Materials PPQs

Question 1

To make concrete suitable for use under large forces steel rods are sometimes embedded in the concrete.
An external force is applied to the steel rods. Concrete is poured into a mould around the rods. Once the concrete has set the external force is removed from the steel rods, placing the concrete in compression.

Explain how this process increases the maximum tensile force that the concrete can withstand before fracture. [4]

Answer 1

• concrete can withstand higher stress under compression
• concrete remains under compression when tensile force is applied
• steel rods take some of the stress
• steel can withstand a larger tensile force

Question 2

To make concrete suitable for use under large forces steel rods are sometimes embedded in the concrete.
An external force is applied to the steel rods. Concrete is poured into a mould around the rods. Once the concrete has set the external force is removed from the steel rods, placing the concrete in compression.

Explain why the external tensile force in the rods must not take the steel beyond its elastic limit. [2]

Answer 2

• steel will be permanently deformed, can’t return to its original shape
• the concrete will not compress as much

Question 3

A student carries out an investigation to determine the Young modulus of a material. Which of the following gives two measuring instruments the student should use?
A calipers and stopwatch
B metre rule and micrometer
C metre rule and stopwatch
D micrometer and calipers

Answer 3

B

Question 4

Define work, give its SI units [2]

Answer 4

Work done is the product of force and distance moved in the direction of the force
Joules (J) / ((kgm^2s^-2))

Question 5

Describe the origin of pressure from a fluid [2]

Answer 5

• when they collide with something, particles in a fluid exert a force on it
• force acting on an area produces pressure

Question 6

Explain why fluid pressure acts at right angles to a surface [2]

Answer 6

• fluid pressure is due to countless particles hitting a surface at random angles.
• The resultant force of all of these random collisions is at right angles.

Question 7

State the equation for pressure due to a column of liquid

Answer 7

Pressure due to a column of liquid = height of column x density of liquid x gravitational field strength

(P=pgh)

Question 8

Explain why ships will float

Answer 8

A ship will float because the weight of water displaced is equal to the weight of the ship.
The ship will sink deeper into the water until this is true.

Question 9

How would a ship look different when sailing in fresh water and salt water

Answer 9

Salt water is roughly 15% more dense than fresh water. So it would produce a larger upthrust.

Question 10

The viscosity of fluids varies with temperature.
Which line of the table correctly shows the change in viscosity with increasing temp?
Oil - dry air
A decreases decreases
B decreases increases
C increases decreases
D increases increases

Answer 10

B

Question 11

A bubble of air is rising through a vertical column of water. Which statement about the motion of the bubble, is correct to a good approximation?
A the bubble has a constant velocity because its weight equals the viscous drag
B the bubble has a constant velocity because the upthrust is equal to the viscous drag
C the bubble has an acceleration because its weight is greater than the upthrust
D the bubble has an acceleration because the viscous drag is greater than the upthrust

Answer 11

B

Question 12

A skydiver steps out of an aeroplane and falls from rest, towards the ground. Her parachute opens a short time after she reaches terminal velocity. Which of the following statements is correct for the vertical acceleration a of the skydiver when her parachute opens?
A a decreases to zero
B a increases to a maximum
C a is constant and equal to g
D a is constant but less than g

Answer 12

A

Question 13

An exhibit in a science museum requires the observer to use a pump to create air bubbles in a column of liquid. The bubbles then rise through the liquid.
A student wishes to determine the total drag force acting on a bubble.
Explain why it might not be possible to use Stoke’s law to calculate the drag force acting on a bubble. [2]

Answer 13

• only for small spheres
• has a non-stationary surface

Question 14

Two identical springs are suspended from a fixed support and a force applied of 8N down. The spring constant of each spring is 80Nm-1

What will the extension of this arrangement be

Answer 14

F = kx
5cm

Question 15

Two identical springs are arranged side by side. When a force of 20N is applied, an extension of 8c, is obtained. A force of 5N is applied to one of the springs on its own. What is the extension obtained

Answer 15

4cm

Question 16

A sample of steel in the form of a wire is subjected to an increasing load.
Which of the following is the best description of the elastic limit of the steel?
A the stress at which the steel undergoes an increase in strain with no increase in stress.
B the stress beyond which the stress and strain are no longer proportional
C the stress beyond which the steel becomes permanently deformed
D the stress at which the steel breaks

Answer 16

C

Question 17

The surface of a golf ball is covered in small dimples. These dimples enable the ball to travel a greater distance when struck by a golf club.
Which of these statements is true
A the dimples result in only turbulent flow
B the dimples result in only laminar flow
C the dimples reduce drag
D the dimples increase drag

Answer 17

C

Question 18

A spring extends by 9cm when a force of 6N is applied. The limit of proportionality is not exceeded.
Another identical spring is joined end to end with this spring and a force of 4N is applied.
The extension for the pair of springs is
A 3cm
B 6cm
C 12cm
D 18cm

Answer 18

C

Question 19

A physics book gives this definition :
A material which shows a large plastic deformation under compression.
This is the definition for
A ductile
B hard
C malleable
D stiff

Answer 19

C

Question 20

The photograph shows a praying mantis hanging from a thin twig. Four of the praying mantis’ six legs are in contact with the twig. The tension in the legs balances the weight to keep the praying mantis stationary.

A student suggests that the tension in each leg in contact with the twig is 25% of the weight of the praying mantis. State why this is not correct. [1]

Answer 20

Tension has horizontal component as well
OR
Only the vertical component of the tension supports the weight.

Question 21

The photograph shows a praying mantis hanging from a thin twig. Four of the praying mantis’ six legs are in contact with the twig. The tension in the legs balances the weight to keep the praying mantis stationary.

The praying mantis moves around the twig so that it is now standing upright on top of the twig. State the difference between the stress in the legs when the praying mantis is beneath the twig and when it is on top of the twig. [1]

Answer 21

When under the twig (the stress/force) is tensile and when on top it’s compressive

Question 22

Explain the difference between compressive strain and tensile strain

Answer 22

Compressive - decrease in length
Tensile increase in length

Question 23

A student carries out an investigation to measure the Young modulus of the material of a wire. He clamps one end of the wire and passes the other end over a pulley.
The student measures the length and diameter of the wire. He hangs masses from the free end of the wire and completes a table with values of mass and extension.
Describe how the data collected should be used to determine the Young modulus using a graphical method. Your answer should include a sketch of the expected graph.
[4]

Answer 23

• force = mg
  Area = pi d/2 ^2
• x and y variables to produce a suitable straight line graph
• correct use of gradient from graph to determine young modulus

Question 24

Air resistance is the greatest source of frction opposing the motion of cyclists.
Three cyclists one after the other
The lead cyclist uses about a third more energy than those behind him. Explain this statement using the idea of the relative velocity between the cyclist and the air around him. [2]

Answer 24

• velocity of lead cyclist relative to air is greater
• more air resistance on lead cyclist

Question 25

As the speed of a cyclist increases, the air resistance acting on him becomes proportional to the square of his speed.
i.e. air resistance = constant x speed ^2
The cyclist has a power output P when travelling at a certain constant speed. He then doubles his speed.
Calculate his new power output as a multiple of P

Answer 25

Power = Fd/t
(Air resistance = force)
Power = (kv^2)d/t
Pnew = k(2v)^2 x 2d /t
Pnew = 8(kv^2 x d)/t

8P

Question 26

When investigating Young Modulus
Explain why the wire chosen should be as long as possible [2]

Answer 26

• the larger the extension
• the smaller the percentage uncertainty

Question 27

A beaker of water is placed on a balance and a rock is hung from a force meter. The initial reading on the balance is R, and the initial reading on the force meter is F. The rock is lowered gently into the beaker of water until it is completely submerged.
Explain any changes in the readings R and F as the rock is lowered into the water. [6]

Answer 27

• there is an upthrust on the rock
• hence net downward force on rock is reduced.
• F decreases to a minimum once rock is fully submerged
• rock pushes down on water
• extra force on balance
• R increases to a max once rock is fully submerged

Question 28

An exhibit in a science museum requires the observer to use a pump to create air bubbles in a column of liquid. The bubbles then rise through the liquid.
It is observed that larger bubbles reach the top of the column of liquid in less time than smaller bubbles. By considering the forces acting on a bubble as it rises, explain this observation. [3]

Answer 28

• upthrust greater for larger bubble
• drag/friction increases
• upthrust increases more than drag

Question 29

Suggest one modification that would produce a greater extension for a given mass for a wire

Answer 29

Thinner wire OR lower spring constant OR lower Young Modulus

Question 30

When soil is allowed to settle in water, the pebbles tend to be found towards the bottom, followed by sand, silt and clay in succession. Explain why this happens. Assume that all particles have the same density. [3]

Answer 30

• larger particles have higher terminal velocity
  OR smaller particles reach terminal velocity quicker
• viscous drag varies in proportion to radius
• velocity proportional to radius squared.

Question 31

Explain what happens to the speed of observed bubbles if the temperature of the liquid increases [2]

Answer 31

Viscosity decreases so speed increases

Question 32

It is possible to create a small bubble followed by a larger bubble. Use the expression to explain why the larger bubble catches up to the smaller one [1]

Answer 32

If radius increases, speed increases

Question 33

Laminar flow [1]

Answer 33

No abrupt change in direction or speed of flow

Question 34

Turbulent flow [1]

Answer 34

Abrupt change in direction or speed of flow
Mixing of layers

Question 35

It is suggested that ‘dimples’ on a gold ball decrease the area over which there is turbulent flow. Explain how decreasing the area over which there is turbulent flow would increase the range of a golf ball. [2]

Answer 35

• smaller area of turbulent flow -> less resistive force
• less KE dissipated. Less work done against air resistance