Weird Blocks

Two blocks A and B each of mass 1 kg placed on a smooth horizontal surface. Two horizontal forces 5 N and 10 N are applied on the blocks A and B respectively as shown in the figure. The block A does not slide on block B. Then the normal reaction acting between the two block is

5 2 \frac { 5 }{ 2 } N 5 N 15 N 5 3 \frac { 5 }{ \sqrt { 3 } } N

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1 solution

Akhilesh Prasad
Jul 19, 2014

This is my first solution so if you like it then please voteup

After the blocks come in contact the net force would be in the direction of the 10 N 10 N force. So, the net force will be ( 10 5 ) N = 5 N (10-5)N = 5N .

After doing some geometry, you will find that the normal reaction on the block on the left hand side by the block on the right hand side makes an angle of 60 ° 60° .After both the blocks come in contact we can treat the blocks as one system so the acceleration of both the blocks becomes F n e t m i n m o t i o n = 10 5 1 + 1 = 5 2 \frac{ { F }_{ net }}{m_{in motion}} = \frac{10-5}{1+1} = \frac{5}{2} As the net force on the block on the left hand side is 5 N 5N in the direction of the 10 N 10N force hence, we get the equation as follows

N c o s 60 ° 5 = m a Ncos60° - 5 = ma

N c o s 60 ° 5 = 5 2 \Longrightarrow\ Ncos60° - 5 = \frac{5}{2}

N c o s 60 ° = 15 2 \Longrightarrow\ Ncos60° = \frac{15}{2}

N 2 = 15 2 \Longrightarrow\ \frac{N}{2} = \frac{15}{2}

N = 15 \Longrightarrow\ N = \boxed{15}

Similarly for the other block, the equation formed will be

10 N c o s 60 ° = 5 2 10 - Ncos60° = \frac{5}{2}

And on solving it we will get

N = 15 N = \boxed{15}

Good solution

Mudasir Mudasir - 4 years, 2 months ago

Explanation is good

sree ammu - 3 years, 10 months ago

Easy to understand

Banday Zaheen - 3 years, 10 months ago

Thanks a lot

Dev Khanduja - 3 years, 7 months ago

Please explain the geometry with which you find the angle as 60 degrees.

Manasi Singh - 2 years, 11 months ago

But where is the friction going ? Pls explain .

Thakumar Jhuli - 2 years, 10 months ago

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Isn't it more like mg balances N than friction No friction mentioned in the problem

Cooldude Goodguy - 2 years, 8 months ago

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