Download momentum

Speed and Velocity
• Speed: a distance traveled over a period of time
• (ex. 5 m/s, 30 mph)
• Velocity is speed with direction
• (ex. 5 m/s to the left, 30 mph North)
• Directions could be right, left, up, down, East, West, North
or South
• It’s a vector (has quantity and direction)
• When the velocity is to the:
–
–
–
–
Right
Left
Up
Down
+
–
+
–
Which takes more force to stop?
• Big player @ 2m/s
Small player @ 2 m/s
• Big player @ 0.6 m/s
Small player @ 6 m/s
• Small player @ 2 m/s
Bullet @ 100 m/s
• Small player @ 100 m/s
Bullet @ 4 m/s
Which takes more force to stop?
• Big player @ 2m/s
Small player @ 2 m/s
Is there ever a time when it would take the
same amount of force to stop both players?
Momentum
• Defined as “a mass in motion” (amount of motion)
– Momentum equals mass times velocity
– A vector quantity (has direction)
• The more momentum an object has
– could mean it has a larger mass or velocity
– it is harder to change its motion
Momentum
• Momentum is different from inertia
– Inertia is the resistance to the change of motion
– Momentum is how much motion there is
• An object not moving has inertia but zero momentum
• An object moving has inertia and momentum
Calculating Momentum
Determine the momentum of a ...
• a. 100-kg halfback moving to the right at 4 m/s.
• b. 70-kg halfback moving to the left at 2 m/s.
Check Your Understanding
A car traveling down the road has momentum of
20,000 kg·m/s. What would be the car's new
momentum if ...
a.its velocity were doubled.
b.its mass were tripled (by adding more passengers
and a greater load)
c.The car turned around.
Collisions and Momentum
• Objects collide
• Objects push apart or explode
• In any system, momentum is conserved
• Total momentum is the same before and after a
collision
•
http://youtu.be/4IYDb6K5UF8
Solving Momentum Problems
• Draw pictures
– Before picture (before the collision)
– After picture
• Need to know the momentum of each object first
(p of all the objects)before = (p of all the objects)after
• A hunter spots his prey off to the right. He waits
patiently and does not move. When the hunter
fires, the bullet has a momentum of 284 kg·m/s.
The hunter feels a kickback after the bullet is fired.
Determine the momentum of the hunter after he
fires the bullet.
• A van traveling East with a momentum of
1500 kg·m/s collides with a car moving West
with a momentum of 800 kg·m/s. After they
collide, they stick together and the van has a
momentum of 500 kg·m/s. What is the
momentum of the car?
• Playing a game of pool, the cue ball is hit so it
moves with a momentum of -0.90 kg·m/s. It
collides with the 1-ball that was at rest. After
they hit, the 1-ball has a momentum of -0.75
kg·m/s. What is the momentum of the cue
ball?
• Two figure skaters are standing next to each
other on an ice rink. The skaters then push off
from one another. One skater has a
momentum of 120 kg·m/s to the right. What is
the momentum of the other skater?
• Playing a game of pool, the cue ball (m = 0.17 kg)
is hit with a speed of 3 m/s toward the 4-ball
(m = 0.16 kg) that was at rest. After the two balls
collide, the 4-ball has a speed of 2 m/s. What is
the speed of the cue ball after the collision?
• An 80 kg hunter spots his prey off to the right. He
waits patiently and does not move. When the
hunter fires, the 5 gram bullet has a velocity of +
120 m/s. The hunter feels a kickback after the bullet
is fired. Determine the speed of the of the hunter
after he fires the bullet.
• A 1000 kg van traveling East with a velocity of
of 1.5 m/s collides with a 600 kg car moving
West with a velocity of 1.5 m/s. After they
collide, they stick together. What is the
velocity of the van and the car?
• Two children on ice skates start at rest and push off
from each other. One has a mass of 30 kg and
moves back at 2 m/sec. The other child has a mass
of 15 kg, but what is their speed?
• Standing on an icy pond, you throw a 0.5 kg ball at
40 m/sec. You move back at 0.4 m/sec. What is your
mass?