# The laws of motion

1. Dec 20, 2013

### haha1234

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
The photo shows below is my attempted solution.
Is there any mistake so I cannot get the correct answer?

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2. Dec 20, 2013

### Andrew Mason

You have the right idea. But you have not taken into account the fact that the mug is moving during this process. So how far does the table cloth have to move?

AM

3. Dec 21, 2013

### haha1234

I still cannot figure out the right answer

4. Dec 21, 2013

### Tanya Sharma

I cannot understand your working .

1) What is the acceleration of the mug ?
2) What is the relative acceleration of the cloth with respect to the mug?
3) How much time does it take to pull the cloth completely out from under the mug?

5. Dec 21, 2013

### haha1234

(1)I don't know if it is correct.
I found the acceleration of the mug is f=maâ†’0.1=0.2aâ†’0.5ms^-1
(2)3-0.5?
(3)I don't know.

6. Dec 21, 2013

### Tanya Sharma

Correct...except the units are ms-2

Correct...So the relative acceleration is 2.5ms-2

Okay...First answer this - What is the distance cloth has to move with respect to the mug so that the cloth is completely out ? (Read the question carefully ,you may get the answer in question itself )

Last edited: Dec 21, 2013
7. Dec 21, 2013

### haha1234

I don't know.
I still cannot understand and still think that the solution is as follow.- -"
0.3=0.5(3)t^2
T=0.447s

s=ut+0.5at^2
=0.5X0.5X0.447^2
=0.05m
However the correct answer is 0.06m

Last edited: Dec 21, 2013
8. Dec 21, 2013

### Tanya Sharma

Correct...

So the cloth has to move .3m with respect to the mug such that it is completely out.The relative acceleration of the cloth is 2.5ms-2 .

Now use the kinematics equation x=ut+(1/2)at2 ,where u=0 ,x=0.3m and a=2.5ms-2 .

This will give you the value of 't' i.e time it takes to pull the cloth completely out from under the mug .

What do you get ? Let the answer be in square root only.

9. Dec 21, 2013

### haha1234

Thank you!
I can get the correct answer now.:rofl:

10. Dec 21, 2013

:thumbs: