## CBSE Class 9 Science Practical Skills – Pressure

EXPERIMENT

AIM
To observe and compare the pressure exerted by a solid iron cuboid on fine sand/wheat flour while resting on its three different faces and to calculate the pressure exerted in the three different cases.

APPARATUS REQUIRED
Solid iron cuboid of appropriate size, spring balance, clamp, stand, trough filled with fine loose sand, scale, temporary marker or chalk and wooden sheet.

THEORY
Pressure of solid on a solid.
The thrust, i.e. the normal force exerted by a solid per unit surface area in contact with the other solid is called pressure exerted by the first solid over the another solid and is given by
P=$$\frac { F }{ A }$$
where, P is the pressure, F is the applied force and A is the surface area where the force is applied.

PROCEDURE

1. Measure the dimensions of a cuboid with the help of scale and mark them l, b and h respectively.
2. Measure the weight of a cuboid with the help of spring balance as shown below. Record your observations in table A.
Note: See Experiment 2 of term II for the procedure of measurement in detail.
3. Level the surface of loose sand in the trough with the help of wooden sheet.
4. Lay the cuboid on the loose sand on its widest base position, l x b, first as shown in Fig. 4.2. Observe the depression of solid in the sand.

5. Mark the point carefully on the cuboid with a marker where it is just above the sand.Remove it and measure the height of the point from the base of cuboid .Let it be xmm
– After levelling the sand, repeat the step 4 when cuboid is on upright position, i.e. with base bxh as shown in Fig.4.3
– cuboid is on side base position, i.e. with base lxh as shown in Fig. 4.4Mark the point carefully on the cuboid with a marker where it is just above the sand.Remove it and measure the height of the point from the base of cuboid .Let it be xmm
6. Note the observations in two different cases of step (6) in a similar manner as in step (5). Let it be xand  x3.
7. Mark the point carefully on the cuboid with a marker where it is just above the sand.Remove it and measure the height of the point from the base of cuboid .Let it be xmm
8. Repeat the experiment two more times for three different cases of cuboid. Record the observations in observation Table B.

OBSERVATIONS
(a) For dimensions of the cuboid

1. Length of cuboid, l =……………… cm
2. Breadth of cuboid, b =…………… cm
3. Height of cuboid, h = ………….. cm

(b) For weight of the cuboid

1. Least count of spring balance – g wt
2. Zero error of spring balance = …… g wt

Table A: Measurement of weight of a given iron cuboid

Mean weight of cuboid=$$\frac { { w’ }_{ 1 }+{ w’ }_{ 2 }+{ w’ }_{ 3 } }{ 3 }$$ g wt = ………g wt=……kg wt
Table B: Measurement of mark on cuboid from base

CALCULATIONS
(a) Mean height of mark on the cuboid from its base when in the

1. widest base position =………… mm
2. upright position =…………. mm
3. side base position =…………… mm.

(b) Force ‘F’ exerted by the cuboid on the sand = Weight of cuboid = kg wt
(c) Calculation of pressure exerted by cuboid in three different cases:

1. Widest base area, A1= l x b =…………………… cm= …………..m2
Pressure, =P=$$\frac { F }{ { A }_{ 1 } }$$=…………………. N/m2
2. Upright position, A2= b x h =…………………… cm= …………..m2
Pressure P2 =$$\frac { F }{ { A }_{ 2 } }$$=…………………. N/m2
3. Widest base area, A1= l x b =…………………… cm= …………..m2
Pressure, P3=$$\frac { F }{ { A }_{ 3 } }$$=…………………. N/m2

PRECAUTIONS

1. Spring balance should be sensitive, stable and error free.
2. Spring balance must be suspended vertically from fixed support of iron stand.
3. Eye should be kept in line (exactly horizontal) while taking the reading of spring balance.
4. Reading should be taken only when oscillation of hanging object dies completely.
5. Zero error, if any, should be taken into account.
6. Loose sand must be used.
7. Same cuboid should be used for the three different cases as different cuboids have different weights and dimensions.
8. Cuboid should be placed gently on the sand.
9. Caution should be taken while marking on the cuboid.
10. Sand must be levelled properly after taking every observation.

RESULT

1. Mean height of mark on the cuboid from the base in three different cases shows that the effect of thrust on sand is larger while standing upright than while lying side base than in widest base.
2. Pressure exerted by cuboid while in
• upright position, P=………..N/m2
• side base position, P2 = ……….N/m2
• widest base position, P3 = …………..N/m2
This confirms that
Smaller the area over which same magnitude of force acts, greater will be the pressure applied by the object.

INTERACTIVE SESSION

Examiner :
What is the aim of your experiment?
Examinee:
To observe and compare the pressure exerted by a solid iron cuboid on sand while resting on its three different faces and to calculate the pressure exerted in the three different cases.

Examiner :
What observation is made by you in this experiment?
Examinee:
The effect of force is greater when the area of contact is smaller.

Examiner :
In which position of cuboid, the effect of force is the greatest?
Examinee:
When the cuboid is in upright position.

Examiner :
Why?
Examinee:
The upright position of cuboid has smallest area of contact which increases the pressure exerted on the loose sand.

Examiner :
When is the effect of force minimum?
Examinee:
When the cuboid is laid on its widest base position.

Examiner :
What is pressure?
Examinee:
It is defined as the force exerted per unit area.

Examiner :
How is the area of contact to the force acting over it?
Examinee:
Since P = F/A, so, for the same magnitude of force, pressure is inversely proportional to area. Hence smaller the area, the greater is the pressure and therefore, the larger is the effect of force.

Examiner :
What is the unit of pressure in SI system?
Examinee:
The unit of pressure is pascal (Pa).
1 Pa = 1 N/m2

Examiner :
Why do nails have pointed tip, knives sharp edge and buildings wide foundation?
Examinee:
According to the basic concept of pressure, the effect of thrust depends on the area on which it acts. Therefore, for the same magnitude of force acting on a smaller area exerts a larger pressure and a smaller pressure on a larger area. This is the basic reason behind these happenings.

Examiner :
If you swing around on one leg of a chair, how much pressure do you put on one point of the floor as you do if you sit properly?
Examinee:
Four times as the total pressure is 4P which is distributed equally among four legs when we sit properly. But now the whole pressure is exerted by one leg of the chair on the floor.

Examiner :
What is the difference between force and pressure?
Examinee:
The force exerted by the object on the surface is equal to its weight while the pressure exerted by the object is the force exerted per unit area.

Examiner :
Do all forms of matter exert pressure?
Examinee:
Yes, all forms of matter— solid, liquid and gas exert pressure.

Examiner :
How?
Examinee:
In case of solid and liquid, pressure is exerted by the weight of an object over the area on which it acts.

Examiner :
And in case of gas?
Examinee:
It is caused by the motion of gas particles which travel through the space collide with the objects (such as wall of the container) and exerts a force on it. The force exerted per unit area of the wall causes gas pressure.

PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA

Question 1:
Which of the following statements is true for the pressure exerted by a solid body?
(a) It depends upon the area of contact for the same magnitude of force
(b) It does not depend upon the area of contact for the same magnitude of force
(c) Greater the area over which the force acts, greater is the pressure
(d) None of the above

Question 2:
To show that the effect of force depends upon the area of contact, a student holds both the ends of a sharpened pencil between the index-fingers of both the hands and press it from both sides. The finger which has a deeper mark of the pencil is
(a) towards the pointed end of the pencil
(b) towards the fiat end of the pencil
(c) on forefinger of both the hands
(d) on thumb of both the hands

Question 3:
To observe and compare the pressure exerted by a solid iron cuboid on sand, two students take different solid iron cuboids having the same weight but different dimensions. After performing the experiment
(a) same conclusion is drawn by them
(b) different conclusions are drawn by them
(c) none of the students draw any conclusion
(d) conclusion can be determined by making the dimension same

Question 4:
To observe and compare the pressure exerted by solid iron cuboid in its upright position, student ‘A’ placed the cuboid gently on the loose sand while ‘B’ placed it on the surface of table top. Who will observe the exerted pressure by the cuboid easily?
(a) Student A only
(b) Student B only
(c) Both students A and B
(d) Neither A nor B

Question 5:
For the greatest and the least pressure exerted by the cuboid in the experiment ‘To observe and compare the pressure exerted by solid iron cuboid on sand while resting on its three different faces’ the position of cuboid should be
(a) upright, widest base
(b) widest base, side base
(c) upright, side base
(d) widest base, upright respectively

Question 6:
In a school trip to Rohtang pass, Manali, student A wears wide base shoes and student B wears narrow base shoes while walking on the ice. Who will be able to walk easily and comfortably if both the students have equal weight?
(a) Neither student A nor B
(b) Only student B
(c) Both students A and B
(d) Only student A

Question 7:
In doing the experiment, a student puts the solid iron cuboid in upright position on loose sand. After that he laid it. In these two cases
(a) thrust is same but effects are different
(b) thrust is same and its effects are also same
(c) effects of thrust are same but different thrusts are there
(d) the effect of thrust on sand is smaller while in upright position than lying

Question 8:
To observe and compare the pressure of solid iron cuboid on sand, a student puts the cuboid in its upright position on loose sand. The force, that is, the weight of cuboid acting on an area equal to area of its base. When he lies it down
(a) the same force acts on area equal to contact area of cuboid
(b) different forces act on an area equal to contact area of cuboid
(c) same force acts on area equal to surface area of cuboid
(d) same force acts on each face of cuboid.

Question 9:
To compare the pressure exerted by the solid iron cuboid, a student took two cuboids having the same dimensions and same nature of material. After performing the experiment with both the cuboids, she found
(a) p1 = 2p2
(b) p1= p2
(c) p2 = 3p1
(d) p2 = 2p1

Question 10:
If a cuboid is mounted on the top of an inverted pointed object, the pressure exerted by the set-up on the sand
(a) remains same
(b) increases
(c) decreases
(d) cannot say

Question 11:
In the activity shown in above question, more impression will be made by the cuboid in the sand in case of
(a) activity (i) only
(b) activity (ii) only
(c) both activities (i) and (ii)
(d) neither in activity (i) nor in activity (ii)

Question 12:
To observe and compare the pressure exerted by three faces of a cuboid on sand, the following apparatus is available in the laboratory:
(i) Iron cuboid (ii) Aluminium cuboid of same dimensions (iii) Coarse sand in a tray (iv) Very fine sand in a tray
The best choice would be
(a) Iron cuboid and coarse sand
(b) Aluminium cuboid and coarse sand
(c) Iron cuboid and fine sand
(d) Aluminium cuboid and coarse sand.

Question 13:
An iron cuboid of dimensions 12 cm x 5 cm x 2 cm is placed on a bed of sand. Which of its surface in contact will apply the maximum pressure on the sand bed?
(a) 12 cm x 2 cm
(b) 12 cm x 5 cm
(c) 5 cm x 2 cm
(d) The pressure applied will be same for all the surfaces.

Question 14:
A student takes an iron cuboid of dimensions 30 cm x 20 cm x 10 cm of 5 kg mass. He places it on the loose sand filled in a rectangular tray. On the basis of his observations he may conclude that the penetration of the iron cuboid into the loose sand is
(a) maximum when it lies on its sides of dimension 30 cm x 20 cm
(b) maximum when it lies on its sides of dimension 20 cm x 10 cm
(c) maximum when it lies on its sides of dimension 10 cm x 30 cm
(d) same in all the three cases as the thrust is same.

Multiple Choice Questions/VSA

1. (a)
2. (a)
3. (b)
4. (a)
5. (a)
6. (d)
7. (a)
8. (a)
9. (b)
10. (b)
11. (b)
12. (a)
13. (c)
14. (b)