Engineering Makerspace 555061, Experiment Manual, Page: 7 / 28

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The Physics of Cars
POWER
Why is it harder to carry a heavy box while you
are running up a flight of stairs than it is while
you are walking up the same flight of stairs? It is
because it requires more power to move the box
when you are running. Power in physics means
the amount of work that is done over time.
You may have heard the term horsepower used to
describe a car. In the late 1700s, the Scottish
engineer James Watt wanted a way to compare
the amount of power that a steam engine could
produce with that of a draft horse. Watt found
that a horse could lift about 33,000 pounds of
coal a distance of one foot in one minute. Thus,
Watt set one horsepower equal to 33,000 foot-
pounds per minute. An average person can
produce about 0.1 horsepower, while a car can
produce 120 or more horsepower.
VELOCITY
AND SPEED
Speed and velocity
are often used
interchangeably, but
in physics they mean
different things. Speed
is just how fast
something is going, while
velocity is both how fast and
in what direction.
For example, if you are moving 30 miles per hour,
then that is your speed. But, if you are moving 30
miles per hour north, then that is your velocity.
The speed of a car is shown by the speedometer
in miles per hour or kilometers per hour.
ACCELERATION
Acceleration is a change in the velocity of an
object. That means that the object could be
speeding up, slowing down, or changing direction
and it would be accelerating. The time it takes for
a car to go from 0 to 60 miles per hour is a
common measure of a car’s ability to accelerate.
WORK
The way that physicists define work is different
from the common usage of the word. Work is
when a force causes a displacement in the same
direction as the motion of an object. For
example, if you were walking around at a
steady velocity with a box in your arms you
would not be performing work. This is because
the force required to hold up the box points in
the upward direction, while the displacement
from your walking around is in the horizontal
direction. However, if you were to push a box
along the floor or lift a box up, you would be
performing work.