# Just joined, not expecting answer to question just how to do it

1. Sep 5, 2006

Alright guys, I just signed up to this forum cause I heard from a friend that you guys can help. I dont really want a solution to the problem, I just need help guiding me towards the solution. The problem is...

A gymnast works out on a trampoline. At the instant that she leaves the trampoline, a point on her waist is 2.3 m above the floor and at the center of the trampoline. At that instant, the point has an upward velocity of 7.8m/s and a horizontal velocity of 3.0 m/s. Write equations that describe the subsequent motion of that point and find its maximum height.

All ive got so far (which is probably wrong is): Xo = 0m VoX = 3m/s Yo = 2.3m Voy = 7.8 m/s as well as ax= 3m/s^2 and ay = -9.8 m/s^2

If that is right what would be the function that I plug these values into, and if not, where did I make my error. Thanks for anyone that can get me a little guidance here!

2. Sep 5, 2006

### Päällikkö

There is no acceleration in the x-direction.

The basic equation:
$$x = x_0 + v_0t + \frac{1}{2}at^2$$

3. Sep 5, 2006

Well doesnt she go 3m/s on x axis, or is there no kind of friction in this problem?

4. Sep 5, 2006

### dmoravec

well, the only error I can see is the ax=3m/s^2. You have an initial horizontal velocity, but there is no horizontal accelaration. You might want to make use of the formula
$$s_{x}(t)=s_{o} + v_{o}t + \frac{1}{2}a_{o}t^2$$
where s is the position, v is the velocity and a is the accelaration.

Last edited: Sep 5, 2006
5. Sep 5, 2006

### dmoravec

damn.... not fast enough at the latex writing yet...

6. Sep 5, 2006

### Päällikkö

Yes, but there certainly are no forces accelerating her movement in the horizontal direction.

7. Sep 5, 2006

so x = 0m + (7.8m/s) + 1/2 * (-9.8m/s^2) ? Im sorta confused how I take the 7.8 m/s and include it with gravity

8. Sep 5, 2006

### dmoravec

well, if you include the time variable as well as your initial hight of 2.3 m
y(t) = 2.3 m + 7.8 m/s*t + 1/2 * -9.8 m/s^2 * t^2
and the output y(t) will be your height at time t

9. Sep 5, 2006

So its basicaly (original height) + (Upward velocity) + effects of gravity over time?
or something like that

Last edited: Sep 5, 2006
10. Sep 5, 2006