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A body is projected up with a speed u an...

A body is projected up with a speed `u` and the time taken by it is T to reach the maximum height H. Pich out the correct statement

A

it reaches `(H)/(2)` in `(T)/(2)` sec

B

It acquires velocity `(u)/(2)` in (T)/(2)` sec

C

Its velocity is `(u)/(2)` at `(H)/(2)`

D

same velocity at 2T

Text Solution

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The correct Answer is:
To solve the problem, we need to analyze the motion of a body projected upwards with an initial speed \( u \) under the influence of gravity. The time taken to reach the maximum height is given as \( T \). ### Step-by-Step Solution: 1. **Understanding the Motion**: - When a body is projected upwards, it moves against the gravitational force until it reaches its maximum height \( H \). At this point, its velocity becomes zero. 2. **Using the Equations of Motion**: - We can use the following equations of motion: 1. \( s = ut + \frac{1}{2} a t^2 \) 2. \( v - u = at \) 3. \( v^2 - u^2 = 2as \) Here, \( s \) is the displacement, \( u \) is the initial velocity, \( v \) is the final velocity, \( a \) is the acceleration (which is \( -g \) for upward motion), and \( t \) is the time. 3. **Finding the Maximum Height \( H \)**: - At maximum height, the final velocity \( v = 0 \). - Using the second equation: \[ 0 - u = -gT \implies u = gT \] - Now, substituting \( u \) into the first equation to find \( H \): \[ H = uT - \frac{1}{2}gT^2 = gT \cdot T - \frac{1}{2}gT^2 = gT^2 - \frac{1}{2}gT^2 = \frac{1}{2}gT^2 \] 4. **Analyzing the Options**: - **Option 1**: "It reaches \( H/2 \) in time \( T/2 \)". - Using the first equation for \( T/2 \): \[ s = u \cdot \frac{T}{2} - \frac{1}{2}g\left(\frac{T}{2}\right)^2 = gT \cdot \frac{T}{2} - \frac{1}{2}g\frac{T^2}{4} = \frac{gT^2}{2} - \frac{gT^2}{8} = \frac{4gT^2 - gT^2}{8} = \frac{3gT^2}{8} \] - Since \( H/2 = \frac{1}{4}gT^2 \), this option is incorrect. - **Option 2**: "It acquires velocity \( u/2 \) in \( T/2 \) seconds". - Using the second equation: \[ v = u - g\frac{T}{2} = gT - g\frac{T}{2} = g\frac{T}{2} = \frac{u}{2} \] - This option is correct. - **Option 3**: "Its velocity is \( u/2 \) at \( H/2 \)". - Using the third equation: \[ v^2 = u^2 - 2g\left(\frac{H}{2}\right) = (gT)^2 - 2g\left(\frac{1}{2}gT^2\right) = g^2T^2 - g^2T^2 = 0 \] - This option is incorrect. - **Option 4**: "The same velocity at \( 2T \)". - The velocity will be equal in magnitude but opposite in direction when it returns to the starting point. Thus, this option is incorrect. ### Conclusion: The correct statement is **Option 2**: "It acquires velocity \( u/2 \) in \( T/2 \) seconds".

To solve the problem, we need to analyze the motion of a body projected upwards with an initial speed \( u \) under the influence of gravity. The time taken to reach the maximum height is given as \( T \). ### Step-by-Step Solution: 1. **Understanding the Motion**: - When a body is projected upwards, it moves against the gravitational force until it reaches its maximum height \( H \). At this point, its velocity becomes zero. 2. **Using the Equations of Motion**: ...
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Knowledge Check

  • A body when projected up with an initial velocity u goes to a maximum height h in time t and then comes back at the point of projection. The correct statement is :

    A
    the average velocity is 2h/t
    B
    the acceleration is zero
    C
    the final velocity on reaching the point of projection is 2u.
    D
    the displacement is zero.
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