Free Fall vs Terminal Velocity
If James is holding a bullet at the same height Emily is
holding a gun, and James drops the bullet at the same
time Emily shoots the gun, which bullet will hit the
They will land at the same time!
Horizontal acceleration does not affect vertical
Both bullets will accelerate vertically at 9.8m/s2
Projectile Motion is the curved path an object follows
when thrown or propelled near the surface of the Earth.
It has two components:
…and they are independent of each other!
Does horizontal Motion affect
It doesn’t, it doesn’t, no seriously…
Okay, enough about that…let’s “bite the bullet” and find
out more about this problem…
I was there too, and I timed it: The bullets took
two seconds to hit the ground.
What was their final speed in the vertical direction? and
What was their final speed in the horizontal direction if
the initial speed in the vertical direction was 100m/s
(Let’s pretend they are in a magical
place with no fluid friction)
For the horizontal…
100 meters per
second, of course!
In the vertical…
What if Emily was holding the gun at an upwards angle
so the path of the bullet looked like
Would the bullet James dropped land first, or the bullet
from Emily’s gun?
This time James’s bullet will land first.
By tilting the gun upwards, Emily gave the bullet an
initial velocity in both the vertical and horizontal
direction. Gravity will have to decelerate the bullet as it
goes up before it pulls the bullet down to Earth.
If you were able to throw a ball really far (beyond the
horizon)and really fast, would it ever hit the ground?
Gravity would pull the ball down towards the surface of
the Earth, but since the Earth is curved, it is always
bending away from the ball.
You could say the ball would be in orbit because it is in
free fall and has horizontal velocity
Free Fall occurs when the only force acting on
something is gravity.
Since outer space is a vacuum, there is no fluid friction
acting on the ball- only gravity
Fluid friction prevents an object from being in free fall on
As an object falls on Earth, the upward force of air
resistance increases until it matches the downward force of
When this happens, the net force is zero, acceleration
stops, and the object falls at a constant velocity.
Terminal velocity is the constant velocity at which a falling
object travels when the size of the upward force of air
resistance matches the size of the downward force of gravity
Person: can be between 53m/s and 76 m/s (or 119170mph)
What factors can you think of that would change
someone’s terminal velocity?
Terminal velocity depends on:
Someone with a small surface area and a high mass
will reach higher speeds than someone with a large
surface area and small mass
It’s raining cats and dogs!
Do cats really have
Or do they have a
velocity of 60mph
because of their
ability to “parachute”
1987 Study from
Journal of American Veterinary Medical
Studied 132 cats
Up to 7 stories, injuries increased with height
At 7 stories and above, injuries sharply declined
After 5 stories of falling, cats reach terminal velocity
One cat survived a fall from 46 stories