Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.6

Example 37: Flat tyre

QUESTION

Kevin is helping his dadreplace the flat tyre on the car. Kevin has been asked to undo all the

wheel nuts. Kevin holdsthe spanner at the samedistance for all nuts, but applies the force at

two angles (90◦and 60◦). If Kevin applies a force of 60 N, at a distanceof 0,3 m away from

the nut, which angle is the best to use? Prove your answer by means of calculations.

F

r

F

60 ◦

F⊥

r

SOLUTION

Step 1 : Identify what has beengiven

The following has beengiven:

• the force applied, F = 60 N


• the angles at which theforce is applied, θ = 90◦and θ = 60◦


• the distance from the centre of the nut at whichthe force is applied, r = 0,3 m
  The quantities are in SIunits.

Step 2 : Decide how to approach the problem

We are required to determine which angle is more better to use. This means that

we must find which angle gives the higher torque. We can use

τ = F⊥· r

to determine the torque. We are given F for each situation. F⊥= F sin θ and

we are given θ. We are also given thedistance away from thenut, at which the

force is applied.

Step 3 : Solve the problem for θ = 90◦

F⊥= F

τ = F⊥· r

= (60N)(0,3m)

= 18N· m

Step 4 : Solve the problem for θ = 60◦

τ = F⊥· r

= F sin θ· r

= (60N)sin(θ)(0,3m)

= 15,6N· m