Motion
Last updated: April 1, 2009.
IIt's hard to believe, but everything in the world is in motion, all the time. Even
things that look perfectly still are packed with atoms that are
vibrating with energy. Understanding how motion works was one of the
great milestones of science and it's credited to the brilliant English physicist Sir Isaac Newton. His laws of motion were so well
stated that scientists still use them in most situations today. Let's
take a closer look at the science of moving things!
Photo: A space rocket is an impressive demonstration of Newton's laws of motion. The force of the hot exhaust gas shooting backward propels the rocket forward. Photo by courtesy of Great Images in NASA.
Newton's laws of motion
Photo: Isaac Newton—the man who put science in motion.
Picture courtesy of US Library of Congress.
Sir Isaac Newton (1642–1727) summarized how things move with three simple laws. They're often
simply called Newton's laws and they apply to pretty much
everything (except very tiny subatomic things and objects moving close to the speed of light):
- Things that are still stay still and things that are moving keep moving with a
steady speed unless a force of some kind pushes or pulls on them.
- When a force acts (pushes or pulls) on an object, it changes the object's speed and makes it accelerate. The
bigger the force, the more the object accelerates.
- When a force acts on an object, there's an equal force (called a
reaction) acting in the opposite direction. This law is sometimes
written "action and reaction are equal and opposite."
How do those laws run in practice? Suppose you're standing on a skateboard. Unless you
kick against something, you'll stand on it forever going nowhere.
That's Newton's first law in action. Now if you kick against the
sidewalk, you start moving: your speed increases, and the harder you kick the faster you accelerate.
There's an example of Newton's second law. And what about the third
law? Kick back against the pavement and the pavement pushes you
with an equal and opposite reaction force that propels you forward.
Speed
A sports car can go 50 times faster than you can walk and 8 times faster than you can
run. That's because its engine turns gasoline into power much more
quickly than your body can burn food to pump your muscles. The faster a car burns gas, the quicker it can go—the more
speed it has.
In science, we define speed as the distance something goes in a second. You can figure out a car's average speed by dividing how far it goes by how long it takes to get there. If a car is going at 100km/h (60mph), it can travel 100km (60 miles) in an hour.
This is the formula for speed:
Speed = d/t
Here, d is distance and t is time.
You can see from this that a car is a kind of time machine: you can use its speed to get somewhere more quickly. If you
go twice as fast, you can arrive in half the time. The faster you go,
the sooner you get there, and the more time you save. Of course, you
can never actually arrive before you leave—that would be taking the
science a bit far!
Velocity
Velocity is not just another word for speed: it means your speed in a certain
direction. When a Formula-1 car races round a tight bend, its
speed stays the same, but its velocity is always changing because
it's turning and changing direction the whole time.
Suppose you drove from the North Pole to the South Pole in a straight line at 100km/h
(60mph) and then drove back again at the same speed. Your average speed would be
100km/h (60mph), but your average velocity would be zero. That's
because your velocity from South to North would be exactly opposite
the velocity from North to South and the two would cancel out.
This is the formula for velocity:
Velocity = d/t
Here again, d is distance and t is time. You'll notice this formula is the same as the formula for speed.
But remember when you have to state which direction the velocity is heading in too.
Photo: These two cars have the same speed but completely different velocities because they're
travelling in opposite directions. Photo by Warren Gretz
courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
Acceleration
If you're a driver, acceleration means putting your foot down to go faster. But if
you're a scientist, acceleration also means stamping on the brakes.
That's because acceleration means any change in your
velocity. Speeding up is an acceleration, but so is slowing down—it's just a negative acceleration. And because your velocity is your
speed in a certain direction, you accelerate every time you go round
a corner, whether you change speed or not.
Photo: This car is going round a curve at constant speed, but its velocity is changing all the
time because its direction is changing. That means it's accelerating as well, even though its speed stays exactly the same!
Photo by Warren Gretz
courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
A car is a heavy lump of metal and it takes a lot of force to get it moving, speed it up, slow
it down, or turn it round. The heavier something is, the more force
it takes to accelerate. That's why trucks take longer to accelerate
than cars, even though they have much bigger engines.
People compare cars by seeing how many seconds they take to accelerate from 0-100 km/h
(0-60 mph). If a car can go from 0-100 km/h in 5 seconds, it changes
its velocity by 100km/h in 5 seconds, so its acceleration is 20 km/h
per second. That's the same as changing your speed by 5.5
meters/second every second. Scientists write that 5.5 m/s/s or 5.5m/s2
(and say it out loud as "five meters per second squared").
This is the formula for acceleration:
acceleration = v/t
Here, v is velocity and t is time.
Momentum
An object's momentum is a measure of how much it wants to keep moving—and how long it can exert a force for when it stops. You can figure out something's momentum by multiplying its mass by its velocity:
momentum = mv
Here, m is mass and v is velocity.
If two things crash together, their total momentum is the same before and
after the collision. This is a basic law of physics called the conservation of
momentum. If a car crashes into a wall, bits of the wall start
moving—so they gain the momentum the car loses and that's what slows
it down.
Photo: If a truck has 10 times the mass of a car,
but moves at the same speed, it has 10 times more momentum.
Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (US DOE/NREL).
Kinetic energy
It takes energy to make something move, and the faster it goes the more energy it needs.
In other words, energy feeds speed. The energy something has when
it's moving is called kinetic energy. You can figure out a
car's kinetic energy from this formula:
kinetic energy = ½mv2
Here, m is the car's mass and v is its velocity. If you double the weight
of a car (by adding a caravan on the back), you need to use twice as
much energy to go at the same speed. If you want to double your
speed, you'll need four times as much energy, because energy is
related to the square of your speed.
Find out more in our main article about energy.
