Laws of Falling Bodies

  • #1

Homework Statement



If a falling object overcome 72 metres in 6 second, how much distance it overcame in first 3 second.

Homework Equations


s = ut+(at^2)/2

The Attempt at a Solution


Here,
s = 72 m
u = 0
t = 6s
a = ?
We know,
s = ut+(at^2)/2
Or, 72 = 0*6 + (a*6^2)/2
Or, 72 = 36a/2
Or, 72 = 18a
Or, a = 4 m/s

In the second part,
u = 0
t = 3s
a = 4 m/s^2
s = ?
Again,
s = ut+(at^2)/2
= 0*t + (4*3^2)/2
= (4*9)/2
= 36/2
= 18 m (ans.)
 

Answers and Replies

  • #2
tnich
Homework Helper
1,048
336

Homework Statement



If a falling object overcome 72 metres in 6 second, how much distance it overcame in first 3 second.

Homework Equations


s = ut+(at^2)/2

The Attempt at a Solution


Here,
s = 72 m
u = 0
t = 6s
a = ?
We know,
s = ut+(at^2)/2
Or, 72 = 0*6 + (a*6^2)/2
Or, 72 = 36a/2
Or, 72 = 18a
Or, a = 4 m/s

In the second part,
u = 0
t = 3s
a = 4 m/s^2
s = ?
Again,
s = ut+(at^2)/2
= 0*t + (4*3^2)/2
= (4*9)/2
= 36/2
= 18 m (ans.)
You have apparently assumed the falling object is on another planet or somewhere in space above the earth's surface where the acceleration of gravity is not the same as it is at earth's surface. Since a object would fall more than 72m in 6s near earth's surface, your assumption seems reasonable, and you have correctly worked out the result based on that assumption. But are your sure that was what was intended in the original question?
 
  • Like
Likes M. M. Fahad Joy
  • #3
You have apparently assumed the falling object is on another planet or somewhere in space above the earth's surface where the acceleration of gravity is not the same as it is at earth's surface. Since a object would fall more than 72m in 6s near earth's surface, your assumption seems reasonable, and you have correctly worked out the result based on that assumption. But are your sure that was what was intended in the original question?
It's from my school exam question.
It's about falling object on Earth. But the gravity of earth is 9.8 as I know.
We know that the gravity doesn't depend on mass. But for the friction of air reduces the acceleration of gravity. So it a problem me seeing the first law of falling bodies.
 
  • #4
tnich
Homework Helper
1,048
336
It's from my school exam question.
It's about falling object on Earth. But the gravity of earth is 9.8 as I know.
We know that the gravity doesn't depend on mass. But for the friction of air reduces the acceleration of gravity. So it a problem me seeing the first law of falling bodies.
The force that air resistance applies to a falling body (called drag) depends on the speed of the falling body. So you could not solve the problem by assuming drag is constant. You might look at the original problem statement and if there is anything you missed.
 
  • Like
Likes M. M. Fahad Joy
  • #5
The force that air resistance applies to a falling body (called drag) depends on the speed of the falling body. So you could not solve the problem by assuming drag is constant. You might look at the original problem statement and if there is anything you missed.
I have checked it again. It is ok.
 
  • #6
tnich
Homework Helper
1,048
336
I have checked it again. It is ok.
In the problem statement you have written "overcome [a distance]". That is not a common expression. I assume that is your translation of the original problem. What do you think it means?
 
  • Like
Likes M. M. Fahad Joy
  • #7
In the problem statement you have written "overcome [a distance]". That is not a common expression. I assume that is your translation of the original problem. What do you think it means?
I am not so well in English. I have translated it from my own language. That's the problem.
 
  • #8
tnich
Homework Helper
1,048
336
I am not so well in English. I have translated it from my own language. That's the problem.
Do you think the object could have started out going upward, reached a maximum height and then fallen?
 
  • Like
Likes M. M. Fahad Joy
  • #9
tnich
Homework Helper
1,048
336
Do you think the object could have started out going upward, reached a maximum height and then fallen?
I didn't mean to criticize your English. I meant to ask if "overcome 72 m" could mean something different than "fall 72m".
 
  • Like
Likes M. M. Fahad Joy
  • #10
I didn't mean to criticize your English. I meant to ask if "overcome 72 m" could mean something different than "fall 72m".
Yes, the translation will be fall 72 metres.
By the way, it was an MCQ.
a) 36m b) 24m c) 18m d) 8m

Most of my friends answered 36 metres. But I have answered 18 metres. That's my confusion.
 
  • #11
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
35,581
6,450
Most of my friends answered 36 metres.
Then they fell into the trap.
Moving half the distance in half the time would be right for uniform velocity, but for uniform acceleration from rest it is a quadratic, so a quarter of the distance in half the time.
 
  • Like
Likes M. M. Fahad Joy
  • #12
Then they fell into the trap.
Moving half the distance in half the time would be right for uniform velocity, but for uniform acceleration from rest it is a quadratic, so a quarter of the distance in half the time.
Are you sure, quarter of the distance in half time?
 
  • #13
haruspex
Science Advisor
Homework Helper
Insights Author
Gold Member
2020 Award
35,581
6,450
Are you sure, quarter of the distance in half time?
½at2. What happens to that if you halve t? Note, this is only for falling from rest.
 
  • Like
Likes M. M. Fahad Joy
  • #14
½at2. What happens to that if you halve t? Note, this is only for falling from rest.
Ok, Thanks.
 

Related Threads on Laws of Falling Bodies

  • Last Post
Replies
9
Views
2K
  • Last Post
Replies
2
Views
3K
  • Last Post
Replies
5
Views
3K
  • Last Post
Replies
4
Views
8K
Replies
7
Views
10K
Replies
3
Views
6K
Replies
5
Views
637
  • Last Post
Replies
10
Views
10K
  • Last Post
Replies
3
Views
1K
Top