ppt_force-and-laws-of-motion for Class 9
- 1. Force and laws of motion
- 2. Force Force is an external effort in the form of pushing, pulling, stretching, compressing etc., which may move a body at rest stop a moving body change the speed of a body change the direction of a moving body changes the size and shape of a body
- 3. Balanced and unbalanced forces Balanced forces: if the resultant of all the forces acting on a body is zero balanced forces does not change the state of motion Example: a wooden block placed on a table. In this case weight mg of wooden block is balanced by normal reaction by the table Unbalanced forces: if the resultant of all the forces is non zero unbalanced forces change the state of motion
- 4. Newton’s laws of motion First law of motion which defines inertia and force Second law of motion gives quantitative relation between force and effect ( acceleration and change in momentum) produced by force. Third law of motion gives the nature of force that equal and opposite forces always exist between pair of objects keeping the harmony in nature
- 5. First law of motion or Law of inertia It states that, a body continues to be in a state of rest or in a state of uniform motion along a straight line, unless some external unbalanced force is applied on the body to change that state. From this law, we may defines force as external effort in the form of push or pull which a) Actually moves or tries to move a body at rest, b) Actually stops or tries to stop a moving body, c) Actually changes or tries to change the direction of the motion of the body
- 6. Inertia Inertia of a body is the inability of the body to change by itself its state of rest or state of uniform motion along a straight line Mass of a body is a measure of inertia of the body. Larger the mass, greater is inertia. Example: it is easier to push an empty box than to push a box full of books. Inertia is of three type: i. Inertia of rest ii. Inertia of motion iii. Inertia of direction
- 7. Types of Inertia Inertia of rest: Inability of a body to change by itself, its state of rest. Examples: when a bus or train starts suddenly, the person sitting inside tends to fall backwards When we shake vigorously a branch of tree, the leaves and fruits fall down. Inertia of motion: Inability of a body to change by itself, its state of motion. Examples: when a bus or train starts suddenly, the person sitting inside tends to fall backwards Inertia of direction: Inability of a body to change by itself, its direction of motion. Examples: when a bus or train takes a sharp turn suddenly, the person sitting inside tends to fall in opposite direction
- 8. Linear Momentum Linear momentum is the amount of motion contained in a moving body It is defined as product of mass and velocity of the body. P=mv where p= momentum, m= mass of body and v= velocity of the body If v=0 then p=0 means an object at rest possesses no linear momentum. Linear momentum is a vector quantity, its direction is same as the direction of velocity SI unit of linear momentum is 𝑘𝑔𝑚𝑠−1
- 9. Second law of motion It states that, the rate of change of linear momentum of a body is directly proportional to the external force applied on the body, and this change takes always in the direction of the applied force. 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚 𝑡𝑖𝑚𝑒 ∝ 𝑓𝑜𝑟𝑐𝑒 𝑎𝑝𝑝𝑙𝑖𝑒𝑑 Suppose m=mass of body, u= initial velocity of the body along a straight line, F= external force applied on body, t= time for which force is applied, v= final velocity of the body along same straight line, after t seconds Change in linear momentum of the body=𝑝2−𝑝1= 𝑚𝑣 − 𝑚𝑢 = 𝑚 𝑣 − 𝑢 Rate of change in momentum= 𝑚(𝑣−𝑢) 𝑡 =𝑚𝑎 𝑚𝑎 ∝ 𝑓 𝑜𝑟 𝑓 ∝ 𝑚𝑎 𝑜𝑟 𝑓 = 𝑘𝑚𝑎 If we define f=1, when m=1 and a=1 then k=1 and 𝑓 = 𝑚𝑎 SI unit of force= 1 newton(N) and 1𝑁 = 1 𝑘𝑔𝑚/𝑠−2
- 10. Application of second law of motion Catching a cricket ball: to catch a fast ball, a player pulls his hands backwards to prevent injury to his hands. High jump: the athletes are made to fall either on a cushioned bed or on a sand bed Use of seat belt in cars: to prevent the injuries to the passengers in case of an accident or in case of sudden application of brakes.
- 11. Third law of motion It states that to every action, there is equal and opposite reaction i.e., the forces of action and reaction are always equal and opposite. These forces act on different objects and never cancel each other. Each force produces its own effect. Examples 1. Walking 2. Swimming 3. Recoiling of gun 4. Man and boat 5. The flying of rockets and jet planes
- 12. Law of conservation of linear momentum When two or more bodies interact with each another, the vector sum of their linear momenta remains constant( i.e. conserved), and is not affected due to their mutual action and reaction. The only condition is that no external unbalanced force should be acting on the system of bodies. i.e. if system is isolated or no unbalanced force is acting on it, then total linear momentum of system remains constant or conserved
- 13. Proof: suppose two balls A and B are moving in the same direction along straight line Let 𝑚𝑎 = mass of ball A 𝑚𝑏 = mass of ball B 𝑢𝑎 = velocity of ball A 𝑢𝑏 = velocity of ball B, 𝑢𝑎 > 𝑢𝑏 The two balls collide with each other, let this collision last for t seconds,during collision suppose 𝐹𝐴𝐵 = force exerted by A on B, and 𝐹𝐵𝐴 = force exerted by B on A 𝑣𝑎 = velocity of A after collision 𝑣𝑏 = velocity of B after collision According to second law of motion, 𝐹𝐴𝐵 = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚 𝑜𝑓 𝐵 𝑡𝑖𝑚𝑒 = 𝑚𝑏 (𝑣𝑏 −𝑢𝑏 ) 𝑡 , 𝐹𝐵𝐴 = 𝑐ℎ𝑎𝑛𝑔𝑒 𝑖𝑛 𝑚𝑜𝑚𝑒𝑛𝑡𝑢𝑚 𝑜𝑓 𝐴 𝑡𝑖𝑚𝑒 = 𝑚𝑎 (𝑣𝑎 −𝑢𝑎 ) 𝑡 According to third law of motion, 𝐹𝐵𝐴 =- 𝐹𝐴𝐵 𝑚𝑏 (𝑣𝑏 −𝑢𝑏 ) 𝑡 = - 𝑚𝑎 (𝑣𝑎 −𝑢𝑎 ) 𝑡 or𝑚𝑏 (𝑣𝑏 − 𝑢𝑏 ) = -𝑚𝑎 (𝑣𝑎 − 𝑢𝑎 ) or 𝑚𝑏 𝑣𝑏 + 𝑚𝑎 𝑣𝑎 =𝑚𝑏 𝑢𝑎 + 𝑚𝑎 𝑢𝑎 = constant
- 14. Application of conservation of linear momentum Flight of planes and rockets Recoiling of gum