## CBSE Class 9 Science Practical Skills – Relationship Between Weight of a Body and Force Required to Just Move it

Basic Building Concept
Friction is a part of our everyday life. Nearly every movement which we make, involves friction in one way or the other. Since our childhood, we have learnt to take advantage of it and analyse what may happen in the absence of it.

Friction
Friction is a force that is comes into act whenever two surfaces move or try to move over each other.

• It opposes the relative motion of two bodies.
• The force of friction always acts in the opposite direction of motion.
• It depends on the way the bodies move over each other.
• It also depends on the texture of surfaces of both the bodies.
• It is independent of the area of contact.
• The liquids and gases also exert friction on the bodies moving through them.

Cause of Friction
According to old view, friction is caused due to interlocking between the irregularities, chumps, cracks, valleys, minute bumps or depressions on the two surfaces of bodies that are in contact.
Modern view of friction is that it is not a fundamental force but occurs because of electromagnetic forces between the charged particles which constitute the surfaces in contact.
Identifying the forces involved in friction:
We know that

• force acts in a certain direction whether it is a pull or push.
• gravitational force i.e., weight of the body always acts in downward direction i.e.towards centre of the earth.
• the direction of frictional force is always parallel to the surface and opposite to the direction of the intended motion of an object.
Suppose a body of weight ‘W’ is kept on the table. The various forces, as discussed above, are as shown in figure.
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According to Newton’s third law of motion, the weight of the body (mg) is balanced by the reaction force ‘N’. If the block is pulled by a force ‘F’, it is compensated by the frictional force ‘Ff’, between the surfaces of body and table in contact. If we increase the applied force slowly, the body does not move till net force (F – Ff remains zero. This means that frictional force Ff increases with the increase in the force applied till the body is at rest.
This opposing nature of force, when one body tends to move over the surface of another, is called static friction.
Note:

1. Static friction does not exist by itself.
2. It is a self adjustment force in nature.
3. When there is no applied force, there is no static friction.

At some value of applied force, the body just starts to move. The magnitude of this force, which is just sufficient to move the body in the direction of applied force is called limiting friction. Therefore, the maximum value of static friction is the limiting friction.
Sliding friction: When a body slides over another, the friction acts between them is called as sliding friction.
Ex: When a body slides over a flat surface or a slope.
Rolling friction: When one body is actually rolling over the surface of another body, the friction acting between them is termed as rolling friction.
Rolling friction is always less than the sliding friction.
Ex: Rolling of wheel on the plane surface.

Friction is Necessary
Force of friction plays an important role in our daily life. In the absence of frictional force, we may not be able to be perform many activities. Some of them are given here:

1. Walking will not be possible without friction.
2. Brakes of the vehicles will not work without friction.
3. Without friction, assembling of furniture is not possible.
4. Without friction, one cannot be able to write on the paper or blackboard.

Friction is Evil

1. Due to opposing nature, extra energy has to be spent in overcoming friction.
2. It causes wear and tear and production of heat in the moving parts of a machine in contact. This could reduce the life of the machine.
Thus, friction is a necessary evil.

Ways to Reduce Friction
Reduction in friction improves the efficiency of a machine. Some ways to reduce the friction are given below:

1. Polish the surfaces to make them smoother.
2. Add a lubricant oil or grease between the two surfaces in contact.
3. Replace sliding with rolling because rolling friction is much smaller than the sliding friction.
4. The streamline shape of body reduces the friction with air.

Aim
To establish relationship between weight of a rectangular wooden block lying on a horizontal table and the maximum force required to just move it using a spring balance.

Apparatus Required
Rectangular wooden block with a hook on one side, a spring balance (0-1000 g), a weight box and a fine thread and five pieces of mass (say each of 100 g).

Theory
Whenever an object is made to slide over another surface by applying force, force of friction opposes its motion. The direction of frictional force is always parallel to the surface and opposite to the intended motion of the object.
As the force applied on the object increases, force of friction also increases and compensates it. Net force on the body remains zero, and therefore body does not move.
On further increasing the applied force, the frictional force increases upto ascertain limit. Beyond this limit, body just begins to move as unbalanced force acts on it. This maximum value of force of friction, before the object just starts sliding over another, is called limiting friction.

Procedure

1. Clean the top of a horizontal table.
2. Take the spring balance. Note its least count and zero error.
3. Measure the weight of rectangular wooden block with the help of the spring balance and record it.
4. Take about 1m long thread. Tie one end of the thread to the hook of wooden block and other end to the spring balance.
5. Place the wooden block on the horizontal top of table in such a way that the spring balance could be held at another end of the horizontal plane surface.
6. Apply a small force on the wooden block by pulling the spring balance as shown in the figure below.
7. Gradually increase the force till the block just begins to slide on the horizontal table. Note the reading of the spring balance. It measures the force applied to the mass of wooden block.
8. Now keep additional weight (say 100 g) on the wooden block and repeat steps 6 and 7.
9. Repeat steps 6, 7 and 8 four more times by increasing the weight 100 g every time.
10. Record the observations in tabular form given below.

Observations
Least count of spring balance = ………… gwt.
Zero error in spring balance = …………. gwt.
Weight of the wooden block, Wb = ………… gwt.

 S.No. Weight placed on the wooden block (W) (gwt) Total weight being pulled (Wb + W) = Normal reaction (N) (gwt) Total force pulling the block = Reading of spring balance (Ff) (gwt) $$\frac { { F }_{ f } }{ N }$$ 1. 0 2. 100 3. 200 4. 300 5. 400 6. 500

Calculations

1. The reading of spring balance gives the maximum value of force of friction acting between the two solid surfaces just before the object sets into motion.
2. Find the ratio of total force applied to pull the block (Ff) to the total weight being pulled (N).
3. Plot a graph between total weight being pulled i.e. normal reaction along X-axis and spring balance reading i.e. limiting friction along Y-axis. The graph becomes a straight line as shown in the figure.

Result
It is observed that as the total weight of wooden rectangular block increases, the maximum value of force of friction also increases in the same ratio.
Thus, the graph as well as constant ratio of Ff and N indicates that the maximum force required to just move the block is directly proportional to the weight of block. This is the required relationship.
Note: Student must also perform the same experiment by taking the different nature of surfaces in contact.
i.e., extra smooth surfaces or rough surfaces. By doing so, they can observe that if there is any change in nature of surface in contact occurs, the value of limiting friction also changes accordingly.

Precautions

1. The string should be inextensible.
2. The portion of the string between the wooden block and a spring balance must be horizontal while force is applied on the block.
3. The surfaces of table and wooden block in contact must be clean and dry.
4. The mass of the rectangular wooden block should not be so large that a large force is required to make the wooden block slide.
5. Reading of the spring balance should be noted only when wooden block just begins to start.
6. Before use, adjust the zero of the spring balance.

Interactive Session

Question 1.
Why are the bodies of automobiles, aeroplanes and space launch vehicles designed in streamline shape?
They are designed specially in streamline shape to minimise the effect of friction of air.

Question 2.
Why we say friction is a self adjusting force?
When no force is applied on the object, friction is zero. As the applied force increases, friction also increases i.e. force of static friction is always equal to the applied force till the block starts moving. Thus, static friction adjust itself in the opposite direction of the applied force.

Question 3.
Is the maximum force required to just move the block also a self adjusting force?
No.

Question 4.
Why is more force required to move a heavier body than to move a lighter body?
For a heavy body, normal reaction force ‘N’ also increases. Therefore, force of friction increases. Hence, applied force also increases.

Question 5.
What are the factors on which the maximum force required to just move over the surface of a body another surface in contact depends?
It depends on the nature of material of the surfaces in contact and their roughness or smoothness.

Question 6.
Why should the mass of rectangular block not be so large?
It is so because the large force is required to make the wooden block slide.

Question 7.
What does happen if the block is too light?
Observation cannot be taken as force of limiting friction may be even less than the applied force. Weight of block should be such that the spring balance shows a larger value of force just before the wooden block begins to slide.

NCERT Lab Manual Questions

Question 1.
In which direction the force of friction acts on the block?
The force of friction acts in a direction opposite to the direction of motion of the block i.e. applied force.

Question 2.
Why is the thread used to move the block kept horizontal to the surface and the wooden block you are using?
Applied force on the block should act in the horizontal direction.

Question 3.
How is the limiting friction between two surfaces in contact affected when grease or oil is inserted between them?
Grease or oil fill up the irregularities of the surfaces, making them smoother. Hence limiting friction decreases.

Question 4.
How can you use the concept of limiting friction to measure force?
The force of friction can increase only upto a certain limit. Beyond this limit, the unbalanced force acts on the body and it begins to move. Therefore, the maximum value of force of friction acting between two solid surfaces just before the object sets into motion is equal to the applied force. Thus, limiting frictional force is used to measure the applied force.

Practical Based Questions

Multiple Choice Questions/VSA (1 Mark)

Question 1.
In an experiment to establish the relationship between weight of a rectangular wooden block lying on a horizontal table and the minimum force required to just move it using a spring balance, which type of string should be used?
(a) Very high elastic
(b) Less elastic
(c) Moderate elastic
(d) Anyone can be used.

Question 2.
The portion of the string that lies between the wooden block and spring balance should
(a) make an angle of 30° with the surface of table.
(b) make an angle of 60° with the surface of table.
(c) remain horizontal and parallel to the surface in contact.
(d) remain in vertical position with the surface of table.

Question 3.
Which of the following statements about friction is true?
(а) Friction can be reduced to zero.
(b) Frictional force is directly proportional to the surface area of contact.
(c) Sliding friction is always greater than rolling friction.
(d) Frictional force can accelerate a body.

Question 4.
The value of minimum force applied on the block to just begin to move depends on
(a) nature of the two surface in contact.
(b) their state of roughness.
(c) normal reaction between the two surfaces
(d) All of the above.

Question 5.
The spring balance used for measuring minimum force required to just slide a wooden block has least count of 1 gwt. The number of divisions between its 50 gwt and 60 gwt mark should be [CBSE 2012]
(a) 50
(b) 60
(c) 20
(d) 10

Question 6.
When a body actually begins to move over the surface of another, the force of
(а) static friction comes into play.
(b) kinetic friction comes into play.
(c) limiting friction comes into play.
(d) static friction is doubled.

Question 7.
The coefficient of limiting friction between two surfaces in contact
(a) has a unit newton/metre.
(b) remains same even when two surfaces are polished.
(c) has no unit.
(d) None of the above

Question 8.
The frictional force
(a) is independent of normal reaction .
(b) opposes the relative motion between two objects.
(c) acts in the direction of applied force.
(d) is independent of nature of surfaces in contact.

Question 9.
A person can easily skate on ice but finds it difficult to move on concrete because the coefficient of friction is
(a) more on ice than concrete.
(b) more on concrete than ice.
(c) same in both.
(d) independent of normal reaction.

Question 10.
Moving objects such as an aeroplane is given streamlined shape to
I. reduce frictional force of air.
II. increase frictional force of air.
III. decrease the efficiency of the moving object.
IV. increase the efficiency of the moving object.
(a) Both I & IV are correct.
(b) Both II & III are correct.
(c) Both III & IV are correct.
(d) Both I & III are correct.

Question 11.
In an experiment, to find the relationship between the minimum force applied on the wooden block to just begin to move over another surface with the weight of body, a girl draws the forces acting on the block in four ways when it remains at rest which are

Identify which of them is correct ?
(a) I
(b) II
(c) III
(d) IV

Question 12.
In the given experiment, a student just noted down the minimum force exerted on the wooden block to just move it. He then increased the speed of sliding of wooden block on a horizontal surface, the reading of spring balance observed by him now is
(a) increases as the speed of block increases.
(b) decreases upto a certain limit as the wooden block is moved with increasing speed.
(c) remains the same as it is independent of the speed of wooden block.
(d) decreases continuously to zero as the speed of block increases.

Question 13.
In the given experiment, a student measured the minimum force required to just move a rectangular wooden block lying on a horizontal table with its largest surface using spring balance. He observed it is as F1. He repeated the experiment with smaller surface area of the same rectangular block and find the force as F2 now. He tried to establish a relationship between these two forces by repeating the experiment with rectangular wooden block of different dimensions. Which of the following relation would be obtained by him?
(a) F1 = F2
(b) F1 > F2
(c) F1 < F2
(d) There is no relation between F1 and F2.

Question 14.
On the basis of observations made while estab-lishing the relationship between weight of a rectangular wooden block lying on a horizontal table and the minimum force required to just move it using a spring balance, the student should find
(a) applied force is directly proportional to the weight of block.
(b) applied force is inversely proportional to the , weight of block.
(c) applied focee is equal to the weight of block.
(d) no definite relationship between them.

Question 15.
In an experiment to establish the relationship between weight of a wooden cuboid lying on the horizontal surface and the minimum force required to move it. Which of the following faces of cuboid should be in contact with the surface to exert the minimum force among the three faces of cuboid to move it?

(a) Face A with area 30 x 20 m2
(b) Face B with area 20 x 10 m2
(c) Face C with area 30 x 10 m2
(d) Any of the face A, B or C

Question 16.
While studying the relationship between weight of rectangular wooden block lying on the horizontal surface and minimum force required to just move it, Bharat applied a force on the wooden block using a spring balance. He observed that the minimum force required to just move it is 50 gwt. The magnitude of force of friction between block and table could be
(a) 50 gwt
(b) – 50 gwt
(c) 0
(d) greater than 50 gwt

Question 17.
Lubricants such as grease or oil
(a) reduce the force of friction
(b) increase the force of friction
(c) have no role in friction.
(d) neither increase nor decrease friction.

Question 18.
In an experiment, a student took two wooden blocks having the same mass but different dimensions. He observed that the minimum force read by the spring balance to just move the block would remain the same. He concluded that
(a) force of limiting friction depends on the area of the surfaces in contact.
(b) force of limiting friction is independent on the area of the surfaces in contact.
(c) force of limiting friction does not depend on the normal reaction.
(d) force depends on dimensions of the body.

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

Question 1.
A force applied on a block with constant mass increases gradually by pulling the spring balance. Will the force of friction increase until a point?
Yes, it would increase till the limiting friction is experienced by the block. Further increase would lead to the block accelerating to the direction in which the force is being applied.

Question 2.
What is limiting friction?
Limiting friction is the maximum value of the opposing force which comes into play when one body is just at the verge of moving over the surface of another body.

Question 3.
On the observation made while establishing the relationship between weight of a rectangular wooden block lying on a horizontal plane and minimum force required to just move it using a spring balance, a student plot the given graph between the force of limiting friction ‘F’ and normal reaction ‘N’ (equal to weight of the block) for the two different blocks ‘A’ and ‘B’ of the same material. Which of them has smooth surface in contact with the plane?

The magnitude of force of limiting friction ‘F’ is directly proportional to the weight of the block, i.e. normal reaction ‘N’,
i.e. F ∝ N
or F = µN
where µ = F/N
= Constant of proportionality
= Slope of F-N curve
The given graph indicates that slope for block ‘B’ is greater. Therefore, for block ‘A’ force of friction is smaller and for block ‘B’, it is larger. Hence, the surface of block ‘A’ in contact with the plane must be smoother.

Question 4.
Why is the less elastic thread used to move the block kept horizontal to the surface in the experiment to establish the relationship between weight of a rectangular wooden block you are using and minimum force required to just move it using a spring balance?
The string should be inextensible, i.e. unable to be stretched or drawn out in length so that total applied force must be exerted only on the block.

Question 5.
Does the force of friction vary with

1. weight of the block?
2. change in the conditions of surface in contact?
3. area of contact?