Transcript Chapter 7.1 Notes

Notes
Linear Momentum
• Newton’s 1st law of motion:
• An object in motion stays in
motion and an object at rest
stays at rest unless acted on by
an outside force.
• Object’s inertia is dependent
upon the object’s mass.
• Objects in motion have
momentum also.
• An object’s momentum
tells us how difficult it
is to stop the object.
• Momentum
depends on two
things:
• Mass
• Velocity
• Imagine catching a softball that is
thrown softly at you and than
catching the same ball thrown fast
at you. Which hurts more? Which
has a greater momentum? Why?
• The fast one
• More velocity = more momentum
• Now compare catching a softball
and an iron shot-put ball thrown
at the same speed. Which would
hurt more? Which has a greater
momentum? Why?
• Iron Shot-put
• More mass = More momentum
• Linear momentum = mass x
velocity
• p = mv
• Linear momentum is a vector
since it has a direction and a size.
• The SI units of momentum is
kg·m/s
• A 10 kg ball is thrown at 40
m/s. What is the
momentum?
• p = mv
• p = 10 kg x 40 m/s
• p = 400 kg·m/s
• Newton’s 2nd law with
momentum states:
• A net force exerted on an
object equals the rate of
change of the object’s linear
momentum.
• Newton’s 2nd law (equation) =
Force = mass x acceleration
• Newton’s 2nd law considering
momentum (equation) = Force
= (mass x velocity) / time
• F = (m x v) / t
• A tractor trailer has a mass of
20,000 kg and is moving 25
m/s, what is the braking force
required in 30 seconds?
• F = (m x v) / t
• F = (20,000 x 25) / 30
• F = 16,667.67 N
• Impulse = Force x
time
• Imp = F x t
• Impulse = momentum
change or mvf - mvi
Impulse Egg
• In a penalty kick a soccer player
increases the speed of the ball from
0 to 40 m/s. The mass of the soccer
ball is .5 kg. What impulse does the
soccer player give the ball?
• Imp = mvf – mvi
• Imp = .5(40) - .5(0)
• Imp = 20 Ns
• If the player above is in contact with
the ball for .1 second, what is the
force exerted on the ball?
• Imp = F x t
• Imp = 20 from problem before
• 20 = F (.1)
• 20 = F (.1)
.1
.1
• 200 N = F
• Newton’s 3rd Law of
Motion:
• Whenever one body exerts
a force on another body,
the second body exerts an
equal and opposite force on
the first.
• Conservation of
momentum occurs in
closed systems, where
there is no outside forces.
• A closed system on which
no net external forces act is
called an isolated system.
• The law of conservation
states that when no external
forces act on a closed
system, the total linear
momentum of the system
remains constant.
• Conservation of
momentum equation =
momentum before =
momentum after or
• MfVf +Mf2Vf2 = MiVi
• A 600 kg ship is traveling at 1000 m/s. The
ship burns off 100 kg of fuel and the fuel is
propelled out of the ship at 2000 m/s. What is
the final speed of the ship after the fuel is
expelled?
• MfVf +MfuelVfuel = MiVi
• Mf = ?
• 500(Vf ) + 100(2000) = 600(1000)
• 500(Vf ) + 200,000 = 600,000
• 500(Vf )=600,000–200,000
• 500Vf = 400,000; Vf = 800