1) A 5kg box is pulled by a force F horizontally (no angle). There is friction between the box and horizontal surface. Mu_k = .25 and Mu_s = .5. If F=20N find the magnitude of the friction force and the acceleration of the block.(adsbygoogle = window.adsbygoogle || []).push({});

I used the formula |f_k| = mu_k * |Normal force|

f_k = .25 * 9.8 * 5

f_k = 12.25 N

The correct answer is that the friction is 20 N, meaning that the block didn't move. Where did I go wrong?

2) A 5kg sled is pulled an an angle 5 degree above the horizontal. Friction between the sled and the snow is described by mu_k = .1 and mu_s = .15. What is acceleration of the sled?

The correct answer is 2.99 m/s^2

This is what I did.

I took the 20 N and multiply by cos (5 degree) to get 19.92.

|f_k| = mu_k * N

f_k = .1*5*9.8

f_k = 4.9

(19.92 - 4.9) / 5 = a

a = 3.004

Did I just round my answer differently or did I do something wrong?

3) A 2kg object is under the influences of several forces. Assume that the net force is constant. At t=0, the object's velocity is 3i + 2j and at t=4s its velocity is 11i - 14j . Find the magnitude and direction of net force acting on the object. Please explain this to me. I have no idea how to start.

Thanks in advance for your help.

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# Homework Help: Newton's Laws problems

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