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9.1 Impulse and Momentum
Definitions
Impulse = a force acting over a time period
force x time
Unit: N s
Momentum = a property an object has based on its mass and velocity
Symbol: p
Why not m?
Unit: kg m/s
Large Impulse = large force over short time or small force over long time.
Example of something with a large impulse:
Example of something with a small impulse:
A change in momentum is impulse
Impulse is the change in momentum
The idea here is that if you are given two momentums (called momenta), you
can figure out what the impulse is, and if you have the impulse, you can
figure out what the change in momentum is
Formulas:
p=mxv
and…
FΔt = p2 – p1
Impulse = FΔt
F=m
Conversions:
Km/hr to m/s
Grams to kg
multiply by 1000, then divide by 3600
divide by 1000
Ex: Stopping a Vehicle
A 2200-kg sport utility vehicle (SUV) traveling at 94 km/h (26 m/s) can be
stopped in 21 s by gently applying the brakes, in 5.5 s in a panic stop, or in
0.22 s if it hits a concrete wall.
a. What average force is exerted on the SUV in each of these stops?
(Hint: solve for force for each of the three situations)
b. What is the change in momentum (Δp) of the SUV?
c. What is the impulse of the SUV if it takes 21 seconds to stop?
d. What is the impulse if it takes 0.22 seconds to stop?
Summary:
-
-
When doing a momentum problem first examine the system before
and after the event.
The momentum of an object is the product of its mass and velocity
and is a vector quantity.
The impulse given of an object is the average net force exerted on
the object multiplied by the time the interval over which the force
acts
The impulse given an object is equal to the change in momentum of the
object.
Key equation: p = mv FΔt=p2 - p1