Chapter No 03
Dynamic
Long Question No 7,8,9 and 10.
Question No 07
What is friction? What are microscopic basis of friction. What is normal force how it affects friction ?
Answer
Friction: the opposite force which decrease the motion of an object is called fraction.
Denotation: fraction is denoted by small (f).
Quantity: friction is a vector quantity.
Examples: 1) when we rub our hands on a table we can feel the friction force opposing this motion.
2) When you try to push a heavy object, such as a box, along the floor, you will experience friction. The friction between the box and the floor makes it difficult to move the object smoothly.
Microscopic basis of friction: Friction at the microscopic level is influenced by the roughness of surfaces, adhesion forces, surface energy, deformation, and interlocking. The irregularities and asperities on surfaces interlock, creating resistance when they come into contact. Adhesive forces between atoms or molecules cause surfaces to stick, increasing friction. Different surface energies affect the strength of adhesion and friction. Deformation and interlocking of surfaces further contribute to resistance and friction. Understanding these factors helps in mitigating friction and optimizing surface interactions.
Normal force: a contact force acting perpendicularly to the contact surface that prevent to objects from passing through one another is called normal force.
Denotation: normal force is denoted by (FN).
Effect of normal force on friction:The normal force has a significant effect on friction. Here's how it affects friction:
1. Increased normal force: When the normal force increases, the frictional force also increases. This means that it requires more force to overcome the friction between two surfaces. For example, if you press a book harder onto a table, the frictional force between the book and the table increases.
2. Decreased normal force: Conversely, when the normal force decreases, the frictional force decreases as well. If you reduce the weight of an object on a surface, it will experience less friction. For instance, a lighter object sliding on a table will have less friction compared to a heavier object.
3. Maximum friction: The maximum frictional force that can be generated is directly proportional to the normal force. This relationship is described by the equation Ffriction = μ × Fnormal, where μ is the coefficient of friction. As the normal force increases, the maximum frictional force also increases.
4. Minimum friction: In some cases, the frictional force can be lower than the maximum friction. This occurs when the applied force is smaller than the force required to reach the maximum frictional force. In such situations, the frictional force is reduced to match the applied force.
Understanding the relationship between the normal force and friction is essential in various scenarios, such as determining the force required to move an object or maximizing the grip between two surfaces.
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Question No 08
Differentiate between static and kinetic friction by giving an example.
Find the expression for the coefficient of friction.
Answer
Types of frictional forces: there are two types of frictional forces.
1) static friction
2) kinetic friction
The differentiation between static fraction and kinetic friction is given below.
Static fraction: the frictional force that tends to prevent a stationary object from starting to move is called static friction.
Denotation: static fraction is denoted by fs.
Example of static friction: let's say you have a wooden block placed on a flat surface. When you try to push the block, initially, it doesn't move. This is due to static friction between the wooden block.
Kinetic friction: the fractional force that acts against during motion of an object in an opposite direction to the motion of the object is called kinetic friction.
Or
The force of friction acting on a body when it is in motion is called kinetic friction.
Denotation: it is denoted by (fk).
Example of kinetic friction:
An example of kinetic friction is when you slide a heavy box across the floor. As you push the box, there is a force of friction between the box and the surface it's sliding on. This force acts opposite to the direction of motion and helps slow down the box. The amount of kinetic friction depends on factors such as the roughness of the surfaces and the weight of the box.
Note:
It's important to note that static and kinetic friction depend on the nature of the surfaces in contact and the normal force between them. The coefficients of friction also play a role in determining the magnitude of these forces.
Expression for the coefficient of friction:
Derivation:
The maximum static fractional force is directly proportional to the normal force.
fs max α FN
To change the proportionality into equality hair or constant ( μs) is included known as the coefficient of static friction and depend upon the nature of surfaces.
fs max =( μs)FN
μsFN = fs,max
μs = fs ,max/FN ..................... equation 1
Similarly for Kinetic Friction.
μk= fk/FN.................... equation 2
As μ is the ratio of forces therefore it has no units.
It is constant for a given pair of surfaces.
Equation 1 and 2 are the expressions for coefficient of friction.
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Question No 09
what are the advantages and disadvantages of friction ? also give methods to reduce and improve friction.
Advantages of friction: there are many advantages of friction some of them are as under .
1)Friction enables us to walk.
2) Air resistance friction helps the birds to fly in the air.
3) friction between paper and pen help us to write.
4) friction help us to fix the nail in the wall.
5) Friction is highly desirable when climbed up a hill.
Disadvantage is of Friction: the following are some of the disadvantages of friction.
1)Friction makes it difficult to slide a heavy object on the floor.
2) friction produces wear and tear.
3) friction produces heat between the moving parts of a machine which causes damage.
4) friction causes damage to the Tyres of vehicles , soles of shoes and many other things.
Methods and ways of reducing friction: some methods of reducing friction are is as under.
1) By polishing: friction can be reduced by polishing the rough surfaces.
2) By applying Lubricants( oil Grease) to surface: add oil to the moving parts of the machine it can reduce friction .
3) By using ball bearing: this method Converts the sliding friction into rolling
Friction by the use of ball bearing this method also reduces friction.
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Question No 10
What is tension ? if two connected bodies of mass m1 and mass m2 are hanging from the ends of a string which is passing over a pulley , find the values of tension and acceleration in it.?
Answer:
Tention: Tension force is the force exerted by a string, rope, cable, or any other flexible connector that is pulling on an object from opposite ends. It is a pulling force that acts along the length of the connector and is transmitted through the entire length of the connector.
Explaination: Tension force arises when an object is connected to a string, rope, or cable and there is a force being applied to the other end of the connector. The tension force acts in such a way that it prevents the connector from breaking or becoming slack.In practical terms, tension force can be observed in various situations, such as when a person pulls a rope, when a weight is hanging from a string, or when a cable is used to support a hanging object. The tension force in the connector is equal and opposite at both ends, as long as the connector is not stretching or breaking.
Values of tension and acceleration in two bodies of mass m1 and mass m2 are hanging from the ends of a string which is passing over a pulley : To find the values of tension and acceleration in the system, we need to analyze the forces acting on the two masses.
Let's assume m1 is the mass hanging on one side of the string and m2 is the mass hanging on the other side. Let the acceleration of the system be a.
1. Tension in the string:
The tension in the string can be different on each side due to the difference in masses. Let's denote the tension on the side of m1 as T1 and on the side of m2 as T2.
On the side of m1:
T1 - m1g = m1a (Equation 1)
On the side of m2:
m2g - T2 = m2a (Equation 2)
Here, g is the acceleration due to gravity.
2. Acceleration of the system:
Since the two masses are connected by the same string passing over the pulley, their accelerations will be the same. Therefore, a = a for both m1 and m2.
Now, let's solve the equations to find the values of tension (T1 and T2) and acceleration (a):
From Equation 1:
T1 = m1(g + a)
From Equation 2:
T2 = m2(g - a)
Summing up the forces on m1:
T1 - m1g = m1a
m1(g + a) - m1g = m1a
m1g + m1a - m1g = m1a
m1g = m1a
Similarly, summing up the forces on m2:
m2g - T2 = m2a
m2g - (m2(g - a)) = m2a
m2g - m2g + m2a = m2a
m2g = m2a
Therefore, the values of tension and acceleration in the system are:
T1 = m1(g + a)
T2 = m2(g - a)
a = (m1g)/(m1 + m2)
Please note that the above calculations assume ideal conditions without any friction or other external forces acting on the system.
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