Student Practical

P11.1 Student practical

Name ...... Class ...... Date ......

Pressure circus

Specification references  P.5.5.1.1 Pressure in a fluid 1; P.5.1.3 Gravity  WS 2.4, 2.6, 3.3  AT 1  MS 1a, 2a, 3b, 3c, 5c.

Background We can learn a lot about a physical situation by understanding the forces which act in that situation. However, the same force can have very different effects depending on the area it acts over. For example, it makes a big difference whether you try to push a drawing pin into a notice board by pushing on the flat head or the sharp point. You might exert the same force both times, but if you push on the point the contact area is smaller and the pressure on your thumb or finger is much greater.

Learning objectives After completing the practical you should be able to:  describe the relationship between pressure, force, and area  explain how, and why, certain objects are designed to exert low or high pressures  calculate the pressure an object exerts, from its weight and area of contact with the ground.

Safety  In Experiment 2, hold the balloon well away from your ears, and do not press hard on the balloon.  Take care when handling both the single pushpin, and the ‘bed of nails’.

Equipment and materials Experiment 1 Experiment 2 Experiment 3  Lump of modelling clay  ‘Bed of nails’  Squared paper  Coin  Single pushpin  Weighing scales  Balloon  Shoe (either your own or provided by teacher)

Name ...... Class ...... Date ...... Method Experiment 1: Coin and modelling clay 1 Place a thick lump of modelling clay, flattened on top, onto the bench. 2 With one finger, gently push a coin, edge on, into the modelling clay. 3 Remove the coin and record the estimated depth of the dent it has made. 4 Repeat with the same amount of force, but with the coin face down (flat). 5 Remove the coin and record the estimated depth of the dent it has made.

Experiment 2: Bed of nails 1 Blow up a balloon to medium full (not fully inflated). 2 Standing back from it to protect your ears, press the balloon gently onto the ‘bed of nails’. 3 Standing back from it to protect your ears, press the balloon gently onto the single pushpin with the same amount of force as in step 2.

Experiment 3: Foot pressure In this experiment you will make the measurements needed to find the pressure which a person exerts when standing on the ground. 1 Estimate the area of a shoe in contact with the floor, in cm2, using the squared paper. 2 Measure or find out the person’s weight in newtons. 3 Record the area and the weight.

Results Record your observations for each experiment.

Questions 1 In Experiment 1 (the coin and modelling clay experiment), what two factors does the pressure on the modelling clay depend on?

(2 marks)

2 Using the ideas of force, area, and pressure: a explain your results from Experiment 1 (the coin and modelling clay experiment)

(3 marks)

Name ...... Class ...... Date ...... b explain your results from Experiment 2 (the bed of nails experiment).

(3 marks)

3 In this question, use your measurements from Experiment 3 (the foot pressure experiment). a Use the person’s mass to calculate their weight, and write the unit.

(2 marks) b Calculate the pressure which the person exerts on the ground, in N/cm2, when the person stands on both feet.

(2 marks) c State whether the pressure will be higher or lower than this when the person is walking, and explain why.

(2 marks) d Did you measure the whole area of the shoe simply by drawing around the outside of the sole, or did you try to work out the actual contact area with the ground? Which would give a more accurate result for the pressure?

(1 mark)

e Show, using a sketch or a written explanation, that 1 m2  10 000 cm2.

(1 mark) f Convert your answer to b into pascals (Pa). (Hint: 1 Pa  1 N/m2.)

(1 mark)

Extension 4 If a person exerts a pressure of 20 000 Pa on the ground, what pressure does the ground exert on the person? Explain how you know.

Name ...... Class ...... Date ...... (3 marks)

5 a Estimate the uncertainty in your measurement of area from the foot pressure experiment. (‘Uncertainty’ means the range within which you are confident that the true value lies. For example, ‘area  12 ± 2 cm2’ means the experimenter has measured an area of 12 cm2 and is confident that the true area lies between 10 cm2 and 14 cm2. The uncertainty in the measurement is ‘± 2 cm2’, pronounced ‘plus or minus 2 cm2’.)

(1 mark)

b Estimate the uncertainty in the measurement of the person’s weight.

(1 mark)

c Which of these two uncertainties do you think has the greater effect on the uncertainty in the calculated pressure? Explain your answer.

(2 marks)

Follow-up Where gravitational field strength (g) is needed in a calculation, use a value of 9.8 N/kg. Write the unit with each numerical answer. 1 A football player who weighs 550 N stands indoors wearing her football boots. The boot’s ‘blades’ (long studs) have a total area of 30 cm2. Only her blades are in contact with the floor. Calculate the player’s pressure on the floor.

(4 marks)

Name ...... Class ...... Date ...... 2 For each of the following, say whether it should have a large or small area of contact, and explain why. a skis

(3 marks) b chef’s knife

(3 marks) c drawing pin

(3 marks) d rucksack straps.

(3 marks)

3 Explain why it is more painful to walk on gravel (small stones) than on a smooth pavement.

(2 marks)

4 A Shaolin monk of mass 60 kg is able to do a ‘finger stand’: he supports his whole weight on his two index fingers, giving him a total contact area of 4 cm2 with the ground. Calculate the pressure he exerts on the ground, and write your answer to two significant figures.

(4 marks) (Warning: do not try this! Only a few people in the world can do it, and an untrained person will almost certainly break their fingers if they try.) 5 The world’s largest gold bar, worth ten million dollars in 2014, has a base measuring 46 cm  22 cm and exerts a pressure of 24 000 Pa on the surface it sits on. a Calculate the weight of the bar. Write your answer to two significant figures.

(4 marks)

Name ...... Class ...... Date ...... b Calculate the mass of the bar. Write your answer to two significant figures. (Hint: use weight  mass  gravitational field strength.)

(3 marks)

Extension 6 A student asks, “How much force can an egg withstand without breaking?” a Explain why it is not possible to answer this question, even for a specific egg.

(1 mark)

b The shell of a hen’s egg can withstand a pressure of up to about 30 N/mm2. i Convert this pressure into pascals.

(2 marks) ii Write your answer to part i in kPa (kilopascals).

(1 mark) iii Write your answer to part i in MPa (megapascals).

(1 mark)

Name ...... Class ...... Date ...... c In an experiment, a cement block of mass 14 kg was placed on top of 8 (carefully arranged) eggs without breaking the eggs. i Calculate the force per egg caused by the weight of the block.

(2 marks) ii If the pressure on each egg is 25 N/mm2, calculate the contact area, in mm, between the block and one egg. Show the steps in your working, and write your answer to a suitable number of significant figures.

(3 marks)

d Many types of baby bird and reptile have a small sharp point on their beak or above their mouth, called an ‘egg-tooth’. They lose this soon after hatching from the egg. Suggest what it is used for, and explain why it is effective.

(3 marks)