Download Factors That Affect Motion

Factors That Affect Motion
Objective 4.05
Determine factors that affect motion including:
Force
Friction
Inertia
Momentum
What Affects Motion?
• Motion is the change in an object’s
location over time. People and machines
can make objects move. Unbalanced
forces, like pushes or pulls, make objects
move.
• We also understand that usually objects
don’t keep moving forever. They can stop
moving or change direction. They can
speed up or slow down. Objects change
their motion because unbalanced forces
act on them. There are four main factors
that affect the motion of objects.
These factors are force, friction,
inertia, and momentum.
Force
• (Review) Ways that forces affect motion
are:
* direction of application—the angle of a
force determine the direction of the
motion of the object.
* strength of force—delivering a lot of
force moves objects a long way.
* mass of object—massive objects require
more force.
• In Figure 15.1, the skater’s direction is
determined by the angle of force he
applies. How far he moves is determined
by how hard he pushes. The lighter he is,
the easier he can glide across the ice.
Figure 15.1 Skater
Friction
• Friction acts in the opposite direction of
the object’s motion. Rubbing often
produces friction. When objects touch,
their surfaces interact. Moving one surface
across another creates friction. It can be
felt if you rub your hands together quickly.
Repeated rubbing of your hands creates
heat. This is one side effect of friction.
Figure 15.2 Rubbing Creates Friction
• Friction can also result from air resistance.
Particles of air bump against the surface of an
object. This slows its motion. Air resistance is a
type of frictional force. Frictional force oppose
Figure 15.3 Frictional Forces
the motion of objects.
on a Ball
• Think about this: a ball rolling across the floor.
Where will friction occur? Some will come from
the ball’s surface rubbing against the floor’s
surface. This will slow the ball’s motion. Friction
will also come from air resistance. These forces
cancel the forward force (and thus the forward
motion) of the ball.
• Friction can be a good thing. It is what
keeps your bike tires (and car tires) on the
road as you go around corners or come up
to a stop. It is also what allows your shoes
to grip the basketball or tennis court when
playing a game. Track runners and rock
climbers use extra sticky shoes to
increase friction and improve performance.
• There are only a few situations where
friction is non-existent or a minor force.
There are also many ways to reduce
frictional forces. Grease or oil inside
machinery is often used to reduce friction.
Inertia
• You may have heard this one before: an object
at rest will remain at rest, and an object in
motion will remain in motion. That is, unless an
outside force acts on the object. When we put
something down, it tends to stay in the same
place unless someone or something moves it.
The tendency of matter to remain at rest or in
motion is another way of explaining inertia.
Inertia is an object’s resistance to a change in
motion. A massive object has a lot of inertia
while a smaller less massive object has a little
inertia.
• We see examples of inertia everyday. One
common example of inertia is felt in a car.
Have you ever been in a car that stops
suddenly? If you have, you might have felt
like you were being thrown forward. That
is because your body resists the stopping
motion of the car. Your body tends to
remain in motion. The outside force, the
stopping car and its seat belt, act to stop
your forward motion.
• Figure 15.4 shows this very situation. The
driver of this car forgot to stop at the stop
sign. He slams on his brakes, causing the
body of the car and himself lean forward.
This is inertia at work. But remember,
inertia is NOT a force. It is a property of
the object.
Figure 15.4 Inertia Can be felt in a Car
• Perhaps you have tried to carry a glass of
water quickly across a room. Stopping
suddenly causes the water inside the
glass to spill. This is because of the water
has inertia. When you stop, the water
wants to keep going, resisting the change
in motion. In Figure 15.5, you can see
some water behaving badly. Stopping the
glass allows the liquid inside to keep
moving according to inertia.
Momentum
• Momentum is an objects mass multiplied by
its speed. In mathematical equations, we
represent momentum with a “p.” The for
momentum is:
p = m (mass of an object) x v (velocity of the object)
• Let’s think about it like this: an object’s
mass multiplied by its speed equals its
momentum. How much matter you have
times how fast it is moving equals the
momentum of the object. In short, the
amount of momentum depends on two
factors: mass and speed. Large objects
moving quickly have a lot of momentum.
Smaller objects moving slowly have less
momentum.
• Simply put, momentum is mass that is in
motion. Any object in motion has
momentum. Increasing the mass or speed
will increase the momentum. Conversely,
decreasing the mass or speed will
decrease the momentum.
• Example, throwing a heavy bowling ball
with a high acceleration will have more
momentum than the same bowling ball
thrown with a lower acceleration. Also,
throwing a lighter ball (lower in mass) will
result in less momentum.
Review
1. According to momentum, which statement is true about a
small ball rolled down a hallway?
A. It has force.
B. It has zero friction.
C. It needs a larger force.
D. It is very heavy.
2. What word best describes friction?
A. stopping
B. accelerating
C. harmful
D. quick
3. What best describes inertia?
A. the tendency to resist mass
B. the ability to move
C. the tendency to resist changes in motion
D. the ability to decelerate
4. Which situation below best demonstrates friction?
A. Markus trips while running, falling forward.
B. Josh throws a strike at the bowling alley.
C. Francis pushes a box across the floor.
D. Sam throws a ball through the air.
5. Kelly is riding her bike down the street. She runs into a pothole and falls
forward off her bike. She hits the ground and dents her helmet. What
causes her to dent her helmet?
A. gasoline
B. friction
C. inertia
D. momentum
The End