Download Forces Accelerate

Forces Accelerate!
Yesterday you multiplied the mass of things in kilograms (kg) times 9.8 to find their
weight in Newtons (N). But why 9.8? It turns out that all forces, including gravity
accelerate things as they push or pull them. Remember that a force is a push or a pull.
Acceleration means a change in speed. So when you stand on a scale, or place a rock on
the triple beam balance gravity is actually accelerating you towards the center of the
Earth. And we can measure that acceleration: it is 9.8 m/s2.
That unit m/s2 looks funny. Here’s how it works. Acceleration is a change in speed.
Speed is measured as distance divided by time, or D/T. In science we use SI units so we
typically measure speed as meters divided by seconds or m/s. If we measure speed
changing, then we are measuring that change in speed over time, or D/T/T, so we get
m/s/s. That is meters divided by seconds, divided by seconds again, or m/s2 .
1. Check it for yourself using what you learned about manipulating units like numbers.
Write out m/s/s so it works out to be m/s2 . Write it out in the blank below!
It should make sense to you that gravity accelerates a rock when you drop it, since you
have learned that gravity increases in strength as two objects of mass get closer and
gravity weakens as the objects move farther apart. At the surface of the Earth, the
acceleration is a constant and is 9.8 m/s2 as we just learned, but if we travel up into space
that acceleration rate changes. Gravity is actually a rather weak force, so at the height of
the International Space Station (ISS) in low-Earth orbit (about 400 km or 250 miles
above Earth), it’s accelerating back to Earth at about 90% of the force of gravity on
Earth’s surface.
2. Calculate the rate of gravity at the height of the ISS by multiplying 0.9 times 9.8 m/s2 .
Make sure that you understand that 0.9 is the same thing as 90%!
Now we know that all forces accelerate objects of mass. On Earth that acceleration due
to gravity is 9.8 m/s2 . Since the force of gravity changes with distance between objects
that acceleration rate changes too.
Now for something weird. You know that acceleration is changing speed. We think of
going faster as acceleration, but going slower and turning is also acceleration! Huh?
Here’s why. Slowing down is also changing speed and is called negative acceleration or
deceleration, which is a typed of acceleration. OK, but why turning? Well, do you feel
a force when you turn in the car or on your bike especially when you are going fast? Do
you have to slow down in the car or on your bike when you turn? Yes! We feel a force
when we change direction. Scientists actually refer to speed as velocity. And velocity is
defined as speed and direction. So if we change velocity we are accelerating!
3. Write the definition of velocity: __________________________________________
4. Write down the three ways that velocity can change (three ways to accelerate).
a. ________________________ b.____________________ c. _____________________
Now check out how fast things go when they fall to Earth. If you fall even only 10
meters you can get hurt badly or even be killed.
5. Calculate your speed if you fall for 10 seconds. Multiply the rate of acceleration of
gravity on Earth times 10 seconds to get the speed. See if you can work out
understanding the units that you should get which, if correct, are m/s. In other words,
multiply 9.8 m/s2 times 10 s to get an answer in m/s.
6. Now do the same calculation if you fell 100 seconds and then 1000 seconds! Be sure
to show your equations below. Write them out neatly below!
a. speed at 100 seconds
b. speed at 1000 seconds
Finally, you learned that the unit of force is a Newton, N. One Newton (N) is defined as:
N = kg x m/ s2
This is about the weight you feel in your hand when you hold a couple of apples. Can
you see now why we multiplied mass in kg times 9.8 to find weight in Newtons? When
you do the calculation you are really multiplying mass times the acceleration of gravity at
the surface of the Earth and the units are N x m/ s2 . We could just leave the units like that
since we can’t combine those units mathematically, but scientists got tired or writing
kg x m/ s2 so they called that unit a Newton! Get it now?
7. Try a few final simple calculations of force below. Remember the units are Newtons!
a. What is the force on a object of mass 2 kg accelerating at 2 m/ s2
b. What is the force on an object of mass 10 kg accelerating at 10 m/ s2