A soccer player kicks a rock horizontally off a 40.0-m-high
cliff into a pool of water. If the player hears the sound of
the splash 3.00 s later, what was the initial speed given
to the rock? Assume that the speed of sound in air is
343 m/s.
WHY in my answer book do they say gravity is NEGATIVE???
the rock is falling DOWN?
I think I understand the -40m = Yf since it goes 40 m BELOW the cliff, BUT WHATS WITH THE NEGATIVE GRAVITY????
btw the answer set up looks like
" Yf=Yi + Vyi t + 1/2 ay t^2
= -40m= 0 +0 + 1/2 (-9.8m/s^2) t^2
my only question is why is 9.8 NEG???
thank you sooo much.
cliff into a pool of water. If the player hears the sound of
the splash 3.00 s later, what was the initial speed given
to the rock? Assume that the speed of sound in air is
343 m/s.
WHY in my answer book do they say gravity is NEGATIVE???
the rock is falling DOWN?
I think I understand the -40m = Yf since it goes 40 m BELOW the cliff, BUT WHATS WITH THE NEGATIVE GRAVITY????
btw the answer set up looks like
" Yf=Yi + Vyi t + 1/2 ay t^2
= -40m= 0 +0 + 1/2 (-9.8m/s^2) t^2
my only question is why is 9.8 NEG???
thank you sooo much.
-
When you use equations like:
Yf=Yi + Vyi t + 1/2 ay t^2
you have to remember displacement, velocity and acceleration are vectors; they have directions.
It is usual to make upwards vectors positive and downwards ones negative. (But you could do it the other way around as long as you were consistent.)
The acceleration due to gravity is caused by an object's weight, so it acts downwards the same direction as th weight; that's why it should be given a negative sign.
If you don't use a negative sign for g, your calculations will show the height getting bigger and bigger! Try it with some made- up figures to see it for yourself.
Yf=Yi + Vyi t + 1/2 ay t^2
you have to remember displacement, velocity and acceleration are vectors; they have directions.
It is usual to make upwards vectors positive and downwards ones negative. (But you could do it the other way around as long as you were consistent.)
The acceleration due to gravity is caused by an object's weight, so it acts downwards the same direction as th weight; that's why it should be given a negative sign.
If you don't use a negative sign for g, your calculations will show the height getting bigger and bigger! Try it with some made- up figures to see it for yourself.
-
Gravity is relative to the person who answers the question.
generally gravity is positive when something is falling and negative when something is raising, however you can say that gravity is negative when an object is falling and positive when raising.
make sure to state your assumption in the given (wheter you used gravity as negative or positive)
generally gravity is positive when something is falling and negative when something is raising, however you can say that gravity is negative when an object is falling and positive when raising.
make sure to state your assumption in the given (wheter you used gravity as negative or positive)