Transcript Momentum, Impulse and Recoil

Momentum, Impulse and
Recoil
Momentum
• A truck is rolling down a hill than a roller
skater with the same speed; which has the
greater momentum?
Momentum
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Momentum is inertia in motion!
Momentum = mass x velocity
Momentum (p) = mv
p= mass x speed (when direction is not
important)
• A greater mass but the same speed will
lead to a greater momentum
Decreasing Momentum
• Why does a car driven into a haystack at the same speed suffer less
damage than one driven into a brick wall?
Impulse
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Impulse = F∆ t
This is the force x time interval
Impulse = change in momentum
F ∆ t = ∆ (mv) [change in impulse =
change in momentum
• If t increases and ∆ (mv) is constant then F
decreases.
• This is the principle of an air bag in a car.
• Why does the
boxer who moves
back with a punch
feel less pain
than the boxer
who moves into
the punch?
Changes in Momentum
• The boxer increases the time taken for the
impact and therefore reduces the force of
the impact.
• Ft = change in momentum
• How does this
runaway truck
ramp use the
laws of
physics?
Bungee Jumping
• How does momentum
and impulse relate to
your changes of
surviving a bungee
jump?
• Ft = Impulse
• Ft = ∆(mv)
• The momentum, mv, is the amount gained
before the cord begins to stretch. Ft is the
impulse the cord supplies to reduce the
momentum to zero.
• Because the rubber cord stretches for a long
time, a large time interval t ensures that a small
average force F acts on the jumper.
• The cord typically stretches to about twice the
original length during the fall.
Starter Questions
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How is the recoil of a rifle or cannon
related to momentum conservation?
A man is stranded in the middle of an ice
rink that is perfectly frictionless. How
can he get to the edge?
Conservation of Momentum
• When the cannon is fired the
force on the cannonball inside
the cannon barrel is equal and
opposite to the force causing
the cannon to recoil.
• Before the firing of the system
is at rest and the momentum
is zero.
• After the firing the net
momentum, or total
momentum, is still zero. Net
momentum is neither gained
nor lost.
Law of Conservation of Momentum
• In the absence of an external force, the
momentum of a system remains
unchanged.
• If a system undergoes changes wherein all
the forces are internal, e.g. in atomic
nuclei undergoing radioactive decay, cars
colliding, or stars exploding, the net
momentum remains constant.
Recoil
“In any interaction between two (or
more objects) the total momentum of
the objects remains constant”
Pi = PF
• A bullet of mass 50g is fired from
a gun of mass 2.0kg at a speed of
450 m/s. Calculate the speed of
the recoil of the gun.
Recoil of Gun
• Total momentum before and after is zero
(as there is no velocity to start with).
• Momentum of bullet = Momentum of gun
Recoil Question
• A bullet of mass 0.01kg moves horizontally
with a speed of 400m/s and embeds itself
in a block of mass 0.39kg that is initially at
rest on a frictionless table. Find the final
velocity of the bullet and the block.
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Runaway truck ramp: http://www.northlightimages.co.uk/content_images/photography_tour/mon_5.jpg
Truck: http://geoimages.berkeley.edu/geoimages/BainCalif/CAL400/runaway.jpg
Roller Skater: http://www.mchenrycountyblog.com/uploaded_images/Roller-skates-715946.jpg
Haystack: http://floridalawfirm.com/haystack.gif
Brick wall: http://www.dailyhaha.com/_pics/car_crash_brick_wall.jpg
Boxer avoids punch: http://cache.daylife.com/imageserve/01iL0EYd2c7Hd/610x.jpg
Boxer lands punch:
http://www.chinadaily.com.cn/sports/images/attachement/jpg/site1/20071205/001aa018ff9c08c0a
6df0c.jpg
Bungee Jumpers: http://uk.geocities.com/osfuk/osf/bungee.html
Cannon firing: http://www.ottoleffler.com/psd/portfolio/Canon/ist2_2437935_fire_away.jpg
Gun Recoil Pic: http://farm1.static.flickr.com/67/157553435_39e61a5113.jpg?v=0