HOOKE'S LAW
5 marks · HardA designer is choosing materials for a chair. The seat uses metal springs under a cushion, while the backrest is made of a plastic sheet. Both materials can be squashed or stretched slightly when someone sits on the chair.
Explain why using springs with elastic behaviour is suitable for the seat, but the plastic sheet is less suitable if elastic behaviour is required for comfort.
State one reason why it is important for the springs to return to their original shape after the load is removed.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Springs with elastic behaviour return to original shape after force is removed, providing comfort and reusability; plastic sheets may deform permanently and not return, making the seat uncomfortable and less durable. · must show Springs are elastic; restore shape, Plastic sheet may deform permanently, Elastic materials provide repeated comfort
(b) So the seat does not become permanently deformed or sag; ensures continued comfort and support for each use. · must show Prevents permanent deformation, Comfort and support maintained
Common mistakes: Confusing elastic with plastic behaviour
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Identifies springs are elastic and return to original shape after load
B1 (a) · 1 mark · States plastic sheet might deform permanently (plastic behaviour)
B2 (a) · 1 mark · Links material choice to repeated comfort, durability or practical function for chair
B3 (b) · 1 mark · States the seat will not become permanently deformed or sag
B4 (b) · 1 mark · Links this to continued comfort or proper support for the user
Worked solution
a) Springs are elastic, so after someone sits and gets up, they return to their original shape. This makes them suitable for seats that need to retain comfort and shape after repeated use. Plastic sheets may not return to original shape after being deformed (plastic behaviour), so they are less suitable as they might stay deformed, causing discomfort. b) This is important so the seat continues to give proper support and remains comfortable after many uses; otherwise, it would become misshapen or sag.