laws of motion part 1 class 11.pptx

Class 11

1. Laws of Motion ….Vidya Gaude

2. Force Force is a push or pull which changes or tries to change the state of rest, the state of uniform motion, size or shape of a body. SI unit is Newton (N) and CGS is dynes Dimensional formula is [MLT -2 ]. Motion: If an object changes its position with respect to time.

3. BALANCED FORCE If the forces on an object are equal and opposite, they are said to be balanced, and the object experiences no change in motion. UNBALANCED FORCE If they are not equal and opposite, then the forces are unbalanced and the motion of the object changes.

4. Various philosophers studied the basic ideas of cause of motion. According to Aristotle, a constant external force must be applied continuously to an object in order to keep it moving with uniform velocity. Later this idea was discarded and Galileo gave another idea According to him, no force is required to keep an object moving with constant velocity. It is the presence of frictional force that tends to stop moving object, the smaller the frictional force between the object and the surface on which it is moving, the larger the distance it will travel before coming to rest. After Galileo, it was Newton who made a systematic study of motion and extended the ideas of Galileo.

5. Sir Isaac Newton (1643-1727) an English scientist and mathematician famous for his discovery of the law of gravity also discovered the three laws of motion. Today these laws are known as Newton’s Laws of Motion and describe the motion of all objects we experience in our everyday lives. Sir Isaac Newton

6. Newton’s First Law An object at rest tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an unbalanced force. To stop a ball rolling down an inclined plane by applying a force against the direction of its motion. To stop the moving car we apply brake.

7. Newton’s First Law is also called the Law of Inertia Inertia : the tendency of an object to resist changes in its state of motion The First Law states that all objects have inertia . The more mass an object has, the more inertia it has (and the harder it is to change its motion). ex: Coin falls into the tumbler when the card is given a sudden jerk

8. Inertia is of three types: Inertia of Rest : When a bus or train starts to move suddenly, the passengers sitting in it falls backward due to inertia of rest. Inertia of Motion: When a moving bus or train stops suddenly, the passengers sitting in it jerks in forward direction due to inertia of motion. Inertia of Direction : When a bus moving along a straight line takes a turn to the right, the passengers are thrown towards left. This is due to inertia which makes the passengers travel along the same straight line, even though the bus has turned towards the right.

9. Newton’s Second Law The rate of change of momentum of a body is directly proportional to the external force applied on it and the change in momentum takes place in the direction of the force. F = ma means that the force of an object comes from its mass and its acceleration.

10. Newton’s Third Law For every action there is an equal and opposite reaction. whenever one body exerts a certain force on a second body, the second body exerts an equal and opposite force on the first. Newton’s third law is sometimes called as the law of action and reaction.

11. Flying of a birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift. The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases. The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

12. Impulse impulsive force: a large force acting for a very short time, to produce a finite change in momentum. (e.g.) The blow of a hammer on a nail, the collision of two billiard balls etc. Impulse = Force × time Impulse of the force = Change in momentum

13. Law of conservation of Linear momentum If no external force acts on a system, then its total linear momentum remains conserved.

14. PROOF: consider an isolated system of n particles , suppose the n particles have masses m 1 , m 2 ,m 3 , …… .. m n and are moving with velocities …… respectively, then total linear momentum of the system is if is the external force acting on the system then according to Newton’s Second Law: for an isolated system , As the derivative of a constant is zero, so = constant

15. EQUILIBRIUM OF A PARTICLE + + + = 0   A  particle is said to be in equilibrium if there is no net force acting on it. 

16. F 1 + F 2 =0 F 1 = - F 2                 F 1 + F 2 + F 3 =0 F 1 + F 2 = - F 3

17. Friction Whenever a body moves or tends to move over the surface of another body, a force comes into play which acts parallel to the surface of contact and opposes the relative motion. This opposing force is called friction. A B FORCE MOTION Frictional force

18. Frictional forces are produced due to intermolecular interactions acting between the molecules of the bodies in contact.

19. No motion FORCE FORCE FORCE J ust start moving S teady motion Static Friction (f s ) Limiting Friction( f s max ) Kinetic/dynamic Friction ( f k )

20. Types of friction: 1. Static Friction (f s ): The force of friction which comes into play between two bodies before one body actually starts moving over the other surface. 2. Limiting Friction( f s max ): The maximum force of static friction which comes into play when body just start moving over the other surface. 3. Kinetic/dynamic Friction ( f k ): The force of friction which comes into play when a body is in the state steady motion over the other surface. Kinetic friction is of two types: (a) Sliding friction (b) Rolling friction

21. Rolling Friction Rolling friction is friction that occurs between surfaces in motion in which one of the surfaces is a wheel, roller, or ball. Examples: Riding a bike – tires and ground Bowling – ball and lane

22. The Limiting friction depends on their nature of surfaces in contact and their state of polish, i.e., smoothness or roughness.

23. The value of Limiting friction do not depend on area and shape of surfaces in contact

24. Laws of limiting friction: The Limiting friction depends on their nature of surfaces in contact and their state of polish, i.e., smoothness or roughness. The value of Limiting friction do not depend on area of surfaces in contact. The Limiting friction acts tangential to the two surfaces in contact and in a direction opposite to the direction of motion of the body. The Limiting friction ( f s max ) is directly proportional to the normal reaction R between the two surfaces. ( f s max ) α R (f s ) = μ s R μ s - coefficient of limiting friction μ s =

25. Is friction required in daily life? Walking : We can walk only if we apply frictional force.  Friction is what holds your shoe to the ground.  Writing –  A frictional force is created when the tip of the pen comes in contact with the surface of the paper. Rolling friction is what comes into play while writing with a ballpoint pen while sliding friction arises when one writes with a pencil.

26. 26 Various tread patterns are designed to provide a firm grip on the road surface Tread pattern on a car tyre Tread pattern on a mountain bicycle tyre

27. Lubricant A  lubricant  is a substance (often a liquid) introduced between two moving surfaces to reduce the friction between them, improving efficiency and reducing wear.  Lubrication is the process or technique of using a  lubricant  to reduce  friction  and  wear and tear  in a contact between two surfaces. Lubricants can be  solid-liquid  (such as  grease ),  liquids  (such as  oil  or  water ). Adequate lubrication allows smooth, continuous operation of  machine elements , reduces the rate of wear, and prevents excessive stresses at bearings. When lubrication breaks down, components can rub destructively against each other, causing heat, local welding, destructive damage and failure.

28. Motion on an Inclined Plane: Angle of repose or angle of sliding: It is the minimum angle of inclination of a plane with the horizontal, such that a body placed on it, just begins to slide down. The forces acting on a block of mass m at rest on an inclined plane are the weight mg acting vertically downwards the normal force N of the plane on the block, and the static frictional force f s opposing the motion. In equilibrium, the resultant of these forces must be zero. Resolving the weight mg along the two directions shown, we have