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# Projectile Motion Question

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1. I'm completely stuck on how to do this question.

A ball is thrown horizontally with speed u from a height of 20m. It hits the ground with a speed 3u. Find the value of u, and the angle at which the ball strikes the ground.
Any help is appreciated.
2. Divide the question into vertical and horizontal components.

Horizontally, final velocity = 3u, acceleration = 0, initial velocity = 0

Vertically, acceleration = 9.8, distance = 20, final velocity = 0

Hope this helps you to work through the question
3. (Original post by foldingstars45)
Divide the question into vertical and horizontal components.

Horizontally, final velocity = 3u, acceleration = 0, initial velocity = 0

Vertically, acceleration = 9.8, distance = 20, final velocity = 0

Hope this helps you to work through the question
the horizontal final velocity will be u... the same as the initial horizontal velocity

the vertical final velocity will not be zero
4. I have worked out the final vertical velocity to be 19.8m/s but I don't know where to go next to find u.

Edit to add: I have realised that I now have the x component to be u, y component to be 19.8 and the hypotenuse to be 3u. So I could easily work out u and then the angle. Am I correct with this?

5. http://img269.imageshack.us/img269/3148/motion.png

Horizontal velocity do not change with the gravity
On Vertical Velocity, motion is like the free fall.
In terms of final Velocity, calculations are about the vectors.
6. Yes. When the object hits the ground its horizontal component is unchanged at u
It's vertical component is the result of falling from rest for 20m with acceleration g
That gives the vertical component as 19.8m/s
3u is the vector sum of u and 19.8m/s at right angles.
From this you can find u and the angle from Pythagoras and trig.
7. (Original post by Haulward)

http://img269.imageshack.us/img269/3148/motion.png

Horizontal velocity do not change with the gravity
On Vertical Velocity, motion is like the free fall.
In terms of final Velocity, calculations are about the vectors.
Thanks.

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