What is the correct formula for solving time in free fall?

[ammora313]

Homework Statement

t=2v/g

the AAMC used this formula to solve a problem that was asking what would happen to time if the gravity becomes g/6. the answer is that the time increases by a factor of 6.

Homework Equations

v=v₀ + at

The Attempt at a Solution

I got the wrong answer by thinking that i can use the free fall equation x=1/2 gt² Thanks in advance.

 

[haruspex]

Homework Statement

t=2v/g

the AAMC used this formula to solve a problem that was asking what would happen to time if the gravity becomes g/6. the answer is that the time increases by a factor of 6.

Homework Equations

v=v₀ + at

The Attempt at a Solution

I got the wrong answer by thinking that i can use the free fall equation x=1/2 gt² Thanks in advance.

You need to post the entire original question.

 

[ammora313]

You need to post the entire original question.

 

[haruspex]

Next time, please try to post the image the right way up.

thinking that i can use the free fall equation x=1/2 gt2

But you don't know x, so how can you use that? Did you assume the height reached would be the same in both gravities?
There are 5 variables in the 5 SUVAT equations. Each equation uses four of them. Given the values of three and the need to determine a fourth, pick the one with those four.
Which three do you know here, and which is to be determined?

 

[ammora313]

I'm sorry about that, I kept trying to make it right side up but it wouldn't budge.

The one I need to determine is the time. The ones I know are acceleration (gravity) and velocity.
There is the equation v=v₀ + 1/2 at² but I'm not sure how to manipulate it to come out to t=2v/g. When I tried to make it equal to t, I get t=v-v₀/a. I don't understand where they get 2v from since the v₀ is added on one side, wouldn't it get subtracted if it moves to the other side?

 

[haruspex]

There is the equation v=v₀ + 1/2 at² but I'm not sure how to manipulate it

There is no such SUVAT equation. Do you mean s= v₀ t + (1/2) at² , or v=v₀ + at?
Also, to what process are you applying this? E.g. the whole process from throwing the ball up to its landing again?

 

[ammora313]

I'm sorry that was a typo. The equation was v=v₀ + at
I was applying it to the whole process, from when the ball was thrown up to it landing

 

[haruspex]

I'm sorry that was a typo. The equation was v=v₀ + at
I was applying it to the whole process, from when the ball was thrown up to it landing

OK, so v₀ is the velocity at which it was thrown up. What's the velocity just before it lands?

 

[ammora313]

the velocity before it lands is v=√2gh

 

[haruspex]

the velocity before it lands is v=√2gh

OK, you can do it that way if you can figure out h. Alternatively, what do you think the relationship is between the velocity it was thrown up with and the velocity it lands with?

 

[ammora313]

OK, you can do it that way if you can figure out h. Alternatively, what do you think the relationship is between the velocity it was thrown up with and the velocity it lands with?

Would the two velocities by equal? 2v?

 

[haruspex]

Would the two velocities by equal?

Not equal, exactly. Remember, a velocity is a vector, it has direction.

2v?

What are you asking?

 

[ammora313]

Not equal, exactly. Remember, a velocity is a vector, it has direction.

What are you asking?

I'm trying to figure out how they came up with t=2v/g

 

[haruspex]

I'm trying to figure out how they came up with t=2v/g

Yes, I know that, but your last post finished with the question "2v?". I have no idea what you are asking there.

 

[ammora313]

Oh ok sorry about that, I didn't know if that's where the 2v in the equation came from. If they're talking about the initial v and the v before it lands.

 

[haruspex]

Oh ok sorry about that, I didn't know if that's where the 2v in the equation came from. If they're talking about the initial v and the v before it lands.

OK, let's get back to where we were. What is the relationship between the initial and final velocities? They are not equal because they are not in the same direction, but it is a very simple relationship.

 

[ammora313]

Are they inversely proportional to each other? Equal but opposite signs?

 

[haruspex]

Are they inversely proportional to each other? Equal but opposite signs?

Two very different propositions. Don't guess - which is it? You should be able to pick the right one from everyday experience.

 

[ammora313]

Ok they are inversely proportional to each other since they are vectors

 

[haruspex]

Ok they are inversely proportional to each other since they are vectors

Do you understand what inversely proportional means? It would mean that one of them equals some constant divided by the other. There is no such process as division by a vector.
If you throw a ball up at 3m/s and catch it at the same height as it comes down, how fast do you think it will be traveling when you catch it?

 

[ammora313]

It'll be traveling at 3 m/s when I catch it. I'm sorry I guess I meant they are proportional to each other but that would be wrong since they are equal with just opposite directions

 

[collinsmark]

It'll be traveling at 3 m/s when I catch it. I'm sorry I guess I meant they are proportional to each other but that would be wrong since they are equal with just opposite directions

Right, they are in opposite directions, so there is a minus sign that needs to fit in there somewhere.

So plug that into v_f = v_i + at noting that one of the velocities is a negative value (and taking appropriate care in whether the acceleration is up or down [positive or negative], with your chosen conventions).

There's another way to approach this derivation you don't like working with the velocity formula. Start with

y = vt + \frac{1}{2}a t^2

where v is the initial velocity. You know that the ball finishes at the same location that it starts. In other words, you know that the ball starts at y = 0 and ends at y = 0. Solve for t. (Substitute whatever you need to in for a, keeping attention to whether it is up or down [positive or negative] given your chosen conventions.)

 

[ammora313]

Ok so if I let V_i be negative:

v_f = -v_i + at

If I move v_i to the other side it becomes 2v

2v=at
2v/a=t
2v/g=t

is that correct?

 

[collinsmark]

Ok so if I let V_i be negative:

v_f = -v_i + at

If I move v_i to the other side it becomes 2v

2v=at
2v/a=t
2v/g=t

is that correct?

That is one valid way to do it, yes. In that convention that you chose, down is positive. Since the ball initially thrown up, you tacked the negative sign on v_i (and this assumes that v_i itself is a positive number, equal to v_f).

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Edit: Or perhaps even better, you could have kept the equation in it's original form, v_f = v_i + at, and then made the substitution that v_f = -v_i. That way the formula remains the same, and it's only the values that change sign.
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You also could have done it using the other convention where up is positive and down is negative. In that case a = -g, the initial velocity is positive and the final velocity is negative.

But if you didn't figure out before-hand that |v_i| = |v_f| then you could have derived the equation with y_f = y_i + vt + \frac{1}{2}at^2 knowing full well that y_i = y_f, using whichever convention that you chose (the convention of whether up or down is positive). So that's yet another way.

 

[ammora313]

Thanks so much for your help and for being patient with me. You're awesome!