What direction is the friction exerted?

Question

if you press a book flat against a vertical wll with your hand, in what direction ist he friction exerted by the wall on the book?
a)downward
b)upward
c)out from the wall
d)into the wall
is it d into the wall?

Answer 1

There are some correct answers above (though more incorrect ones), so let's make it clear. Friction is a resistive force. Unlike the forces you have come to know and love, it cannot cause positive acceleration, it can only slow something down. Seems pretty obvious, but it is a common mistake made by physics students.

So how is this negative acceleration created? Obviously, not in the direction of motion. That makes the object move faster. The clear, logical direction is then the opposite direction (force vector time negative 1). In fact, we find that this concept applies to ALL motion when friction is involved.

Back to your book. My book ain't moving, you might say, this doesn't apply. But your book WANTS to move, and friction is keeping it from moving (we call this "static friction"). So the force exerted by friction must be in the direction opposite of where the book WANTS to move. And direction the book ought to be going. Down. Force of gravity. Opposite of down is UP. First grade stuff. Choice B.

In this case, force of friction equals the force of gravity, "cancelling" it out to keep the book against the wall. Note that the friction due to you hand is also involved, as well as the force of you pushing against it. Fortunately, they are all irrelevant to the question.

If you are ever in doubt as to the direction of forces, draw a free body diagram. Here, you could have no clue to what is going on and still get the answer if you drew a diagram and instantly saw that something was needed opposite of gravity.

Answer 2

Static friction oppose movement which by the weight should be downward so the direction is upward(b)

Answer 3

Opposite of the direction of the force due to gravity.

I didn't want to answer your homework question but I'd rather you have the correct answer than an incorrect one given by someone else. So the answer is b. upward.

Answer 4

Frictional force is parallel to the surfaces in contact.

Answer 5

c.. if the friction is exerted by the wall, it cant be exerted into the wall..
the wall cant exert force on its self..

Answer 6

out from the wall and
into the wall