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What is the initial speed of the ball?

  • Thread starter elitespart
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  • #1
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A ball is projected horizontally from the edge of a table that is 1.00m high, and it strikes the floor at a point 1.20m from the base of the table.

a) What is the initial speed of the ball?
b) How high is the ball above the floor when its velocity vector make a 45.0° angle with the horizontal?

...So far all I got is that it take .102 seconds for the ball to hit the ground and I think average speed is 11.8 m/s. Please help.
 

Answers and Replies

  • #2
radou
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elitespart said:
A ball is projected horizontally from the edge of a table that is 1.00m high, and it strikes the floor at a point 1.20m from the base of the table.

a) What is the initial speed of the ball?
b) How high is the ball above the floor when its velocity vector make a 45.0° angle with the horizontal?

...So far all I got is that it take .102 seconds for the ball to hit the ground and I think average speed is 11.8 m/s. Please help.
How did you get the 0.102 seconds? Doesn't look correct. Could you present the way you got that result? Since the ball is thrown horizontally, the equation of displacement for the y-direction is pretty simple.
 
  • #3
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oh my bad. That's wrong. But then how would I find it?
 
Last edited:
  • #4
radou
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elitespart said:
oh my bad. That's wrong. But then how would I find it?
From the equation y(t) = 1/2*g*t^2.
 
  • #5
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ok im not getting what im supposed to do with this equation. can u help me out please
 
  • #6
radou
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This equation represents the displacement of the ball in the y-direction (vertical), right? And you know the height of the table (i.e. the distance from the top of the table to the ground). So, just plug it in the equation and calculate the time it takes for the ball to fall on the floor.
 
  • #7
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ok so it'd be like 1.0=1/2(9.81) x t^2? and then i jus isolate the t^2.
 
  • #8
radou
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elitespart said:
ok so it'd be like 1.0=1/2(9.81) x t^2? and then i jus isolate the t^2.
Exactly. After getting the time, use x = v0 * t for the initial speed.
 
  • #9
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Alrite so i got 2.21 seconds. What's v0?
 
  • #10
radou
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elitespart said:
Alrite so i got 2.21 seconds. What's v0?
You know the displacement in the x-direction equals 1.2 m, and you just calculated the time. And you saw what is written in my post above. I think that's enough. :smile:
 
  • #11
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k so initial speed is 4.42 m.s?
 
  • #12
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no ... t = 2.21 seconds is incorrect, it should be t = 0.452 seconds.
 
  • #13
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ok yea i got that. so then initial speed would be .542 m/s.
 
  • #14
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no again ... (delta x)/(time) = speed

1.2/0.452 = ?
 
  • #15
95
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2.65 m/s. damn im stupid. thx bro. appreciate it. now i jus gotta figure out b.
 
Last edited:

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