* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download force-2 - CBSEcare.in
Survey
Document related concepts
Classical mechanics wikipedia , lookup
Jerk (physics) wikipedia , lookup
Specific impulse wikipedia , lookup
Fundamental interaction wikipedia , lookup
Modified Newtonian dynamics wikipedia , lookup
Nuclear force wikipedia , lookup
Equations of motion wikipedia , lookup
Fictitious force wikipedia , lookup
Centrifugal force wikipedia , lookup
Newton's theorem of revolving orbits wikipedia , lookup
Rigid body dynamics wikipedia , lookup
Mass versus weight wikipedia , lookup
Classical central-force problem wikipedia , lookup
Transcript
Www.cbsecare.in CLASS-iX CHAPTER - force NEWTON'S SECOND LAW OF MOTION . (a) Statement Newton's second law states that "the force acting on a body is directly proportional to the product of the mass of the body and the acceleration produced in it, the acceleration being in the direction of the force applied." (b) Expression (equation for force) Let a force F acting on a body of mass m produce in it an acceleration a. Then, according to Newton's second law of motion. Force α mass x acceleration i.e., F α ma Or F=kma Where k is a constant of proportionality. Unit force is that force which produces unit acceleration in a body of unit mass, i.e., if m = 1 unit and a = 1 unit Then, F= 1 unit Putting, values in equation (1) We get, 1 = k x 1 x 1, k=I Equation 1 becomes, F=ma i.e. Force = mass x acceleration ... (1) (c) Unit of Force (i) The S.I. unit of force is newton (N). One newton is that force which when applied on a body of mass 1 kg produces in it an acceleration of 1 ms-2. 1 newton = 1 kilogram x 1 m s-2 i.e., or 1 N = 1 kg x 1 m s-2. (ii)The C.G.S. unit of force is dyne (dyne). One dyne is that force which when applied on a body of mass 1 g produces in it an acceleration of 1 cm s-2. i.e. or 1 dyne = 1 gram x 1 cm s -2 1 dyn = 1 g x 1 cm s-2. (iii) Relation between dyne and newton. 1 N = 1 kg x 1 m s-2 = 1000 g x 100 cms-2 = 105 g cm s-2 Or 1 N = 105 dyne. [Note] Newton and dyne are the absolute units of force. Gravitational units. Gravitational unit of force is the force which produces acceleration equal to the acceleration due to gravity in a unit mass. (i) Kilogram weight (kg. wt.) = g N = 9.8 N (ii) Gram weight (g. wt.) = g dyne = 980 dyne Conversion of gravitational units into absolute units To obtain the value of force, in absolute units, if it is known in gravitational units, we have simply to multiply by the value of g' in that system. Thus, Www.cbsecare.in CLASS-iX CHAPTER - force xg wt =x x 980 dyn. xkgwt =x x 9.8N. Relation between Force and Momentum From Newton's second law of motion, F = ma If a force F applied on a moving body of mass m , changes its velocity from u to u in time t. or Force = Rate of change of momentum. Thus we see that force applied on a body is equal to the rate of change of its momentum and this change in momentum takes place in direction of force. IMPULSE OF A FORCE (a) Introduction In previous article, we learnt that a moving body has momentum and that an effort (a force) is needed to stop it. It is our common experience that a smaller force takes more time to stop the body whereas a bigger force stops the same body in lesser time. This observation gives concept of a new quantity, force × time, which is named impulse. (b) Definition The product of the magnitude of a force applied on a body and the time for which it is applied, is called impulse of the force. It is represented by the symbol I. i.e., Impulse = Force × Time Or I=F. t The S.I. unit of impulse is Newton-second (N s) and the C.G.S. unit is dyne-second (dyn s). (c) Impulse and Momentum From Newton's second law of motion, This relation is called impulse equation. (d)Applications of Impulse (or Newton's second law of motion) in Daily Life (i) Catching the ball by a cricketer. While catching a fast moving cricket ball, the player moves his hands backward after catching the ball. By moving his hands backward, the cricketer increases the time. As a result he has to apply a small force on the ball. In reaction, the ball also Www.cbsecare.in CLASS-iX CHAPTER - force applies lesser force and the hands of the player are not injured. (ii) Jumping on a heap of sand. If someone jumps from a height on a heap of sand below, his feet move inside the sand very slowly. His momentum changes slowly requiring a lesser force of reaction from the sand. Thus man is not injured. (iii) Springs in vehicles. The vehicles are fitted with springs to reduce the hardness of the shocks. When vehicles move over an uneven road, they experience impulses exerted by the road. The springs increase the duration of impulse and hence reduce the force. (iv) Springs in seats. The seats are also fitted with springs to reduce their hardness. When we sit on them all of a sudden, the seats are compressed. The compression in creases duration of our coming to rest on the seat. The reaction force of seats becomes negligible. BASANT’S physics CLASS-iX 9777702608/909045548 CHAPTER - force PH-