Question

You are holding your physics book against the wall by pressing on it with a force directed straight into the wall. (a) You are pressing hard enough so that the book doesn’t move. Drawing a free-body diagram for the book and identifying all the forces that are acting on the book. Assuming the force from the hand is directly to the right, 1) Is there a normal force on the book from the wall (Y for "yes", N for "no", or U for "unknown") 2) The direction for the normal force on the book from the wall is (R, L, U, D, 0 for right, left, up, down, none) 3) Is there a friction force on the book from the contact with the wall (Y for "yes", N for "no", or U for "unknown") 4) The direction for the friction force on the book from the contact with the wall is (R, L, U, D, 0 for right, left, up, down, none) 5) Is there a gravitational force on the book from the Earth (Y for "yes", N for "no", or U for "unknown") 6) The direction for the gravitational force on the book from the Earth is (R, L, U, D, 0 for right, left, up, down, none) 7. What relations are there among the forces in your diagram? That is, which forces or sums of forces have to be equal? How do you know? (this question will not be auto-graded, answer fully)

8. You begin to get tired and the book begins to slide down. The book begins to slide down, and you respond so it slides down at a constant velocity. How do each of the forces you have identified change from their magnitudes in part (a)? Explain how you know. (this question will not be auto-graded, answer fully)

9. If the book has a mass of 2 kg, the coefficient of friction between the sliding book and the wall is 0.4, how hard do you have to press on the book if it is sliding down with a speed of 2 cm/s? D2M

10. BCD on 9

Answer 1

(a)

(i) Y

As the hand exerts a force in right direction, the wall will exerted same amount of force in left direction to balance the weight

(ii) answer: L. As stated in (i) the normal force will be towards left to balance the hand force towards right.

(iii)Answer Y

There is friction force acting in the book. This force balances the weight bo the book.

(iv) Answer: U

As the friction force prevents the book from sliding down, it has to act upwards.

(v) answer: Y

Gravitational force will act on the body which we call weight.

(vi) Answer: D

As earth will attract the body towards it. Gravitational force acts downward.

(vii)

Along the horizontal direction we can write,

F=N.

.......(because there is no acceleration in the horizontal direction. The book is at rest)

Along vertical direction we can write,

f=W

.... because there is no acceleration in vertical direction either.

(Viii) as we press less hardly, the force F decreases. As a result N decreases. So f also decreases because in case of sliding, f=μN

Now if the velocity is constant, it's acceleration is zero. So the net force in vertical direction is also zero.

So

f=W

now, f=μN, so N=W/μ

and since F=N so F is also equal to W/μ

(ix) as we saw in 8, F is given as,

F=W/μ= mg/μ=2×9.8/0.4=49N

Value of F asked here is not dependent in the speed. It will be same for any constant velocity.