Subscribe to our Youtube Channel - https://you.tube/teachoo

Last updated at June 7, 2019 by Teachoo

Transcript

NCERT Question 18 A ball thrown up vertically returns to the thrower after 6 s. Find (a) the velocity with which it was thrown up, (b) the maximum height it reaches, and (c) its position after 4 s. Max Height Reaches Highest Point Max Height And comes back to the same point Total Time Taken = 6 seconds Since it takes 6 seconds for the ball to return Therefore, ∴ Time taken to reach maximum height = 6/2 = 3 seconds Now, solving all parts the velocity with which it was thrown up, We need to find Initial Velocity Let Initial velocity = u At the highest point of motion, velocity becomes 0 Therefore Final velocity = v = 0 m/s And Acceleration = −g = −9.8 m/s2 Time = 3 seconds We know v, a and t (Acceleration is negative because ball is thrown upwards) Finding u by 1st equation of motion v = u + at 0 = u + (−9.8) (3) 0 = u − 29.4 u = 29.4 m/s Thus, initial velocity is 29.4 m/s (b) the maximum height it reaches, and Calculating maximum height (s) We know u, t and at Finding distance using 2nd equation of motion s = ut + 1/2 at2 = 29.4 × 3 + 1/2 (−9.8) (3)2 = 88.2 − 1/2 × 9.8 × 9 = 88.2 − 44.1 = 44.1 m Thus, maximum height it reaches is 44.1 m (c) its position after 4 s. The ball reaches highest point in 3 seconds. At 4 seconds, it will be moving downward. This is same as A ball is dropped from highest point We need to calculate distance it moves in 1 second When ball is dropped, Initial velocity = 0 m/s Time = 1 second Acceleration = g = 9.8 m/s We need to find s We know u, a and t Finding s using 2nd equation of motion. s = ut + 1/2 at2 s = 0 × 1 + 1/2 × 9.8 × (1)2 (Positive acceleration as it is moving down) s = 9.8/2 s = 4.9 m So, Position of ball is 4.9 from the top Therefore, ∴ The initial velocity of the ball is 29.4 m/s The maximum height it reaches is 44.1 m It’s position of ball at t = 4s is 4.9 m (from top) (from the top)

NCERT Questions

NCERT Question 1

NCERT Question 2

NCERT Question 3

NCERT Question 4

NCERT Question 5

NCERT Question 6

NCERT Question 7

NCERT Question 8

NCERT Question 9

NCERT Question 10

NCERT Question 11

NCERT Question 12

NCERT Question 13

NCERT Question 14

NCERT Question 15

NCERT Question 16

NCERT Question 17

NCERT Question 18 You are here

NCERT Question 19

NCERT Question 20

NCERT Question 21

NCERT Question 22

Class 9

Chapter 10 Class 9 - Gravitation

About the Author

Davneet Singh

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.