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19, CHAPTER 2, , FORCE AND LAWS OF MOTION, , A. Multiple Choice Questions:, , 1. An unbalanced force acts on a body. The body, (a) must remain at rest (b) must move with uniform velocity, (c) must be accelerated (d) must move along a circle, 2. Lubricating the moving parts of machine, (a) increases friction decreases friction, (c) increases limiting friction (d) none of these, 3. Action and reaction forces do not balance each other because they, (a) act on the same body do not act on the same body, (c) are in opposite direction (d) are not equal, 4. A horse rider falls forward, when the horse suddenly stops. This is due to, (a) the inertia of the horse : (b)) the inertia of the rider, (c) large weight of the horse (d) losing balance, 5. A body of mass 10 kg and velocity 10 m/s collides with a stationary body of, mass 5 kg. After collision, both bodies stick to each other. The common, velocity would be, (a) 3/20 m/s (6) 20/3 m/s _— (c) 10/3 m/s (d) 3/10 mis, 6., , A force of 200 N is applied at one end of a spring balance and an equal force is, applied at the other end at the same time. The reading on the balance will be, , (a) 200N ((b) Zero (c) 100N (d) SON., ANSWERS : | (c), 2 (b), 3 (b), 4 (b), 5.(b), 6 (b),

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cgqumie-- ----. a a, , 20, , Sol:, , Sol:, , Sol:, , Sol:, , Sol:, , Sol:, , QSecondary Science [x, , NS, VERY SHORT ANSWER TYPE QUESTIO, , with equal force so tha, , rope, In a tug of war, two teams are pulling the rop at will be the magnitud,, , Sua 0 z Wh, ribbon on the rope is held in the same position, , of the resultant force acting on the rope?, Zero, , _ Define One Newton., Ans:, , i f 1 Qi, One newton is defined as the force which produces an acceleration of 1 m/s? jp, a body of mass 1 kg., , What is the S.I. and C.GS. unit of momentum?, S.I. unit of momentum — kgm/s, C.GS. unit of momentum — gcm/s, , What is the physical quantity that measures the quantity of motion?, Momentum, , Why is it easier to stop a tennis ball than a cricket ball moving with the same, speed?, , Because the tennis ball has lesser momentum than the cricket ball., , SHORT ANSWER QUESTIONS, , State.Newton’s first law of motion. Give example,, , : Newton’s first law of motion states that, , an object at rest or in uniform motion, will remain at rest or in uniform motion, , ) on unless an unbalanced force acts on it., Example: A book lying on the table will remain at rest and will not move on its, own., , : 0 1 >rtia Of rest, apple remains at rest. As a result, the br rest and so the, , * 4: - vay lea i he }, behind. This detaches the apple from the tree and the apple a i | tne — :, to gravity. wnwards due

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Se eC rt Ct, EET LDP II Wt se eee, he De, he Pee ne gh nye eee Tee eS ee, - 8 TR TERT Ty ee Ne por TEES Ae y wry: ey i Sens : 5.2 CTE ee POD, , Guide, , 4. Whatis the momentum, velocity of 2 m/s?, , Sol: Momentum = Mass x Velocity = 40 kg 2 m/s = 80 kgm/s, , 5. A force of 20 N towards east is balanced by an unknown force F. What will, , be the magnitude and direction of unknown force?, Sol: 20 N towards west., , 21, of a boy of mass 40 kg when he walks with a uniform, , 6. Calculate the change in momentum of a car of mass 1500 kg when its speed, , increases from 18 km/h to 90 km/h uniformly., Ans: Here,, , Mass (m) = 1500 kg, Initial velocity (wv) =18 km/h = 18%1000m _,, , 3600 s, Final velocity (v) = 90 km/h = ger elO08 ma. = 25 m/s, 3600 s, Change in momentum = Final momentum — Initial momentum, = mv—mu, = m(v—u), = 1500(25 —5), = 1500 20, = 30000 kg m/s, , 7. For how long should a force of 100 N act on a body of mass 20 kg so that it, acquires.a velocity of 25 m/s?, Sol: Here,, , Initial velocity (u)=0 m/s, Final velocity (v) = 25 m/s, Force (F')=100 N, Mass (m) = 20 kg, Now, F=ma, F100, , ca eco oe le, m 20

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OSescondary Science x, , From first equation of motion, we have, , v=utat, 25=0+5, St = 25, al, 5, , Hence the force must act on the body for 5 seconds., , A body of mass m moving with a constant velocity v hits another body of the, Same mass moving with the same velocity v but in opposite direction an, , sticks to it. What is the velocity of the compound body after collision?, Sol: Zero, , 9. A cricket ball of mass 0.32 kg moving with a speed of 40 m/s is brought to, , rest in 0.2 sec. What is the average force applied by the player?, Sol: Here, , Mass (m) = 0.32 kg, , Initial velocity (uv) = 40 m/s, Final velocity (v) = 0 m/s, Time (¢) = 0.2 s, , , , Required force is 64 N. The negative sign shows, , that the force is in opposite, direction to the motion of the ball., 10. A body of mass 10 kg is moving with an acceleration Of 5 m/s. Calculate the, force acting on it. | ' |, Sol: Here |, , Mass (m) = 10 kg, Acceleration (a) = 5 m/s?, , Now, f =mxa=10x5=50N, Hence the force acting on the body is 50 N, , 11. What is friction?, , Sol: Friction is an obstruction to all kinds of motion,, , It iS an Opposin fi {, orce tha, appears whenever two bodies come in contact with €ach other. ;

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Guide, , D., I., , Sol:, , Sol:, , Sol:, , 4., , SHORT ANSWER QUESTIONS, , Write two effects produced by a force when it acts on a body. Give an example, of each effect., , (i) Force can move a body at rest. For example, the force of our hands can, make a stationary ball move from rest., , (ii) Force can accelerate or decelerate a body. For example, the force of our, hands can slow down a moving ball., , Show that the second law of motion gives the measure of a force., Newton’s Second Law of Motion states that the rate of change of momentum, , is directly proportional to the force applied on the body and takes place in the, same direction as the force., , Suppose a body of mass m is moving with a velocity u which becomes v in time, , t. Therefore, the initial and final momentum will be Pp, = mu and p, = mv, respectively., , Change in momentum =p,— p, =mv — mu = m (v— u), According to Newton’s second law of motion,, Force&dirate of change of momentum, , m(v—u), t, , Fo, , m(v—u, t, F=kma ., If the units of measurement are chosen in such a way that k = 1,, , then F= ma, , Thus, Newton’s second law of motion gives the measure of force in terms of, the mass and the acceleration produced on the body., , F=k where & is the constant of proportionality., , Derive mathematical relation between force and acceleration., Same as Q.no. 2, , A force of 50 N acts on a body initially at rest. Find the momentum of the, body at3 second and 6 seconds after the force starts acting., , Sol: Initial velocity (u) = 0 m/s, , ' Force (F) = 50 N, , (i) Momentum of the body at3 second (¢ =3 s)