Question

Can you explain how number 31 is 60N?

Answer 1

As frictional force is given as mu N for above.
where mu is coefficient of friction
N is normal force which is 100N in this case as N =mg

Frictional force = 0.6×100 = 60N

Hope you get your answer.

Answer 2

Heya user,

Given the Mass = 10kg, force being applied = 20N
--> We calculate the velocity as ---> 20 / 10 = 2 m per sec

Now, given that the body experiences a velocity 4m per sec in -ve direction,

The net deceleration = [ 2 - [-4] ] m per sec | Note : Direction is -ve ]
.'. Net decel. = 6 m per sec²

.'.The Frictional Force acting = mass * accel. = 6 * 10 = 60N

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Other method ---> 

The frictional force is given by μ * N, where μ is Coefficient of Friction and N is the Normal Force...

The normal force here = Mass of Body * Acceleration due to gravity
-------------------------------> = 10 * 10 = 100N
And the value of μ given = 0.6

.'. The Frictional Force acting = 0.6 * 100 = 60N
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Note: - Whichever way you want, you'll get your marks and the answer too