# Finding Time For Projectile Motion

1. Dec 15, 2011

### phhysicssai

1. The problem statement, all variables and given/known data

A marble launcher shoots a marble with a launch velocity of 6.89 m/s @15.0 degrees above horizontal.

-Find the marble's range.

(I tried to do this but I need to be able to find the time for horizontal motion... how would I do that?)

2. Relevant equations

3. The attempt at a solution

2. Dec 15, 2011

### PeterO

The time is calculated by considering the vertical component of the motion, and treating it like any other vertical motion example.

Or you could have a read of the

http://en.wikipedia.org/wiki/Range_of_a_projectile

then you only need a substitution - provided your projectile is landing at the same level it was launched from and not from/onto a hill/cliff/building.

3. Dec 16, 2011

### jack343

Because its 6.89m/s @ 15.0deg, you can use trigonometric functions to find the initial vertical and horizontal speed:

vertical:
sin(15.0) = o / 6.89

horizontal:
cos(15.0) = a / 6.89

Since the horizontal velocity is constant as gravity does not affect it, the time the projectile is in the air can be found by:

v = a*t

where

v = vertical
a = acceleration due to gravity
t = time of projectile

Once you have the time of the projectile its simple.

We know that distance = velocity * time

-> d = v * t

where d is the "range" we are trying to solve for
where v is the HORIZONTAL velocity
where t is the time of the projectile

Once you have done this, post your work/solution so I can help you further. Nobody here is going to do you homework for you, we can only HELP you if you are trying.

4. Dec 17, 2011

### LawrenceC

jack343, you are forgetting that the projectile goes up AND down so the time of flight is double what you calculate.