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# Answer to Question #6413 in Mechanics | Relativity for parth

Question #6413
a box of mass 50kg is pulled on a incline 12m long and 2m high by the constant force of 100N from rest.it acquires velocity of 2m/s on reaching ,work done against frictional force is ???
1
2012-02-10T07:58:41-0500
Let&#039;s consider all the forces acting on a box along an incline:

F1 = mg/sin&alpha; - the gravity force (here sin&alpha; = 2m/12m = 1/6);
F2 = 100N - pulling force;
F3 - unknown friction force.

Now let&#039;s write down the second Newton&#039;s law:

F1 + F2 - F3 = ma.

Let&#039;s guess what acceleration a is equal to.

S = V*t/2 ==&gt; t = 2*S/V = 2*12/2 = 12 s (the moving time).

We know that

S = a*t&sup2;/2 ==&gt; a = 2*S/t&sup2; = 2*12/12&sup2; = 2/12 = 1/6 m/s.

So,

F3 = F1 + F2 - ma = mg/sin&alpha; + 100 - ma = 50*9.8/6 + 100 - 50/6 = 520/3 = 173.(3) N.

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