Bell Work: Types of Machines
Answer with: lever, inclined
plane, pulley, or wheel and axel
1.
2.
3.
4.
5.
A pencil sharpener is a _____ and _____.
A crow bar is a ____ and _____.
A can opener is a _____, _____ and _____.
An axe is a _____ and _____.
A block and tackle is a ______.
Impulse and Momentum
Momentum
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Inertia in motion
Combination of mass and velocity
Symbol for momentum is p
𝑝 = 𝑚 ∙v
Momentum units are 𝑘𝑔 ∙m/s
Big momentum created by big mass, big
velocity, or both.
Examples:
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cruise ship moving 2 m/s
pebble in orbit around the earth
cruise ship in orbit around the earth
NO VELOCITY = NO MOMENTUM
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Impulse
change in momentum because of a
change in speed (acceleration
occurs)
it takes force to change the speed
(2nd law)
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to change momentum, exert an impulse
Symbol for impulse is Δp
∆𝑝 = 𝐹 ∙ 𝑡
Impulse units are 𝑁 ∙ 𝑠
Momentum and impulse can be
combined: 𝑚 ∙ ∆𝑣 = 𝐹 ∙t
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Δv = change in velocity
To increase momentum: exert more
force or exert force for a longer
period of time
Ex: follow through in sports –
tennis, hockey, etc.
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To decrease momentum: force must
be in opposite direction of motion
Ex: friction, air resistance, tree
slowing down a car
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Changes in momentum caused by
cushy objects have less force and
more time than those caused by
hard items.
Double time,
cut force in half
(inversely related)
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Ex: Jump off a 3 meter platform onto
concrete – time to slow down your
motion is very small once you contact
the cement so the force is great.
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Jump off a 3 meter platform onto a
thick mat – time to slow down your
motion is extended because the matt
gives so the force is lessened.
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Ex: Car crumple zones increase the
time of impact so that less force is
exerted causing fewer injuries to
passengers.
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More Examples of increasing time to
decrease force:
bending legs when landing
 moving away from a punch
 stretchy cord when bungee jumping
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Practice Problem:
What is momentum of a 1500 kg car moving
at 30 m/s?
Given Unknown Equation
1500kg = m
30 m/s = v
p=?
p=mv
Substitution
p=1500x30
Solution
45,000 kg·m/s
What is momentum of the car at rest?
0 kg·m/s
If the car (going 30 m/s) slams on its brakes
and stops in 10 seconds, how much force was
exerted by the brakes?
Given Unknown Equation
10 s = t
45,000 = Δp
F=?
Δp=Ft
Substitution
45,000=F(10)
Solution
4,500 N
An air bag increases the time of impact to
0.9 seconds in a car crash. If the above car
crashed when moving 30 m/s, how much
force acts on the 75 kg driver with the air
bag?
Given Unknown
0.9 s = t
75 kg = m
30 m/s = v
F=?
Equation
Substitution
Solution
mΔv=Ft 75(30)=F(0.9) 2,500 N
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Conservation of Momentum
For a closed system (no external forces),
total momentum remains the same.
Momentum cannot be created or
destroyed, but can be transferred from
one object to another.
Firing a Cannon
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Ex:
Cannon with cannon ball ready to fire
has 0 kg·m/s of momentum.
Cannon firing  cannon exerts force on
cannon ball, cannon ball exerts same
force back on the cannon.
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These forces occur over the same time
period.
Cannon ball and cannon have equal and
opposite impulses.
Because the cannon ball has less mass,
its speed changes more than the
cannons.
The acceleration of the cannon is called
recoil – it moves backwards because the
cannon ball is shot out forward.
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Collisions
Total momentum before a collision equals the total
momentum after a collision.
Momentum transferred from one object to another.
𝑚𝑣𝑏𝑒𝑓𝑜𝑟𝑒 = 𝑚𝑣𝑎𝑓𝑡𝑒𝑟
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𝑚1 𝑣1 + 𝑚2 𝑣2 = 𝑚1 𝑣1 + 𝑚2 𝑣2
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Two Kinds of Collisions
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Elastic – the objects hit and bounce off one another
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The velocities change for each object
The mass will stay the same for each object
Momentum and kinetic energy are conserved.
Inelastic – the object collide and stick together
The mass becomes a combination of the two objects
 There is only one velocity for the two objects stuck
together
 The kinetic energy isn’t conserved.
 Momentum is conserved.
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Ex: Trains: 1st boxcar has a mass of 65000 kg.
The 2nd boxcar has a mass of 92000kg After the
collision, the boxcars move forward together.
Before
After
10 m/s
1.3m/s
0.8m/s
? m/s
𝑚𝑣𝑏𝑒𝑓𝑜𝑟𝑒 = 𝑚𝑣𝑎𝑓𝑡𝑒𝑟
65,000 1.3 + 92,000 0.8 = 157,000𝑣𝑎𝑓𝑡𝑒𝑟
𝑣𝑎𝑓𝑡𝑒𝑟 =1.01m/s