First slide

Question on vertical

Question

Initial height of the body from the ground can be calculated by using the formula h = – ut + (1/2)gt² then
a) A body projected vertically with velocity 'u' from the top of tower, reaches the ground in 't' sec.
b) A body dropped from a balloon moving up with uniform velocity, reaches the ground in 't' sec
c) A body dropped from a helicopter moving up with uniform velocity, reaches the ground in 't' sec
d) A body projected vertically from the ground reaches the ground in 't' sec.

Moderate

A

a, b and c are correct
B

a, b, c and d are correct
C

a is only correct
D

b and d are correct

Solution

$\large h = - ut + \frac{1} {2}gt^2$

This equation is written under the assumption that upward physical quantities are negative and down ward physical quantities are positive, when projected from ground, for all values of 'u', calculated value of h should be negative. But if u < gt/2, calculated value of 'h' will be positive, which is not possible. so above equation cannot be used when projected from ground, so option (2) is correct.

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