Getting to Know: Straight Line Motion
Our lives are filled with motion. Think about how we travel in cars, ride bikes, and fly in airplanes to get from one location to another. We also perform smaller motions—our fingers move across a keyboard, our foot taps in time to the beat of our favorite song, and our chest rises and falls as we breathe. We are constantly in motion. What is motion?
When a train is in motion, it is changing
Motion is a change in position over time
position over time.
measured in relation to a fixed point, or a
reference point. When you roll a ball from one
place to another, or lift a bag of groceries, motion has occurred. This change in position occurs
over time. For example, it takes time for a train to go from one station to another or for a ball
to roll from one location to another on a soccer field.
What causes motion?
To understand motion, first you have to
understand something about the forces that
surround us. A force is a push or pull on an
object. One force that affects nearly all motion
in the universe is gravity. All objects exert a
gravitational force, but objects with a lot of mass
such as planets and stars exert a significant
gravitational force. For example, Earth’s gravity
is the force that pulls objects toward Earth and
keeps the moon in its orbit, whereas the Sun’s
gravity is the force that keeps the planets in orbit.
Like gravity, friction is another unseen force that
occurs around us all the time. Friction is a force
that occurs between objects that are touching or
rubbing against one another.
Gravity is the force that pulls the roller
coaster car down the track.
Misconception 1: Objects can start moving even when additional force is
not applied.
You may have noticed that some objects fall or move downhill without anything
visible having pushed them. However, an unbalanced force is always needed to start a
stationary object moving, so although you can’t see it, gravitational force acts to pull
objects downward to Earth’s surface.
Concept: Straight Line Motion
Getting to Know
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In addition to gravity and friction, there are other,
more obvious, forces at work in the world. For
example, you exert a force on something when
you lift it, push it, or pull it.
Motion always results from unbalanced forces.
Consider arm wrestling, for example: if the two
people who are arm wrestling are exerting the same
amount of force, neither arm moves because the
forces are balanced. If one person starts to exert
a greater force than the other, the forces become
unbalanced, and both arms move.
How do I know forces cause motion
when I can’t see them?
Some forces are more visible than others. If someone
slams a door shut, it is clear that the person applied
a force to the door, making it move. Other forces
are invisible, and we know they are there because
of their effects. In fact, that’s how Isaac Newton
began to be interested in studying gravity and
eventually came up with his three laws of motion.
He understood that forces such as gravitational force
must exist because of the way objects move. We
can’t see gravitational force, but we can understand
how gravitational force works because we can see
its effects.
Motion always results from unbalanced
forces.When forces are balanced, motion
does not occur.
What kinds of laws did Isaac Newton
describe?
Newton’s three laws of motion describe the ways that various forces cause motion and cause
changes in motion. You might already know about Newton’s first law—sometimes called
the law of inertia. The first law explains that objects in motion tend to stay in motion, and
objects at rest tend to stay at rest unless acted on by an unbalanced force. This means that
an unbalanced force is required to cause an object at rest to begin to move. It also means
that an unbalanced force is required to cause an object in motion to slow down or stop.
Misconception 2: A force is required to keep an object moving in a straight line.
Actually, any moving object will continue moving in a straight line at a constant speed
until an unbalanced force acts on that object. Of course, on Earth’s surface, there are a
variety of forces acting on every object all the time. We don’t see these forces, but we do
see their effects. In general, it is often the forces of gravity and friction that act to slow or
stop moving objects.
You’ll learn more about Newton and his three laws as you study forces and motion.
Concept: Straight Line Motion
Getting to Know
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© Discovery Education. All rights reserved.
Discovery Education is a subsidiary
of Discovery Communications, LLC.