(C) The ruler drops under free fall. Therefore,the initial velocity $u = 0$ and acceleration $a = g = 9.8 \; m/s^2$.The distance travelled $d$ and the

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(C) The ruler drops under free fall. Therefore,the initial velocity $u = 0$ and acceleration $a = g = 9.8 \; m/s^2$.The distance travelled $d$ and the reaction time $t_r$ are related by the kinematic equation:$d = ut_r + \frac{1}{2}gt_r^2$Since $u = 0$,we have:$d = \frac{1}{2}gt_r^2$$t_r = \sqrt{\frac{2d}{g}}$Given $d = 21.0 \; cm = 0.21 \; m$ and $g = 9.8 \; m/s^2$:$t_r = \sqrt{\frac{2 \times 0.21}{9.8}}$$t_r = \sqrt{\frac{0.42}{9.8}}$$t_r = \sqrt{0.0428} \approx 0.207 \; s$Rounding to one significant figure or the nearest given option,the reaction time is approximately $0.2 \; s$.

Class 11 Physics Motion in Straight Line Motion Under Gravity

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 the 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 the reaction time in seconds. (in $; s$)

A
$0.1$
B
$0.25$
C
$0.2$
D
$0.15$

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Class 11 Questions

Class 11

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Chapter

Motion in Straight Line

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Motion Under Gravity

Write the equation for the distance covered by an object falling freely during the $n^{th}$ second.

Easy

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The ratio of times taken by a freely falling body to travel the first $5 \ m$, second $5 \ m$, and third $5 \ m$ distances is:

MediumTS EAMCET 2025

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$A$ body thrown vertically upwards from the ground reaches a maximum height $H$. The ratio of the velocities of the body at heights $\frac{3H}{4}$ and $\frac{8H}{9}$ from the ground is

EasyAP EAMCET 2024

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$A$ boy standing at the top of a tower of $20 \, m$ height drops a stone. Assuming $g = 10 \, m/s^2$,the velocity with which it hits the ground is ......... $m/s$.

EasyAIPMT 2011

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$A, B, C$ are points in a vertical line such that $AB = BC$. If a body falls freely from rest at $A$, and $t_1$ and $t_2$ are the times taken to travel distances $AB$ and $BC$ respectively, then the ratio $(t_2 / t_1)$ is:

MediumTS EAMCET 2015

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Write the equation for the distance covered by an object falling freely during the $n^{th}$ second.

The ratio of times taken by a freely falling body to travel the first $5 \ m$, second $5 \ m$, and third $5 \ m$ distances is:

$A$ body thrown vertically upwards from the ground reaches a maximum height $H$. The ratio of the velocities of the body at heights $\frac{3H}{4}$ and $\frac{8H}{9}$ from the ground is

$A$ boy standing at the top of a tower of $20 \, m$ height drops a stone. Assuming $g = 10 \, m/s^2$,the velocity with which it hits the ground is ......... $m/s$.

$A, B, C$ are points in a vertical line such that $AB = BC$. If a body falls freely from rest at $A$, and $t_1$ and $t_2$ are the times taken to travel distances $AB$ and $BC$ respectively, then the ratio $(t_2 / t_1)$ is:

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