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Contact Force, Contact force is a force that is applied by objects in contact with each other. The, contact force acts on a point of direct contact between the two objects. Pushing, a car up the hill, writing your name with a pen, or picking up a textbook from, the table are all examples of contact force., Types of contact force:, 1)Frictional Force: Friction is a force exerted by a surface against the motion, of a body across its surface. Friction is the reason why your car comes to a, standstill if you don’t press the accelerator. The frictional force is opposing the, rotation of the wheels of your car. Therefore, if you leave the accelerator, the, friction slowly removes all the force from the car and when it has removed the, last bit of force, the car comes to a standstill. Another example, if a book slides, across the surface of a desk, then the desk exerts a friction force in the opposite, direction of its motion causing it to stop sliding after a finite distance. Friction is, a type of contact force only., , 2) Normal Force: The normal force is also called Normal reaction. The normal, force is the support force exerted upon an object that is in contact with another, fixed or stable object. For example, if a book is resting upon a surface, then the, surface is exerting an upward force upon the book in order to support the weight, of the book. The solid ground provides the normal force for all of us live. The, normal force is proportional to the weight of the body. Larger the weight of the, body larger the normal force. If a very high weight is kept on a table and it cannot, provide a normal force that large, then it breaks., , Friction, Friction is defined as the resistance offered by the surfaces that are in contact, when they move past each other.

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What Causes Friction?, Friction is caused due to the irregularities on the two surfaces in contact. So,, when one object moves over the other, these irregularities on the surface get, entangled, giving rise to friction. The more the roughness, the more, irregularities and more significant will be the friction., Static friction: Static friction is the friction that acts between the surfaces when, they are at rest with respect to each other., • Force that resists initiation of motion of one body over another with, which it is in contact., • Opposes Impending motion, • Denoted by fs, Let the ball be at rest initially., Applied force, Fa = 0; Static friction, fs = 0, , Later, applied force, Fa = F, then fs also increases but only up to a certain limit., As soon as Fa becomes greater than fs, the ball starts to move., , fs acts when a body is at rest. Hence called Static friction, Limiting value of fs depends on Normal reaction, and is independent of the area, of contact

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fs max ∝ N, fs max = constant  N, fs max = μs N, where μs is the coefficient of static friction, This coefficient depends on the nature of surfaces in contact., According to the Law of Static friction, Static friction is always less than or, equal to the limiting value of fs, fs  fs max, fs max = μs N, fs  μ s N, Sliding Friction, Sliding friction is defined as the resistance that is created between any two, objects when they are sliding against each other., Examples Of Sliding Friction, Following are the examples of sliding friction:, •, •, , Sliding of the block across the floor, Two cards sliding against each other in a deck, , Rolling Friction, Rolling friction is defined as the force which resists the motion of a ball or, wheel and is the weakest types of friction., , Examples Of Rolling Friction, Following are the examples of rolling friction:, • Rolling of the log on the ground, •, , Wheels of the moving vehicles, , As the area of contact is less in the case of rolling than in the case of sliding,, rolling friction is less than the sliding friction., Ways to reduce friction:, 1) Lubrication: We can use lubricants such as oil, water, grease to reduce friction between, surfaces., 2) Ball Bearing: We use ball bearings to reduce the friction between objects while rolling., 3) Polishing: Polishing the surface makes the surface smoother as a result reduces the friction.