Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

Example 18: Normal Forces 2

QUESTION

A man with a mass of 100 kg stands on a scale (measuring newtons) insidea lift that is moving

downwards at a constant velocity of 2 m·s−^1. What is the reading onthe scale?

SOLUTION

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

We are given the mass of the man and the acceleration of the lift. We know the

gravitational acceleration that acts on him.

Step 2 : Decide which equationto use to solve the problem

Once again we can useNewton’s laws. We know that the sum of all theforces

must equal the resultantforce, Fr.

Fr= Fg+ FN

Step 3 : Determine the force due to 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

The scale measures thisnormal force, so once we have determined it we will

know the reading on the scale. Because the lift is moving at constantvelocity

the overall resultant acceleration of the man onthe scale is 0. If we write out the

equation:

Fr = Fg+ FN

ma = Fg+ FN

(100)(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.

In the previous two examples we got exactly thesame result because thenet acceleration on theman
was zero! If the lift is accelerating downwards things are slightly different and now we will get amore
interesting answer!