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# Impulse-Momentum

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```									Impulse-Momentum

January 6, 2009
Newton’s 3rd law
• Equal and opposite forces – what does this
mean?
– 4 conceptual questions
3rd law demos
• Tug of war
• Tug of war on roller skates
Momentum
• Momentum: mass in motion

• p = mv
• Momentum: mass in motion

• p = mv

• Change momentum by exerting a force over a
period of time (impulse)
Impulse-Momentum
• Impulse = Ft
• Impulse = m∆v
Examples of Impulse-Momentum
• Determine the direction of the impulse
– Cart on a track
– Projectile motion
– Circular Motion
• Determine the direction of momentum change
– Cart on a track
– Projectile motion
– Circular Motion
Impulse-Momentum
• Impulse and momentum change are the same
thing!!!
• Impulse is always in the same direction as the
force (so is momentum change)
– Impulse is a vector (it has direction)
• If I drop two identical balls from the same
height, which will exert the larger force on the
ground: the one that bounces or the one that
does not bounce? Why?
• Impulse = F*t

• Impulse = m∆v
Rank the impulse from most positive to most negative (negative
numbers are smaller than positive)

10 m/s    20 m/s       20 m/s    20 m/s        30 m/s      20 m/s

10 m/s    0 m/s        10 m/s    -10 m/s       -10 m/s   -20 m/s
Rank the change in momentum from most positive to most negative
(negative numbers are smaller than positive)

10 m/s         20 m/s       20 m/s     20 m/s         30 m/s      20 m/s

10 m/s        0 m/s         10 m/s     -10 m/s        -10 m/s   -20 m/s
Rank the following situations in order
of greatest to least momentum change
V = 2m/s          V = 4m/s
C
A

V = -2m/s          V = -4m/s

V = 2m/s           V = 4m/s

B
D

V =0               V=0
Rank the following situations in order of
greatest to least impulse
V = 2m/s         V = 4m/s
C
A

V = -2m/s       V = -4m/s

V = 2m/s        V = 4m/s

B
D

V =0            V=0
Impulse Investigation
• A cart holding a force sensor is pulled from rest
across a frictionless surface.
– If we want to determine the impulse the person will
experience, what measurements should we take?
What equation will you use?
Impulse Investigation
• A person holding a force sensor is pulled from
rest across a frictionless surface.
– If we want to determine the impulse the person will
experience, what measurements should we take?
What equation will you use?
– Is there more than one answer to the previous
question?
Force-time Graphs
• Jumping on a force plate
Two methods of determining impulse
• Force Sensor – area under curve
• Motion sensor – m∆v
Impulse from Multiple Forces
• Ex – book sitting on a table
Impulse from multiple forces
• A man weighing 800 N gets in an elevator. As
the elevator is accelerating upward, the scale
– Draw a FBD of the man
– Determine the net impulse he experiences during
the first 2 seconds of motion.
– How fast is the man moving after 2 seconds?
Conservation of momentum
• Equal and opposite impulses (just like equal
and opposite forces)

• Yesterday’s collision simulation
Calculating final velocity (inelastic
collisions)

mava + mbvb = (ma + mb) v’
Example
• A car with a mass of .5 kg and a velocity of .5
m/s collides with a car of mass 1 kg and
velocity -.25 m/s. Determine the final velocity
of the carts after they collide, assuming they
stick together after the collision.
Thursday, January 14
• Conservation of momentum principles
What do you think will happen to
the speed of the cart when a mass
is dropped on top of it?
Conservation of momentum
• http://www.physicsclassroom.com/mmedia/
momentum/cbb.cfm
• http://www.physicsclassroom.com/mmedia/
momentum/fcb.cfm
• http://www.physicsclassroom.com/mmedia/
momentum/creti.cfm
“reverse” collisions
• When you shoot a gun, why does it “recoil”?
Explain using the law of conservation of
momentum.
Hint:
• When objects START together and separate
(shooting a gun, throwing a baseball, pushing
off from someone on ice skates) simply flip
the conservation of momentum equation

(ma + mb) v = mava + mbvb
Practice
• A 15-kg medicine ball is thrown at a velocity of
20 m/s to a 60-kg person who is at rest on ice.
The person catches the ball and subsequently
slides with the ball across the ice. Determine
the velocity of the person and the ball after
the collision.
• Suppose that you have joined NASA and are
enjoying your first space walk. You are outside
the space shuttle when your fellow astronaut
of approximately equal mass is moving
towards you at 2 m/s (with respect to the
shuttle). If she collides with you and holds
onto you, then how fast (with respect to the
shuttle do you both move after the collision?
• http://www.physicsclassroom.com/mmedia/
momentum/ast.cfm
Tomorrow’s lab
• Predict what velocity-time graphs will look like
for:
– Two cars pushed toward each other, sticking
together after the collision
– Two carts pushed toward each other, bouncing off
of each other after the collision
– One cart initially at rest, while the other is pushed
toward it. The carts stick together.
– One cart initially at rest, while the other is pushed
toward it. The carts bounce off of one another.
Tuesday, January 19, 2010
• TEST tomorrow!
• Go over last week’s homework
• On the test:
– Impulse (m∆v, Ft, change in momentum)
– Direction of impulse = direction of force
– Force-time graphs
– Conservation of momentum
Describing the motion….
“reverse” collisions (explosions)
• You are standing at rest on a frictionless sheet
of ice. You throw a .25-kg baseball forward at
a speed of 18 m/s. If you have a mass of 55
kg, at what speed (and in what direction) are
you moving after you throw the baseball?

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