Warm-Up: December 4, 2015
Consider a baseball bat hitting a baseball. Assume the
baseball was thrown from the pitcher’s mound and the
baseball bat is at home plate.
1. How does the force of the bat on the ball compare to
the force of the ball on the bat?
2. How do the directions of motion of the ball and bat
compare before the collision?
3. How do the directions of motion of the ball and bat
compare after the collision?
4. Which object has the larger acceleration during the hit?
Why?

Forces Project



Due Monday
If you designed your own project, resubmit your rubric
with your project.
Digital files may be emailed to mszwast@sandi.net if
small, or brought to school on a USB drive and
transferred during lunch (room 454) or after school
(room 410)
Forces Unit Test Answers
#
1
2
3
4
5
6
7
8
9
10
11
Ver.
1
B
Ver.
2
A
B
D
D
C
B*
D
A
E
C
B
C
C
B
C
C
C
B
B
D
D
#
12
13
14
15
16
17
18
19
20
21
22
Ver.
1
B
Ver.
2
C
A
C
C*
B
C
B
B
B
C*
D
D
C
C
B
B
A
B
B
D
C*
#
23
24
25
26
27
28
29
30
31
32
Ver.
1
E
Ver.
2
D
A
E
D
E
E
A
B
C*
C
D
D
A
E
D
A
B*
C
D
Forces Test Questions?
Momentum
Chapter 9
Momentum




p  mv
The momentum of an object is equal to its mass times
its velocity
Is represented by a lower case “p”
Is a vector (has magnitude and direction)
Has units of kg· m/s

Equivalent to N· s
Example 1

Calculate the momentum of a 103 kg linebacker running
towards the quarterback at a speed of 7.5 m/s
You-Try #1

Calculate the momentum of a 141 kg lineman “running”
at a speed of 2.1 m/s
Warm-Up: December 7, 2015
Rank the following from highest magnitude of momentum
to lowest magnitude of momentum.
A. A mosquito flying at its top speed.
B. A car parked in a parking lot.
C. A large boulder falling at its terminal velocity.
D. Mr. Szwast running a marathon.

Impulse



J  F  t
The impulse acting on an object is equal to the force
acting on it multiplied by the time interval over which it
acts
Is represented by a capital “J”
Has same units as momentum, kg· m/s = N· s
Example 2

A 25 N force acts on a wooden box for 1.2 seconds.
Calculate the impulse.
You-Try #2

A man pushes a heavy sofa for 3.7 seconds with an
average force of 275 N. Calculate the impulse.
Impulse-Momentum Theorem

The net impulse acting on an object is equal to the
object’s change in momentum
J net  p
Fnet t  mv f  mvi
Example 3

A baseball has a mass of 0.145 kg. A pitcher throws it
with a velocity of -41.3 m/s. The bat hits the ball with a
force of 130 N for 0.100 seconds. What is the ball’s
velocity after it leaves the bat?
You-Try #3

A hockey puck has a mass of 0.115 kg. A player hits it
towards the goal with a velocity of -28.7 m/s. The puck
misses the goal, and instead hits the goalpost with a force
of 73.6 N for 0.070 seconds. What is the puck’s rebound
velocity?
Momentum Worksheet




On website
You have the rest of class to work with your partner on
the worksheet.
For #10, be careful with directions and positives and
negatives.
If you have your project on a USB drive, bring it to Mr.
Szwast. (one at a time)
Warm-Up: December 8, 2015

A rocket sled is gliding across horizontal frictionless ice.
It is moving at 120 m/s before it turns on a 4.0 second
rocket boost that provides an average thrust of 5500 N.
After the boost, the rocket is traveling at 210 m/s. What
is the mass of the rocket sled?
When Balls Collide…



Ball A and Ball B are rolling towards each other and
collide. What happens?
It depends on the mass and velocity of each ball.
It also depends on the conditions of the system
Types of Collisions



Elastic collisions conserve kinetic energy (we’ll talk
about this next week).
Inelastic collisions do not conserve kinetic energy.
Perfectly inelastic collisions are where the colliding
objects stick together and have the same final velocity.


A bullet shot into a wooden block
Two balls of clay that stick together
System Definitions



System: The set of objects which are being analyzed.
Closed system: A system in which no mass enters or
leaves the system.
Isolated system: A system with no net external forces
(or small enough to be negligible).
Conservation of Momentum


The momentum of any closed, isolated system does not
change.
For two objects in one dimension:
m1v1  m2v2 initial  m1v1  m2v2 final
Example 4

A black ball with mass 1.4 kg and velocity 1.7 m/s is
rolling towards a stationary gold ball with mass 0.40 kg.
After the collision, the gold ball’s velocity is 1.5 m/s. What
is the final velocity of the black ball?
You-Try #4

A white ball with mass 1.0 kg and velocity 1.0 m/s is
rolling towards a blue ball with mass 0.75 kg and
velocity -1.5 m/s. After the collision, the white ball’s
velocity is -0.15 m/s. What is the final velocity of the
blue ball?
Example 5

A bullet with mass 0.025 kg and velocity 550 m/s is shot
into a 20.0 kg stationary wooden block that is on
frictionless ice. The bullet gets lodged in the block and
they travel together. What is the speed of the block and
bullet?
You-Try #5

Two cars are driving towards each other. One has a mass
of 1200 kg and a velocity of 24 m/s. The other has a mass
of 1350 kg and a velocity of -19 m/s. The collision causes
the two cars to be stuck together. What is the velocity of
the two cars after the crash?
Example 6

Two clay balls are on a collision course. One
ball, with mass 1.5 kg, is traveling north at 4.8
m/s. The other, with mass 2.3 kg, is traveling east
at 3.2 m/s. The two balls of clay stick together
when they collide. What is the velocity of the
clay after the collision?
Example 6
Assignments

Classwork: Conservation of Momentum Worksheet



On website
Work with your partner
Homework :



Read Chapter 9
Page 235 #7-9
Page 250 #32-34, 38, 40, 57, 73, 80
Warm-Up: December 9/10, 2015

A 1.7 kg rock is thrown into a stationary ball of clay at a
speed of 3.2 m/s. The rock gets stuck in the clay and they
move together at a speed of 2.3 m/s. What is the mass of
the clay?
Momentum and Impulse Worksheet
1) 3.8 10
4 kg m
s
2) 260 kg
3) 7.4
4)
5)
6)
7)
8)
m
s
220 N  s
23 N
0.21 s
2s
70. N
9) 6.0
m
s
10) 1.1103 kg
Collisions in Two Dimensions



Momentum in conserved for each dimension
Use conservation of momentum equation for 𝑥 direction,
and again for 𝑦 direction.
Add the resulting vectors (using trig) to find the final
velocity (magnitude and direction).
Example 6

Two clay balls are on a collision course. One
ball, with mass 1.5 kg, is traveling north at 4.8
m/s. The other, with mass 2.3 kg, is traveling east
at 3.2 m/s. The two balls of clay stick together
when they collide. What is the velocity of the
clay after the collision?
Example 6
You-Try #6

A 35.0 gram bullet is shot north at 475 m/s at a
moving target. The target is a 25.0 kg block of
wood that is sliding to the east at 4.2 m/s along
frictionless ice. The bullet gets stuck in the
block of wood. What is the final velocity of the
block of wood?
You-Try #6
Assignments

Classwork: Conservation of Momentum Worksheet



On website
Work with your partner
Homework :



Read Chapter 9
Page 235 #7-9
Page 250 #32-34, 38, 40, 57, 73, 80