# Tutor profile: Berik U.

## Questions

### Subject: Physics (Newtonian Mechanics)

Is friction always opposes the motion?

No. For example, when you walk, your feet applies force backwards against the ground. And by Newton's third law, the ground exerts force equal in magnitude but in opposite direction. So the ground exerts friction forward! So friction, in this case, is in the same direction as you walk and it helps you walk.

### Subject: Calculus

What is integral of derivative of x?

Trick question: it is "x". Integral is the opposite of derivative, so they cancel each other out. When you take derivative of x with respect to x, i.e., d/dx ("x"), you get one ("1"). When you take integral of "1", you get back "x" again.

### Subject: Physics

Why we do not see the dark side of the Moon?

The Moon has two rotations: rotation about the Earth and rotation about itself (i.e., spinning). The Moon rotates the Earth once in 27.3 days. Also, it takes approximately 27 days for the moon to spin once. Hence, from Earth, only the bright side of the Moon is seen. Alternative answer: We do see when we watch "Transformers: Dark of the Moon". :)

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