Main content

## AP®︎/College Physics 1

### Course: AP®︎/College Physics 1 > Unit 1

Lesson 7: Objects in freefall# Freefall review

Review the key concepts and skills for acceleration due to gravity, including analyzing motion of objects in freefall.

## Key terms

Term | Meaning | |
---|---|---|

Acceleration due to gravity | In the absence of air resistance, all objects fall with constant acceleration |

## Analyzing motion for objects in freefall

Freefall is a special case of motion with constant acceleration, because acceleration due to gravity is always constant and downward. This is true even when an object is thrown upward or has zero velocity.

For example, when a ball is thrown up in the air, the ball's velocity is initially upward. Since gravity pulls the object toward the earth with a constant acceleration $g$ , the magnitude of velocity decreases as the ball approaches maximum height.

At the highest point in its trajectory, the ball has zero velocity, and the magnitude of velocity increases again as the ball falls back toward the earth (see figure 1).

## Common mistakes and misconceptions

**People mistakenly think the final velocity for a falling object is zero because objects stop once they hit the ground.**In physics problems, the final velocity is the speed just before touching the ground. Once it touches the ground, the object is no longer in freefall.

## Learn more

For deeper explanations of freefall, see our videos plotting projectile motion and impact velocity from a given height.

To check your understanding and work toward mastering these concepts, check out our exercises freefall concepts and graphs and using kinematic formulas to solve freefall problems.

## Want to join the conversation?

- An astronaut holds a rock at 100m above the surface of Planet X. The rock is thrown upward with a speed of 15m/s. The rock reaches the ground 10s after it is thrown. The atmosphere of Planet X has a negligible effect on the rock when it is in free fall.

Based on my calculations, I found that the acceleration due to gravity of the rock when it is on Planet X is -1.5m/s^2. How does the speed of the rock when it reaches the ground vd compare to the speed of the rock when it is thrown upward vu?

____vd > vu____vd = vu____vd < vu(15 votes)- *note: the _ symbol refers to a subscript. So, v_f refers to the final velocity.

Sorry, but I believe the acceleration is -5m/(s^2).

x_f = x_i + v_i(t) + 0.5at^2

0m = 100m + (15m/s)(10s) + 0.5a(10s)^2

-100m = 150m + 50s^2a

-250m = 50(s^2)a

a = -5m/s^2

Now to the answer. The velocity as the rock reaches the ground will be greater than the initial velocity. Just try substituting values to test this out.

v_f = v_i + at

v_f = 15m/s - 5m/s^2(10s)

v_f = 15m/s - 50m/s

v_f = -35m/s

speed_f = |v_f|

speed_f = |-35m/s|

speed_f = 35m/s

Therefore, speed_f > speed_i.(32 votes)

- "People mistakenly think the final velocity for a falling object is zero because objects stop once they hit the ground. In physics problems, the final velocity is the speed just before touching the ground. Once it touches the ground, the object is no longer in freefall." This tripped me up a lot. Take note people!(20 votes)
- What is the difference between Terminal Velocity and Constant Speed?(4 votes)
- CONSTANT VELOCITY =>If you say that velocity is constant, is just a way to say that the velocity does not change with time...;)

TERMINAL VELOCITY => When an object which is falling under the influence of gravity which increases with velocity, it will ultimately reach a maximum velocity where the drag force equals the driving force. This final, constant velocity of motion is called a "terminal velocity", a terminology made popular by skydivers..:)

HOPE U GET IT ;)(14 votes)

- How do you calculate air resistance?(13 votes)
- Does anyone here like tacos?(11 votes)
- Can an object be in free fall if it is moving upwards?(5 votes)
- How do you determine whether the acceleration (due to gravity) is positive or negative.(2 votes)
- If you dont mind me answering, we know that acceleration due to gravity is positive or negative when the question we are doing tells us that either upwards is positive or negative. For example if they tell us that upwards is positive that means downwards is negative and acceleration due to gravity is always downwards (in this context of 1d motion ofcourse) So the positive or negative sign is determined by the convention of the question.

I hope that helped. anyone in the forum please correct me if i am wrong.

thx(9 votes)

- How do you know how to rearrange the equation?? I list out my variables and pick the right equation to use, I plugin all the variables and still get the answer wrong. I look at the hints and they rearrange the equation. I don’t understand why they do that, when we learned to do that, or how to do that. Can someone please explain or point me to the right lesson where they showed how to rearrange kinetic formulas.(1 vote)
- Rearranging equations is part of algebra. There are also lots of videos on Youtube. Basically, if 1+2=3 then, 2=3-1. So if the equation is v=v0+at you would have to do Commutative property to change the subject to equal a. To make a the subject you would have to subtract v0 from both sides, so it would look like this v-v0=at. Then you would have to divide both sides by t, and that would look like this (V-V0)/t=a. Now you can plug in the values and solve for a. (Whatever you do to one side of the equal sign you have to do on the other side too). Hope this helps! ^~^(7 votes)

- This was aggravating. I need this last sentence for more letters in my sentence because it was too brief.(3 votes)
- Hey, what is the connection between gravity and air resistance?(3 votes)