Everything Science Grade 11

12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

Example 16: Newton III - book andwall

QUESTION

Tumi presses a book against a vertical wall as shown in the sketch.

1. Draw a labelled force diagram indicating allthe forces acting on thebook.


2. State, in words, Newton’s Third Law of Motion.


3. Name the action-reaction pairs of forces acting in the horizontal plane.

SOLUTION

Step 1 : Draw a force diagram

A force diagram will look like this:

Upwards frictional forceof wall on book

Downwards gravitationalforce of Earth on book

Applied force on girl on book Force of wall on book

Note that we had to drawall the forces acting on the book and not the action-

reaction pairs. None ofthe forces drawn are action-reaction pairs, because they

all act on the same object (the book). When you label forces, be as specific as

possible, including the direction of the force andboth objects involved, for exam-

ple, do not say gravity (which is an incomplete answer) but rather say ’Downward

(direction) gravitationalforce of the Earth (object) on the book (object)’.

Step 2 : State Newton’s Third Law

If body A exerts a forceonto body B, then bodyB will exert a force equal in

magnitude, but oppositein direction, onto bodyA.