Good morning. The acceleration due to gravity, little g, is positive. Here
on Earth, it has a value of roughly 9.81 meters per second squared. And, again,
little g is positive. I don’t get it.
Yeah, he keeps saying that. But, I don’t get it either.
But that is what he says, right? ♪ Flipping Physics ♪
Now, I do understand that many students question that g is positive. And I think this is why. For
the first month or so of physics class, every time we use the acceleration due to gravity,
it is as a part of the acceleration experienced by an object in free fall. For example: We have
thrown a medicine ball up in the air. We have dropped dictionaries. From a moving car, we have
dropped a ball and had it land in a bucket. And we have launched a ball from a nerd-a-pult and landed
the ball in a basket. Every time we did that the acceleration of the object in the y-direction was
equal to negative 9.81 meters per second squared. Exactly. Negative 9.81 meters per second squared.
Right. Negative. I think we should keep listening.
Eh. So, again, I understand, because the acceleration
of the object was always negative 9.81 meters per second squared in each of these problems,
students tend to infer that the acceleration due to gravity is also negative 9.81 meters per
second squared. It is not. Little g is positive 9.81 meters per second squared. Now that we
have learned Newton’s Second Law, we can better understand why little g equals 9.81 meters per
second squared near the surface of planet Earth. Let’s begin by looking at the simple example of
a ball in projectile motion. This is a video of a ball in projectile motion at real speed.
And this is the same video shown at 40 times slower than real speed. Bo, please draw a free
body diagram of all the forces acting on the ball while it is in projectile motion.
Sure. Well, clearly the force of gravity is acting down on the ball.
And there is a force normal acting up on the ball. What surface is causing that force normal?
Right. There is no force normal. The force of friction is acting in the …
The ball is flying through the vacuum you can breathe, so there is no air resistance.
Right. So, no force of friction. Is the force of gravity the only force acting on the ball?
Yes Bo. The only force acting on any object in projectile motion is the force of gravity
which acts straight down towards the center of the planet. Bobby, please sum the forces
in the y-direction on the ball. Uh. The net force in the y-direction
equals just the force of gravity which is negative because it is down. Net force equals mass times
acceleration in the y-direction. … Everybody brought mass to the party!
Yeah! ♪ Everybody
brought mass!♪ That means the acceleration
in the y-direction equals negative little g. Oh, and because the acceleration due to gravity
near the surface of planet Earth equals positive 9.81 meters per second squared,
the acceleration in the y-direction of any object in projectile motion equals negative 9.81 meters
per second squared! Sure.
Right, and that free fall acceleration is caused by the force of gravity.
The reason free fall acceleration is negative, is because the force of gravity is down or toward the
center of the planet. Notice, the acceleration due to gravity, little g, is not the acceleration of
the object, but rather, it is the value by which you multiply the mass of an object to get the
force of gravity the Earth exerts on an object. Oh So, the acceleration of an object and
acceleration due to gravity are two different things.
Yeah. Exactly.
Notice that an object at rest on a surface will still have the force of gravity acting downward on
it. You still multiply mass times the acceleration due to gravity to get the magnitude of the force
of gravity acting on the object, however, in this case, because a force normal acts upward
on the object preventing the object from falling through the table, the acceleration of the object
is zero. Again, the acceleration of the object and the acceleration due to gravity, or little g,
are two different things. And the acceleration due to gravity near the surface of planet Earth
is positive 9.81 meters per second squared. (Fine.) (Okay.) (We believe you!)
Thank you very much for learning with me today, I enjoyed learning with you.