Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.3

Example 15: Newton III - forces in alift

QUESTION

Tammy travels from theground floor to the fifthfloor of a hotel in a lift.Which ONE of the

following statements is TRUE about the force exerted by the floor of the lift on Tammy’s feet?

A It is greater than the magnitude of Tammy’s weight.

B It is equal in magnitude to the force Tammy’sfeet exert on the floor.

C It is equal to what it would be in a stationary lift.

D It is greater than what it would be in a stationary lift.

SOLUTION

Step 1 : Analyse the situation

This is a Newton’s ThirdLaw question and not Newton II. We need to focus on

the action-reaction pairsof forces and not the motion of the lift. The following

diagram will show theaction-reaction pairs that are present when a person is

standing on a scale in alift.

F 1 : force of feet on lift (downwards)

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F 2 : force of lift on feet (upwards)

F 3 : force of Earth on person(downwards)

F 4 : force of person on Earth(upwards)

lift

F 1

F 2

F 4

F 3

Figure 12.7: Newton’s action-reaction pairs in alift

In this question statements are made about theforce of the floor (lift) on

Tammy’s feet. This force corresponds to F 2 in our diagram. The reaction force

that pairs up with this one is F 1 , which is the force that Tammy’s feet exerts on

the floor of the lift. Themagnitude of these twoforces are the same, butthey act

in opposite directions.

Step 2 : Choose the correct answer

It is important to analyse the question first, before looking at the answers as the

answers might confuseyou. Make sure that you understand the situation and

know what is asked before you look at the options.

The correct answer is B.