Question

15. You want to calculate the displacement of an object thrown over a bridge. Using -10 m/s2 for acceleration due to gravity, what would be the total displacement of the object if it took 8 seconds before hitting the water?

Answer 1

Do you know the formula
\[x_{final} = x_{initial}+v_{initial}(t) + \frac{1}{2}a^2\]
The initial position is at 0.
The initial velocity is also at 0.
It says to use -10m/s^2 for acceleration.
It also says it took 8 seconds to hit the water.

Plug that into the equation we get:
\[x = 0+0(8) + \frac{1}{2}(-10)^2\]

Can you solve for x here?

Once you solve that displacement has a direction, and since the direction the ball is falling is downwards, it would be negative.

Answer 2

why is the initial velocity = 0 ?

"dropped", maybe works.

and the sign convention - is upwards or downwards positive

i think those might be the more important questions

Answer 3

Because before you throw it the ball isn't moving

so initial velocity is 0

Upwards is positive downwards is negative

Answer 4

yes

Qu: why is downwards negative?

Answer 5

Basically in an XY coordinate plane going downwards is negative.

Answer 6

for every quantity ?

Answer 7

If you're talking about an object moving then yes

Answer 8

can't we define acceleration as upward?

Answer 9

If the ball was thrown upwards then yeah it'll be going positive.
But gravity pulls the ball downwards

Answer 10

forgive me: but on earth, you can only ever have one co-ordinate system for kinematic problems?

Answer 11

i think that you're really talking about vectors