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Throwing a ball upwards

  • Thread starter robax25
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  • #1
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Homework Statement



A child tosses a ball directly upward. Its total time in the air is T. Its maximum height is H. What is its height after it has been in the air a time T/4? Neglect air resistance.

Homework Equations


y=vot +0.5 at²

The Attempt at a Solution


It is 1/4 H but I don't get exact answer.
 
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Answers and Replies

  • #2
stockzahn
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Homework Statement



A child tosses a ball directly upward. Its total time in the air is T. Its maximum height is H. What is its height after it has been in the air a time T/4? Neglect air resistance.

Homework Equations


y=vot +0.5 at²

The Attempt at a Solution


It is 1/4 H but I don't get exact answer.
What answer do you get? Please show.

At what time do you think the ball reaches the maximal height H?
 
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  • #3
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t/2 second
 
  • #4
stockzahn
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t/2 second
What value are you able to obtain with that information and the equation you posted in section 2 of the template in your first post?
 
  • #5
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y=v(t/2)-0,5a(t/2)² when the ball is at maximum height and vo is equal to gt/2
 
  • #6
Ray Vickson
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y=v(t/2)-0,5a(t/2)² when the ball is at maximum height and vo is equal to gt/2
So, you can find ##t## from the first equation (using ##y = H##, a given input in the problem); then, after finding ##t## you can figure out ##v_0##. Take it from there.

By the way: either use ##g## or ##a##, but not both notations in the same problem, given that they are both supposed to be the same quantity.
 
  • #7
stockzahn
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y=v(t/2)-0,5a(t/2)² when the ball is at maximum height and vo is equal to gt/2
Stay with the definition that ##v_0## is the velocity of the ball when it leaves the child's hand, which is what you want to calculate to find the answer of the original problem (##v_0=f\left(g, T, H\right)##).
 
  • #8
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There is no need to find the initial velocity.
t/2 second
What is t? Why did you add "seconds"?
I guess you mean T/2 - just half of the total air time. That is correct.

Consider the second half of the motion for a moment: How can you describe how far it fell down from the highest point as a function if time?
It drops down by H over a time of T/2. How much does it drop down in half of that time, in T/4? While that is not the final answer to the problem you can transfer that result to get the right answer.
 

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