KCL Section 2: Solids (Materials)

Question 1

State three SI base units

Answer 1

m
kg
s

Question 2

State 2 SI derived quantities

Answer 2

kgm^-1s^-1

kgms^-2

Question 3

Define upthrust

Answer 3

The weight of the displaced fluid

Question 4

Draw a free-body force diagram for a particle accelerating upwards through a fluid

Answer 4

^
 I
 I   Upthrust
 I
O
 I   Drag + weight
 ↓

Question 5

Explain why a raindrop will reach a maximum velocity

Answer 5

Drag will equal weight

Question 6

_____ - how difficult it is to scratch the surface
_____ - how much it deforms under large forces
_____ - how much energy from impacts it can absorb without breaking
_____ - how much it will shatter or crack when subjected to a shock
_____ - how much breaking stress it can withstand
_____ - how easily it can be formed into thin sheets
_____ - how easily it can be drawn into a wire

Do not use any of these terms unless you are confident you are using the correct term.

Answer 6

Hardness 
Stiffness 
Toughness 
Brittleness
Strength 
Malleability 
Ductility

Question 7

What did Hooke discover?

Answer 7

Hooke discovered that the extension of a stretched wire is directly proportional to the force acting on the wire.
ΔF = kΔx
where k is the force constant (stiffness of the wire).

Question 8

Can this formula be used for objects that aren’t springs?

Answer 8

This formula can be applied to all solids, up to their limit of proportionality.
The limit of proportionality varies for different materials.

Question 9

Describe Hooke’s law in words

Answer 9

Force is directly proportional to extension

Question 10

Describe the difference between x and Δx

Answer 10

x is the length of the spring whereas Δx is the extension

Question 11

Express a newton in SI base units

Answer 11

kgms^-2

Question 12

Describe what the constant k represents

Answer 12

Spring constant

Question 13

Describe what is meant by the limit of proportionality

Answer 13

A limit that when surpassed the object will no longer extend proportionally to the force exerted.

Question 14

A 350 g mass is hung on a piece of wire. What force is acting on the wire?

Answer 14

A weight of 3.4N

Question 15

A 13 N force is applied to a piece of metal (k = 120 Nm-1) fixed to a table. Calculate the expected extension of the metal.

Answer 15

0.1m

Question 16

A 4.3 N force is applied to a piece of metal (k = 830 Nm^-1) fixed to a table. Calculate the expected extension of the metal.

Answer 16

0.0052m

Question 17

A wire is 1.1 m long and has a force constant of 550 Nm^-1. Calculate the new length when a force of 9.2 N is applied.

Answer 17

1.117m

Question 18

A spring has a stiffness constant of 50Nm^-1 and is 3.0 cm long naturally. How long would it be if a 200g mass were hung from it?

Answer 18

6.9cm

Question 19

A 0.70 kg mass is attached to a spring. The spring extends by 84 cm. What is the force constant?

Answer 19

k = 8.175 Nm^-1

Question 20

What is the magnitude of the force required to stretch a 20.0 cm long wire, with a force constant of 590 Nm-1, to a length of 21.0 cm?

Answer 20

F = 5.9 N

Question 21

What is the force constant of a spring that needs a force of 3 N to be compressed from 40 cm to 35 cm?

Answer 21

k = 60 Nm^-1

Question 22

A spring is extended by 11.5 cm and held in place with a force of 380 N. What is the force constant of the spring?

Answer 22

k = 3300 Nm^-1

Question 23

A metal wire is extended to a length of 65.4 cm using a force 4030 N. The force constant of the material is 1280 Nm-1; what was the original length or the wire?

Answer 23

x = 0.623 m