# Kinematics Free falling acceleration Physics question

• Brownk
In summary, the first rock falls faster than the second rock and both rocks hit the ground at the same time.
Brownk

## Homework Statement

Okay so the question is..A stone falls from rest from the top of a cliff. A second stone is thrown downward from the same height 3.99999 seconds later with an initial speed of 78.3998 m/s. They hit the ground at the same time.

How long does it take the first stone to hit the ground? The acceleration of gravity is 9.8m/s^2.

and

How high is the cliff?

## Homework Equations

vf= vi + a*t
vf^2 = vi^2 +2*a*d
d = vi + (1/2)*a*(t^2)
where vf= final velocity, vi= initial velocity, a = acceleration, d = distance, t = time

## The Attempt at a Solution

Okay so this is my first time posting on this forum and forgive me if I do something wrong but here is what I tried to do.
For the first rock,
vf= ?, vi= 0m/s, t=?, a= 9.8m/s^2, and d=?
And the second,
vf=?, vi=78.3998m/s, t=?, a=9.8m/s^2, and d=?
Am I missing something or am I missing 3 variables for each of the rocks?? I don't really know where to go without another variable for one of the two...Thanks for any help

I'm not sure how to approach this one. Is that all the question gives?

If I had to guess, I'd assume that for stone 2, initial velocity = final velocity.

But I don't think that is the correct way to approach this.

By doing that, you could calculate the height of the cliff and then calculate the time it takes for the first stone to hit the ground using tstone1 = tstonge 2 + 3.9999

are you allowed to use conservation of energy? or is this purely kinematics?

It sounds like kinematics, given you have no masses to work with.

jarednjames said:
I'm not sure how to approach this one. Is that all the question gives?

If I had to guess, I'd assume that for stone 2, initial velocity = final velocity.

But I don't think that is the correct way to approach this.

By doing that, you could calculate the height of the cliff and then calculate the time it takes for the first stone to hit the ground using tstone1 = tstone 2 + 3.9999

Yeah, that's the question word for word.
Okay I'll try approaching it that way and edit in a few. Thanks, and yeah it's just kinematics.

Try this.

1st ball:
Given:
v1 = 0
a= 9.8m/s^2
Δd = v1Δt + 1/2aΔt^2
Δd = 4.9t^2

2nd ball:
Given:
v1=78.4m/s
a=9.8m/s

Δd = v1Δt + 1/2aΔt^2
Δd = 78.3998t + 4.9t^2

Plug equations into each other and solve for t.

Saterial said:
Try this.

1st ball:
Given:
v1 = 0
a= 9.8m/s^2
Δd = v1Δt + 1/2aΔt^2
Δd = 4.9t^2

2nd ball:
Given:
v1=78.4m/s
a=9.8m/s

Δd = v1Δt + 1/2aΔt^2
Δd = 78.3998t + 4.9t^2

Plug equations into each other and solve for t.

But the first balls time is tball2 + 3.9999. So the t values between the two equations aren't the same. So I don't think you can do it like that without include the plus 3.9999.

So first equation should be Δd = 4.9(tball2+3.9999)^2

jarednjames said:
But the first balls time is tball2 + 3.9999. So the t values between the two equations aren't the same. So I don't think you can do it like that without include the plus 3.9999.

So first equation should be Δd = 4.9(tball2+3.9999)^2

If I set those 2 equations equal then don't I have two variables? Sorry I'm still confused this problem makes no sense

You know the distance each ball falls is the same, so you can set the two equations equal to each other and solve for time.

The equation would be:
$$78.3998t_{ball2}+4.9t_{ball2}^2=4.9(t_{ball2}+3.99999)^2$$

The times in each equation are different, but are related in that the second ball is released at 3.99999 s after the first ball is released. So, at that time, $$t_{ball1} = 3.99999\,\mathrm{s}$$ and $$t_{ball2}=0$$. If we put everything in terms of $$t_{ball2}$$, then $$t_{ball1} = 3.99999\,\mathrm{s} + t_{ball2}$$, which can be verified by setting $$t_{ball2}=0$$.

Thus, we have two equations with two unknowns and we can substitute in and solve.

Last edited:

## What is kinematics?

Kinematics is a branch of physics that studies the motion of objects without considering the forces that cause the motion.

## What is free falling acceleration?

Free falling acceleration is the acceleration an object experiences when it is falling due to the force of gravity. On Earth, this acceleration is approximately 9.8 meters per second squared.

## What is the formula for calculating free falling acceleration?

The formula for calculating free falling acceleration is a = g, where a is the acceleration and g is the acceleration due to gravity.

## How does air resistance affect free falling acceleration?

Air resistance can act as a force in the opposite direction of an object's fall, which can reduce the object's acceleration. This is why objects with a larger surface area, such as a parachute, fall at a slower rate than objects with a smaller surface area.

## What is the difference between free falling and free fall?

Free falling refers to the motion of an object that is falling under the influence of gravity. Free fall, on the other hand, refers to the condition where the only force acting on an object is gravity. In other words, free falling is a type of motion, while free fall is a specific scenario in which that motion occurs.

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