Friction

Question 1

Which of these are NOT an example of useful friction?

Using shoes for more grip
Using snow chains on tyres
Rockets heating up on re-entry
Using oil on a bike chain as lubricant

Answer 1

Rockets heating up on re-entry

Question 2

Which of these is not an example of friction?

A person lifting a heavy object
A car skidding to a halt
A rocket burning upon re-entry to Earth’s atmosphere 
Sliding around on ice

Answer 2

A person lifting a heavy object