Reaction time : When a situation demands our immediate action, it takes some time before we really r

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2.Motion in Straight Line

easy

Reaction time : When a situation demands our immediate action, it takes some time before we really respond. Reaction time is the time a person takes to observe, think and act. For example, if a person is driving and suddenly a boy appears on the road, then the time elapsed before he slams the brakes of the car is the reaction time. Reaction time depends on complexity of the situation and on an individual.

You can measure your reaction time by a simple experiment. Take a ruler and ask your friend to drop it vertically through the gap between your thumb and forefinger . After you catch it, find the distance $d$ travelled by the ruler. In a particular case, $d$ was found to be $21.0\; cm$. Estimate reaction time in second.

$0.1$

$0.25$

$0.2$

$0.15$

Solution

The ruler drops under free fall. Therefore, $v_{e}=0,$ and $a=-g=-9.8 \mathrm{m} \mathrm{s}^{-2} .$ The distance travelled $d$ and the reaction time $t_{r}$ are related by

$d=-\frac{1}{2} g t_{r}^{2}$

Or, $t_{r}=\sqrt{\frac{2 d}{g}} \mathrm{s}$

Given $d=21.0 \mathrm{cm}$ and $g=9.8 \mathrm{ms}^{-2}$ the reaction time is

$4 \sqrt{\frac{28\times 0.21}{28}}{=0.2}\;s$

Standard 11

Physics

A body is projected with a velocity $u$. It passes through a certain point above the ground after $t_1$ second. The time interval after which the body passes through the same point during the return journey is

hard

A stone dropped from the top of a tower is found to travel $\left(\frac{5}{9}\right)$ of the height of the tower during the last second of its fall. The time of fall is ……. $s$

medium

A boy throws balls into air at regular interval of $2$ second. The next ball is thrown when the velocity of first ball is zero. How high do the ball rise above his hand is …… $m$ [Take $g=9.8 \,m / s ^2$ ]

easy

A lift is coming from $8^{th}$ floor and is just about to reach $4^{th}$ floor. Taking ground floor as origin and positive direction upwards for all quantities, which one of the following is correct?

medium

A particle is projected from ground in vertical upward direction at $t = 0$, with initial velocity $48\, m/s$ and the distance travelled by particle in $5^{th}$ second is……$m$ $(g = 10\, m/s^2)$

normal