 2nd Law:
F = ma = mDv/Dt
p
Impulse-Momentum Theorem
FDt = p
If this girl throws a 0.2 kg
snowball at 20 m/s…at you, and
it impacts your skull for 0.05 s,
what is the force
of the impact?
FDt = mDv
F = mDv/Dt
F = (0.2 kg)(20 m/s)/0.05 s
F = 80 N
Stopping Distance
A 2500 kg car brakes to slow from
25 m/s to 10 m/s in 6 s. What was the
force of braking?
F = mDv/Dt = (2500 kg)(10 m/s – 25m/s)/6s
F = -6250 N
How far will it go in that time?
Dx = ½(vi + vf)Dt = ½(25m/s + 10m/s) 6s
Dx = 105 m
Conservation of Momentum
The total momentum before equals the
total momentum after, if there are no
external forces.
m1v1i + m2v2i = m1v1f + m2v2f
80 kg Schoettle steps out of
his 100 kg boat with a
velocity of 2 m/s. What is
the boat’s velocity?
0
0
m1v1i + m2v2i = m1v1f + m2v2f
v2f = -m1v1f/m2
v2f = -(80 kg)(2 m/s)/100 kg
v2f = 1.6 m/s
Types of collisions (two things hitting each other)
Perfectly Inelastic: the two things stick together
m1v1i + m2v2i = (m1 + m2)vf
10 kg
v1i = 3 m/s
5 kg
What is the velocity after they stick?
m1v1i + m2v2i = (m1 + m2)vf
m1v1i = (m1 + m2)vf
vf = 2 m/s
Centripetal Acceleration
An object moving in a
circle is accelerated
toward the center
Forces that maintain circular motion
Something moving in a
circular path has a tendency
to want to continue in a
tangent path. A force must
hold it in its circular path.
This is called Centripetal Force. What provides the
centripetal force in the picture?
What keeps a car on a circular path moving in a
circle?
Universal Gravitation
All matter creates a force of gravity with
other matter
Mass of two objects
m1m 2
Fg  G 2
r
Force of Gravity
G  6.673x10
11
Nm2
kg 2
Distance between two objects
m1m 2
Fg  G 2
r
If I double m1, what happens to the force of gravity?
If I triple m2, what happens to the force of gravity?
If I double the distance between the masses, what
happens to the force of gravity?
If I halve the distance between the masses, what
happens to the force of gravity?
If I triple the distance, what happens to the force?
m1m 2
Fg  G 2
r
Fg = mg
m1m 2
mg  G 2
r
mE
g G
rE 2
Rotation: the spinning of an object
Revolution: the orbital motion
```