Selina solutions concise physics class 8 icse Solutions for Chapter 3 force and pressure

Q1) Find the moment of force of 20N about an axis of rotation at distance 0.5m from the force.

Q2) The moment of a force of 25N about a point is 2.5Nm. Find the perpendicular distance of force from that point.

Q3) A spanner of length 10 cm is used to open a nut by applying a minimum force of 5.0 N. Calculate the moment of force required.

Q4) A wheel of diameter 2 m can be rotated about an axis passing through its center by a moment of force equal to 2.0 N m. What minimum force must be applied on its rim?

Q5) A normal force of 200N acts on an area of 0.02m^2 . Find the pressure in pascal.

Q6) Find the thrust required to exert a pressure of 50000 pascals on an area of 0.05m^2 ?

Q7) Find the area of a body which experiences a pressure of 50000 Pa by a thrust of 100 N?

Q8) Calculate the pressure in pascal exerted by a force of 300N acting normally on an area of 30cm^2

Q9) How much thrust will be required to exert a pressure of 20,000 Pa on an area of 1cm^2.

Q10) The base of a container measures 15 cm \times^{ } 20 cm. It is placed on a table top. If the weight of the container is 60N, what is the pressure exerted by the container on the table top?

Q11) Calculate the pressure extended on a surface of 0.5m^2 by a thrust of 100 kgf.

Q12) A boy weighing 60 kgf stands on a platform of dimensions 2.5 cm \times 0.5 cm. What pressure in pascal does he exert?

Q13) Figure shows a brick of weight 2kgf and dimensions 20 cm \times 10 cm \times 5 cm placed in three different positions on the ground. Find the pressure exerted by the brick in each case.

Q1) Write true or false for each statement (a) The S.I unit of force is kg.

(b) A force always produces both the linear and turning motions.

(c) Moment of force = force perpendicular distance of force.

(d) Less force is needed when applied at a farther distance from the pivoted point.

(e) For a given thrust,pressure is more on a surface of large area.

(f) The pressure on a surface increases with an increase in the thrust on the surface.

(g) A man exerts same pressure on the ground whether he is standing or he is lying.

(h) It is easier to hammer a blunt nail into a piece of wood than a sharply pointed nail.

(i) The S.I unit of pressure is pascal.

(j) Water in a lake exerts pressure only at its bottom.

(k) A liquid exerts pressure in all directions.

(l) Gases exert pressure in all directions.

(m) The atmospheric pressure is nearly 105 pa.

(n) Higher we go, greater is the air pressure

Q4) Select the correct alternative

(a) SI. unit of moment of force is

1. N

2. N cm

3. kgfm

4. N m

(b) To obtain a given moment of force for turning a body, the force needed can be decreased by

1. applying the force at the pivoted point

2. applying the force very close to the pivoted point

3. Applying the force farthest from the pivoted point

4. none of the above

(c) The unit of thrust is

1. kgf

2. kg

3. g

4. m\ s^{-1}

(d) The unit of pressure is ……………….

1. N x m

2. kgf

3. N m^{-2}

4. kgf m^2

(e) The pressure and thrust are related as

1. pressure = thrust

2. pressure = thrust x area

3. pressure= thrust /area

4. pressure = area/thrust

(f) A body weighing 5 kgf, placed on a surface of area 0.1 m2, exerts a thrust on the surface equal to

1. 50 kgf

2. 5kgf

3. 50 kgf m^2

4. 5 kgf m^{-2}

(g) The feet of lizards act like ……………………………………………………..

1. moving pads

2. drilling pads

3. Suction pads

4. none of the above

(h) Pressure exerted by a liquid is due to its ……………….

1. Weight

2. Mass

3. Volume

4. Area

(i) Pressure inside a liquid increases with

1. Increase in depth

2. decrease in depth

3. decrease in density

4. none of the above

(j) The atmospheric pressure at sea level is nearly

1. 10 Pa

2. 100,000 Pa

3. 100 Pa

4. 10,000 Pa

(k) Nose bleeding may occur at a high altitude because

1. The atmospheric pressure decreases

2. the oxygen content of atmosphere decreases

3. the atmospheric pressure increases

4. there are strong air currents at the high altitude

Q2) Fill in the blanks: (a) 1 kgf = ____ (nearly)

(b) Moment of force = _____ x distance of force from the point of turning.

(c) In a door, a handle is provided _____ from the hinges.

(d) The unit of thrust is __\.

(e) Thrust is the ______ force acting on a surface.

(f) Pressure is the thrust acting on a surface of _____ area.

(g) The unit of pressure is _\.

(h) Pressure is reduced if ______ increases.

(i) Pressure in a liquid _____ with the depth.

(j) The atmospheric pressure on earth surface is nearly _\.

Q3) Match the following:

Q1) Define force. State its S.I. unit.

Q2) State two effects of a force when applied on a body.

Q3) How does the effect of force differ when it’s applied on (a) a rigid body (b) a non-rigid body ?

Q4) State the effect of force F in the following diagram.

Q5) Define the term moment of force.

Q6) State the S.I. unit of moment of force.

Q7) State two factors which affect the moment of force.

Q8) In fig. A force F is applied in a direction passing through the pivoted point O of the body. Will the body rotate? Give reason to support your answer.

Q9) Write the expression for the moment of force about a given axis of rotation.

Q10) State one way to decrease the moment of a given force about a given axis of rotation.

Q11) State one way to obtain a greater moment of a given force about a given axis of rotation.

Q12) What do you mean by the clockwise and anticlockwise moment of force?

Q13) Explain the following:

(i) The spanner (or wrench) has a long handle.

(ii) The steering wheel of a vehicle is of large diameter.

(iii) The hand flour grinder is provided with a handle near the rim.

(iv) It is easier to open the door by pushing it at its free end.

(v) A potter turns his wheel by applying a force through the stick near the rim of the wheel.

Q14) What is thrust?

Q15) State the unit of thrust.

Q16) On what factors does the effect of thrust on a surface depend?

Q17) Define the term ‘pressure’ and state its unit.

Q18) How is thrust related to pressure?

Q19) Name two factors on which the pressure on a surface depends.

Q20) When does a man exert more pressure on the floor: while standing or while walking?

Q21) Why do camels or elephants have broad feet?

Q22) A sharp pin works better than a blunt pin. Explain the reason.

Q23) Why is the bottom part of the foundation of a building made wider?

Q24) It is easier to cut with a sharp knife than with a blunt one Explain.

Q25) A gum bottle rests on its base. If it is placed upside down,

(a) How does the thrust change?

(b) how does the pressure change?

Q26) Explain the following:

(a) Sleepers are used below the rails.

(b) A tall building has wide foundations.

Q27) Describe an experiment to show that a liquid exerts pressure at the bottom of the container in which it is kept.

Q28) Describe a suitable experiment to demonstrate that a liquid exerts pressure sideways also?

Q29) Describe a simple experiment to show that at a given depth, a liquid exerts same pressure in all directions.

Q30) State two factors on which the pressure at a point in a liquid depends.

Q31) Describe an experiment to show that the liquid pressure at a point increases with the increase in height of the liquid column above that point.

Q32) Which fact about liquid pressure does the diagram in figure illustrate?

Q33) Describe an experiment to show that liquid pressure depends on the density of liquid.

Q34) A dam has broader walls at the bottom than at the top. Give a reason.

Q35) What do you mean by atmospheric pressure?

Q36) Write the numerical value of the atmospheric pressure on the earth surface in pascal.

Q37) We do not feel uneasy even under the enormous atmospheric pressure. Give a reason.

Q38) Describe a simple experiment to illustrate that air exerts pressure.

Q39) Describe the crushing tin can experiment. What do you conclude from this experiment?

Q40) Give reason for the following:

(a) A balloon collapses when air is removed from it.

(b) Water does not run out of a dropper unless its rubber bulb is pressed.

(c) Two holes are made in a sealed oil tin to take out oil from it.

Q41) How does the atmospheric pressure change with altitude?