Projectile Motion Question

  • Thread starter jumbo123
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  • #1
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Homework Statement


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.


Homework Equations


The equations of motion.

The Attempt at a Solution


I know that I must deal separately with the horizontal and vertical components of the motion.
I have worked out that the final vertical velocity will be 19.8m/s but I don't where to go next to find u.
 

Answers and Replies

  • #2
gneill
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Write an expression for the speed at which it hits the ground. Set it equal to the required condition.
 
  • #3
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I have realised that I now have a triangle where the x component is u, y component is 19.8 and the hypotenuse is 3u. So I could easily work out u and then the angle by using simple trig.

Is this correct?
 
  • #4
gneill
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20,913
2,862
Looks good! Perhaps consider using Pythagoras.
 

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