# Physical Significance of Numbers

gamemania1986

## Main Question or Discussion Point

Consider this case: A ball is allowed to fall freely with a constant acceleration of 10 m/s^2. What will its speed be after travelling a distance of 10 m?

We get that:
v[initial] = 0 m/s
d = -5 m (downward direction is taken as negative)
a = -10 m/s^2

To find v[final], we will use the formula d = (v[final]^2-v[initial]^2) / (2a)

Rearranging, we will get:

By plugging the numbers, we will get:

v[final]^2 = 100 m^2/s^2

By mathematics, there are 2 solutions to this problem, that is v[initial] = +10 m/s and v[initial] = -10 m/s. I can only think of the situation where the ball has a downward velocity. Is the other answer (the one with +10 m/s velocity) possible/does it has a physical reality? If so, can anyone explain the situation? Thanks a lot!

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I think to obtain your first formula, the identity
vf - vi = a*t
is used.
Next, you divide by a to get t, and plug that into d = a/2 * t^2.

But strictly speaking, d, v, and a are vectors.
So you are in fact not allowed to divide by a.

You could work around this by saying you just talk about one component (the z-component) of each vector. But this implies that (vf-vi) has the same sign as a (i.e., minus).

Of course, this is still true in the answer.

Hi,
u consider a stationary ball 'b' directly below the falling ball and calculate the velocity of 'b' in + with respect to free falling ball.

Else, if free fall is not crucial, vertical take off of a rocket (containing the ball) with acceleration 10 m/s^2. v = u + at = final velocity of both rocket & ball.

Last edited:
Hurkyl
Staff Emeritus
Gold Member
By mathematics, there are 2 solutions to this problem, that is v[initial] = +10 m/s and v[initial] = -10 m/s. I can only think of the situation where the ball has a downward velocity. Is the other answer (the one with +10 m/s velocity) possible/does it has a physical reality? If so, can anyone explain the situation? Thanks a lot!

The -10 m/s velocity occurs when you trace the evolution of the system going forward in time. Consider the trajectory of the ball before it had the 0 m/s velocity.

Hurkyl

gamemania1986
Aaaah..... I got it! Thanks a lot Hurkyl!