(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 a 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

Physics Questions

Chapter

Motion in a Straight Line

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Motion Under Gravity

$A$ person sitting on the ground floor of a building notices through a window of height $1.5 \; m$ that a ball dropped from the roof of the building crosses the window in $0.1 \; s$. What is the velocity (in $m/s$) of the ball when it is at the topmost point of the window? (Take $g = 10 \; m/s^2$)

MediumNEET 2020

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$A$ man throws a ball vertically upward and it rises to a height of $20 \; m$ before returning to his hands. What was the initial velocity $(u)$ of the ball and for how much time $(T)$ did it remain in the air? $[g = 10 \; m/s^2]$

Difficult

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From the top of a tower,two stones,whose masses are in the ratio $1:2$,are thrown: one straight up with an initial speed $u$ and the second straight down with the same speed $u$. Neglecting air resistance,which of the following is true?

Easy

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It is a common observation that rain clouds can be at about a kilometer altitude above the ground.
$(a)$ If a rain drop falls from such a height freely under gravity,what will be its speed? Also calculate in $km/h$ $(g = 10\, m/s^2)$.
$(b)$ $A$ typical rain drop is about $4\, mm$ in diameter. Momentum is mass $\times$ speed in magnitude. Estimate its momentum when it hits the ground.
$(c)$ Estimate the time required to flatten the drop.
$(d)$ Rate of change of momentum is force. Estimate how much force such a drop would exert on you.
$(e)$ Estimate the order of magnitude force on an umbrella. Typical lateral separation between two rain drops is $5\, cm$.
(Assume that the umbrella is circular and has a diameter of $1\, m$ and the cloth is not pierced through.)

Medium

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$A$ ball is thrown up vertically with a certain velocity so that it reaches a maximum height $h$. Find the ratio of the times at which it is at height $\frac{h}{3}$ while going up and coming down respectively.

DifficultJEE MAIN 2021

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$A$ person sitting on the ground floor of a building notices through a window of height $1.5 \; m$ that a ball dropped from the roof of the building crosses the window in $0.1 \; s$. What is the velocity (in $m/s$) of the ball when it is at the topmost point of the window? (Take $g = 10 \; m/s^2$)

$A$ man throws a ball vertically upward and it rises to a height of $20 \; m$ before returning to his hands. What was the initial velocity $(u)$ of the ball and for how much time $(T)$ did it remain in the air? $[g = 10 \; m/s^2]$

From the top of a tower,two stones,whose masses are in the ratio $1:2$,are thrown: one straight up with an initial speed $u$ and the second straight down with the same speed $u$. Neglecting air resistance,which of the following is true?

$A$ ball is thrown up vertically with a certain velocity so that it reaches a maximum height $h$. Find the ratio of the times at which it is at height $\frac{h}{3}$ while going up and coming down respectively.

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