Gravity, Free Fall and Projectile Motion
Objectives- define each term; describe the forces acting in each
Gravity is a force of attraction between objects. It pulls objects together. Interesting first two
sentences. Do they challenge the way you have thought of gravity? We tend to think gravity as a force
that pulls things down. But that limits what gravity does to just our perception. Gravity does more.
There are two factors that affect how much gravitational pull an object has. One is its mass.
The larger an object, the more gravitational pull it has. Realize we are talking big things. There is one
asteroid that is big enough to hold a smaller asteroid in orbit around it. The larger asteroid is called Ida.
It has a mass of approximately 4.2 x 1016kg and is 58 x 23km (36 x 14miles) in size. Consider the next
example to see how much gravity it has. You know the moon has less gravity than the earth and that if
you jumped on the moon you would go farther and higher than you would on earth. If you jumped on
Ida, you could go from one end of the asteroid to the other (36 miles- take that Michael Jordan!). The
other factor that affects the amount of gravity is how far apart the objects are. Ida’s weak gravity can
only hold on to its moon which is 90km (56 miles) away. If it was farther away, it could not hold onto it
and it would drift away.
As we have seen before, sometimes terms are not the same but end up being used to mean the
same- such as the terms velocity and speed. The terms mass and weight are the same way. Mass is the
amount of “stuff” present. If you went to the moon, your mass would be the same. But as you probably
know, you would weigh less. Weight is affected by gravity. Since the moon has less gravity, you weigh
less.
When an object is falling and the only force acting on it is gravity, the object is said to be in free
fall. Since there is no force opposite it, gravity is an unbalanced force. On earth (sea level) the
acceleration of gravity on an object is 9.8m/s/s. This means that for every second, the object falls
9.8m/s faster. The object goes 9.9m/s faster every second. Regardless of the object, gravity pulls on all
objects the same.
Now let’s consider a second force acting on a falling object- air resistance. Air resistance pushes
up on the object. As an object falls, it will encounter more particles of air and more air resistance which
will slow its rate of acceleration. Eventually air resistance can equal the pull of gravity and forces will be
balanced. The object will no longer fall faster but fall at a constant speed. The highest speed an object
will fall at is called terminal velocity. A couple of factors affect what the terminal velocity will be an
object including its density (mass and volume).
Have you ever wondered which will hit the ground faster a bullet dropped or one shot from the
same height? “Myth Busters” tested this and the results may surprise you. An object thrown
horizontally (we will save objects thrown with
a vertical motion for Physics- you can thank
me later!) has a couple forces acting on it.
The forces are inertia (its forward motion
given to it by the gun going off, the ball being
thrown…) gravity and air resistance. The
bullet (in this case) is moving forward and will
do so for every according to Newton’s First Law. But since gravity is pulling it down- it can’t keep going
straight, it begins to drop. The farther it goes the more air resistance will slow its forward speed and the
horizontal distance it covers becomes less. But gravity does not change, so it keeps falling at the same
rate. The result of all this is an elliptical shaped path.
Questions
1. What does gravity do?
2. What determines how much gravitational pull is felt by an object?
3. How are weight and mass different?
4. What is the acceleration of gravity at sea level?
5. How fast will an object in free fall be going after 4 seconds?
6. In free fall, which will fall faster, a brick or a feather?
7. Define terminal velocity.
8. What factors affect terminal velocity?
9. So which will hit the ground first, a bullet dropped or one shot from the same height? Explain.
10. How could going to the moon be a diet tip??