1
what is momentum?
obje*be,
Define and describe how to calculate
Let's look at the two bowling balls described
earlier. Suppose the velocity of each ball is 20 m/s.
Find the momentum of the 5-kg ball.
momentum.
Key Tutl,s
momentum: a property of all moving objects
law of conservation of momentum: total
momentum of any isolated system always
remains the same
Momentum Picture a bowling ball with a mass
of 5 kg rolling toward the pins at the end of the
alley. In the next alley, aball with a mass of 8 kg is
rolling toward the pins with the same velocity.
Which ball do you think is likely to knock over
more pins? If you answered the ball with the
greater mass/ you are correct. As long as the two
bowling balls are moving with the same velocity,
the ball with the greater mass will strike the pins
with greater energy. The combined effect of the
mass and velocity of an object is momentum.
Momentum is a property of all moving objects.
5 kg
x 20 mts:
100 kg-m/s
The S-kg ball has a momentum of 100
Now find the momentum of the 8-kg ball.
8 kg
x 20 mts :
kg-*/r.
160 kg-m/s
The B-kg ball has a momentum of 160 kg-m/s.
The ball with more momentum will knock over
more prns.
Find the momentum of a 10-kg
object moving at a velocity of 20 rn/s.
CALCULATE:
Conservation of Momentum When one
moving object collides with another object, the
motion of both objects changes. For example, when
a bowling ball strikes the pins, the bowling ball
slows down. It loses momentum. The pins move.
The pins gain momentum. The important thing to
remember is that the total momentum of the ball
and the pins remains the same. In any isolated
system, momentum can be transferred but carLnot
be lost. This is the law of conservation of
momentum.
Figure 13-9 demonstrates this idea. If a sphere on
the left is swung and strikes the row of spheres, a
sphere on the other end will move. The momentum
of the first sphere is transferred through the row of
spheres to the sphere at the other end. No
momentum is lost.
Figure 13-8 Momentum is transferred from the ballto the pins.
IDENTIFY: What two factors determine
momentum?
Calculating Momentum The momentum of
*
an object can be found by multiplying its mass by
its velocity.
/
momentum = fftoss x velocity
Figure 13-9 Momentum is conserved as it is
transferred from sphere to sphere.
Now suppose two spheres are allowed to strike
the remaining row of spheres. Figure L3-10 shows
what would happen.
bowling ball strikes the pins, the ball
momentum.
If the velocity of a car traveling at 50 km/h
changes to 30 km/h, the momentum of the car
2. When
3.
a
will
4. If several objects are traveling at the same
velocity, the object with the greatest mass will
have the greatest
I
,,r Figure 13-10 Momentum is still conserved.
5.
DESCRIBE: Describe the momentum changes that
might occur whena large glass marble rolling
across a smooth surface makes a direct hit on a
smaller glass marble that is not moving.
6.
CALCULATE:
Find the momentum of a25-kg
mass moving with a velocity of 25 m/sec.
What will happen if three spheres are
allowed to strike the row of spheres?
INFER:
Design an experiment to solae the following problem.
lnclude a hypothesis, uariables, a procedure, nnd a type
CHECKING CONCEPTS
1. The momentum of an object depends on its
of datn to study.
PROBLEM: How can you show that the
momentum of an object is related to its mass?
and its velocity.
Li,fs
I soi,en os
nes
ANIMALS AND MOMENTUM
Most birds can fly. Their hollow bones make for a
light body weight. The most difficult parts
of a bird's flight are the takeoff and the landing.
Both require a change in momentum.
ln order to take off, birds have to build up
Figure 13-11 Flamingos during takeoff
enough speed so that the lift from their wings
is greater than their body weight. Small birds
can take off with a hop and a flap of their wings. Larger birds, like the flamingos
in Figure 13-1'1, have more of a problem. Because they have more mass, they need
more momentum to reach the speed needed to take off. They do this by running
as fast as they can while they flap their wings.
For some birds, landing is even harder than taking off. Birds cannot just stop
flapping their wings. They would drop like a stone. lnstead, they twist and spread
their wings so that they slow down gradually. ln other words, they lose
momentum slowly enough to allow them to make a safe landing.
Thinking Critically Why do large birds need more momentum than small birds
do in order to take off?
CHAPTER 13:
Motion
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