If the crate shown here is moving at a constant speed in a straight line and the force applied is 310 n, what is the magnitude of the force of friction?

a.310 n
b.0 n
c.620 n

the answer is A.

Explanation:

Using the laws of newton:

∑F = ma

where ∑F is the sumatory of forces acting in the system, m the mass and a the acelertion of the system.

Then, if the block is moving with constant velocity, its aceleration is equal to 0, so:

∑F = m(0)

∑F = 0

It means that:

F - = 0

where F is the force applied and is the friction force. Replacing the value of F, we get:

310N - = 0

Finally, solving for :

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