# Calculating initial Velocity

1. Sep 26, 2011

### doc2142

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

See image
2. Relevant equations

See image
3. The attempt at a solution

I have tried, but I honestly have no Idea how to start this problem.

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2. Sep 26, 2011

### Staff: Mentor

Start by listing kinematic equations that might be useful. What's the maximum height of a launched projectile? How much time does it take to reach its maximum height?

The problem doesn't explicitly say what height the potato may have when it passes over the screen, so I suppose that you could assume that it just clears it at the screen's height.

Hint: can you find the equation for the trajectory of a "reverse" potato launched from the center of the wall opening and just clearing the screen?

3. Sep 26, 2011

### doc2142

I am a bit confused about your hint. a reverse? what is that exactly mean like wouldn't it need a velocity to go from the hole in the wall to the top of the screen?

4. Sep 26, 2011

### Staff: Mentor

Yup. One property of projectile motion is that its trajectories are "reversible". You can launch a projectile and plot its trajectory to its landing point, and if you note its landing angle and speed, you can launch it from there back along its trajectory by using the same speed and angle to do so.

So what velocity (x and y components) would take a potato from the center of the window to the top of the screen as the trajectory apex?

5. Sep 27, 2011

### JHamm

Could you simplify this somewhat by removing 10cm of height from everything, setting your axis inline with where the projectile begins? Then perhaps consider the system as symmetric about the screen?

6. Sep 27, 2011

### Staff: Mentor

Yes! Those are two easy simplifications that hardly require any elbow grease to carry out

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