Everything Science Grade 11

12.3 CHAPTER 12. FORCE,MOMENTUM AND IMPULSE

Example 17: Normal Forces 1

QUESTION

A man with a mass of 100 kg stands on a scale(measuring newtons). What is the reading on

the scale?

SOLUTION

Step 1 : Identify what information is given and what isasked for

We are given the mass of the man. We know thegravitational acceleration that

acts on him is 9,8 = m·s−^2.

Step 2 : Decide what equationto use to solve the problem

The scale measures thenormal force on the man. This is the force thatbalances

gravity. We can use Newton’s laws to solve the problem:

Fr= Fg+ FN

where Fris the resultant force onthe man.

Step 3 : Firstly we determine the force on the man dueto gravity

Fg = mg

= (100kg)(9,8m· s−^2 )

= 980 kg· m· s−^2

= 980N downwards

Step 4 : Now determine the normal force acting upwards on the man

We now know the gravitational force downwards. We know that thesum of

all the forces must equal the resultant acceleration times the mass. The overall

resultant acceleration ofthe man on the scale is 0 - so Fr= 0.

Fr = Fg+ FN

0 =−980N + FN

FN = 980N upwards

Step 5 : Quote the final answer

The normal force is 980N upwards. It exactly balances the gravitational force

downwards so there is no net force and no acceleration on the man. Thereading

on the scale is 980 N.

Now we are going to add things to exactly the same problem to show how things change slightly. We
will now move to a lift moving at constant velocity. Remember if velocityis constant then acceleration
is zero.