Gravity on the Fly
At the end of the last unit, we made a special point of saying how important it is that we know how far away the Sun and planets are. The obvious reason we need to know this is so we know how long we have to travel to get to a planet.
The mass of a planet tells us how strong the gravity of
a planet is - including Earth's gravity field. As you already know,
gravity is what keeps us firmly planted on Earth. To put satellites
in orbit around our planet, we must carefully balance against the force
of gravity. To send a probe or human mission to another planet, we must
escape from Earth's gravity.
So how does gravity affect an object in motion? We all know that if you throw something straight up it falls back to the ground. Let's look at how Newton explained it. He wrote of firing a bullet, but we will use something a bigger and less threatening - a baseball. It will have to travel as fast as a bullet, in the end, but it is a friendlier image to begin with!
Imagine a baseball pitcher on a hundred-foot high (30.48
m) pitcher's mound. If a pitcher throws a baseball straight (parallel
to the ground), the ball will curve down to the ground and stop when
it hits the ground. In other words, we can clearly see the results of
gravity acting on the baseball. But let's take a step back and examine
exactly what is happening.
Let's picture the same scene as above where a baseball pitcher is on a hundred-foot high (30.48 m) pitcher's mound. This time, we will assume 1) that there is no atmosphere and 2) that there is no gravity from any source.
So what would a baseball trajectory look like with just an atmosphere or just gravity?
Case 1: No Gravity - All Atmosphere
In the pretend case where there is no gravity but somehow we were still in an atmosphere, the baseball would fly straight ahead but slower and slower. Finally, it would stop as the atmospheric drag takes away the ball's energy of motion.
We have just introduced a very important concept - the
effects of atmospheric drag. We'll come back to that later when we look
at how to slow down when we get to Mars in another unit. Atmospheric
drag is the force of friction the atmosphere exerts on an object traveling
through that atmosphere.
Case 2: Gravity with no Atmosphere
In the pretend case where there is gravity but no atmosphere, the ball would still curve down under the force of gravity until the Earth gets in its way.
Putting it together: In the real case of a pitched ball, the path curves to Earth because gravity is pulling on it, and at the same time it slows down because it is moving through air.
The point to remember here is that when a baseball, or any object that is thrown into the air for that matter, falls back to Earth, it does so under the influence of both gravity and atmospheric drag. Your intuition about what will happen has been developed here on Earth. When it comes to moving in space, where there is little or no atmospheric drag, your intuition may be wrong.
Galileo is famous for trying to demonstrate how two bodies
of different masses should fall at the same rate. The Apollo astronauts
did this experiment on the Moon where it worked perfectly. On Earth
it is very hard to do this and have it turn out because the atmospheric
drag has more effect on the lighter mass.