Lesson 12: Describing Motion
In this lesson, we will talk about ways to
describe motion and look at different factors that affect motion, including
gravity, momentum, and force. We will also learn about the difference between
mass and weight, and discover what causes weightlessness in space.
There are three basic terms that can be used to describe motion.
If we use the example of a car, the *speed* tells us how fast it will go in
a certain amount of time. For example, this could be 60 miles per hour (or 100
kilometers per hour).
The *velocity* of the car tells us both its speed and
direction. For example,
traveling 60 miles per hour due north would describe velocity.
Finally, the car has an *acceleration* if its velocity is changing in any way,
whether it is a change in speed, direction or both. In everyday usage,
acceleration refers to an increase in speed, but in science, acceleration can
be positive or negative, and can also refer to a change in direction.
Acceleration caused by gravity is one of the most important types of
acceleration. Acceleration of Gravity, simply called "g", causes objects to
fall faster with time. For example, if you drop a rock from the top of a
building, its speed initially is 0 meters per second, but after 1 second, the
speed increases to about 10 meters per second and by 2 seconds, it is around 20
meters per second. If there is no air resistance, the object will continue to
accelerate at the rate of 9.8 meters per second.
It is important to remember that
gravity accelerates all objects equally, regardless of their mass. A small
feather and a giant boulder both have a 'g' of 9.8 meters per second squared.
Why then, does a rock fall to the ground much faster than a feather? The answer
is air resistance. If you were to drop the two objects on the Moon, where there
is no air, they would fall at exactly the same rate.
When looking at interactions between objects, we need to look at momentum and
force.
An object's momentum is the product of its mass and
velocity. In other words, momentum = mass times velocity.
You can visualize momentum if you think about a large truck running into your
car versus an insect running into your car at the exact same speed. Both would
be moving at the same velocity, but the greater mass of the truck means that it
would transfer much more momentum to your car, exerting a lot more force.
The only way you can change an object's momentum is to apply a force to it. The
net force or overall force is the combined effect of all forces put together on
an object, so with a car, the net force would be the result of the force
generated by the engine, working against the forces of gravity, air resistance,
and road friction. If you are on a steep hill, the force of the engine may not
be enough to overcome the other forces acting on the car, but assuming the net
force is not zero, the car will accelerate.
We know that
momentum equals mass times velocity, so this acceleration represents a change
in the object's momentum, since the velocity is changing.
In the context of astronomy,
planets are always accelerating as they orbit the Sun - their speed and
direction are constantly changing. Isaac Newton identified the force causing
this acceleration to be the force of gravity.
When we are
talking about objects on Earth, we rarely make a distinction between mass and
weight, but they actually measure two very different things.
Mass is the amount of matter in
your body. It does not change no matter where you are.
Weight, on the other hand, is the force that a scale measures when you stand on
it. The force you exert on the scale depends on your mass, but it is affected
by forces such as gravity that are acting on your mass.
So for example, an astronaut's mass stays the same when they go to the Moon,
but they weigh less there since there is less gravity acting on their mass.
Another example would be if you were to stand on a scale in
an elevator. When the elevator accelerates upwards, the floor exerts a greater
force upwards on you, making you temporarily have a greater weight. When it
accelerates downwards, you would temporarily be lighter. If the elevator cable
were to break, you would temporarily be weightless as you experience free-fall
- your mass is unchanged, but you are not exerting any downward force on the
scale.
When you see videos of astronauts floating weightlessly in space, you are not
witnessing an absence of gravity. What you are actually seeing is a perpetual
state of free-fall. Free-fall is a state of falling without any resistance to
slow you down. When you jump off of a chair, for example, you are in free-fall
and therefore weightless for a split-second.
All orbiting objects are
constantly falling. The Earth's gravity pulls the object down, but it is kept
in orbit by its speed. It is continuously falling, but it is moving fast enough
that at the same rate it is falling, the Earth is curving from underneath it.
That is why satellites and
other objects eventually fall to the Earth if they are
not periodically given an altitude boost. Friction with gas molecules in the
Earth's atmosphere gradually reduces the velocity of the object until
eventually, it is not moving fast enough to counteract gravity's downward pull.