Force & Laws of Motion (Physics) Q1. What is a force? Ans. Force is a push or pull which tries to bring the change in the state of rest or of uniform motion in a straight line. Q2. What are the effects of force? Ans.1. Force can be used to change the magnitude of velocity of an object (that is, to make the object move faster or slower) 2. Force can change its direction of motion. 3. Force can change the shape and size of objects Q3. Differentiate between balanced and unbalanced forces? Ans. When the resultant of a set of forces acting on a body is zero, the set is said to be a set of balanced forces. Balanced forces do not change the state of rest or of motion of an object They can change the shape of the object When the resultant of a set of forces acting on a body is not zero, the set is said to be a set of unbalanced forces. An unbalanced force acting on an object can bring change in its state of motion, direction or magnitude of velocity. Q4. State Newton laws of motion. Ans. The first law of motion is stated as: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction Q5. Why does a gun recoil? Ans. When a gun is fired, it exerts a forward force on the bullet. The bullet exerts an equal and opposite reaction force on the gun. This results in the recoil of the gun Since the gun has a much greater mass than the bullet, the acceleration of the gun is much less than the acceleration of the bullet For more Downloads logon: www.homestudyportal.com (99 88 156 156) Q6. State and prove the law of conservation of momentum. Ans. Suppose two objects (two balls A and B, say) of masses mA and mB are travelling in the same direction along a straight line at different velocities uA and uB, respectively and there are no other external unbalanced forces acting on them. Let uA>uB and the two balls collide with each other During collision which lasts for a time t, the ball A exerts a force FAB on ball B and the ball B exerts a force FBA on ball A. Suppose vA and vB are the velocities of the two balls A and B after the collision, respectively the rate of change of momentum of ball A during the collision will be= the rate of change of momentum of ball B during the collision will be Since (mAuA + mBuB)is the total momentum of the two balls A and B before the collision and(mAvA+ mBvB) is their total momentum after the collision the total momentum of the two balls remains unchanged or conserved provided no other external force acts. The sum of momenta of the two objects before collision is equal to the sum of momenta after the collision provided there is no external unbalanced force acting on them. This is known as the law of conservation of momentum. Q. 7. Define One Newton. Ans. one unit of force is defined as the amount that produces an acceleration of 1 m s-2in an object of 1 kg mass. Q.8. While catching a fast moving cricket ball, a fielder in the ground gradually pulls his hands backwards with the moving ball? Ans. In doing so, the fielder increases the time during which the high velocity of the moving ball decreases to zero. Thus, the rate of change of momentum of the ball is decreased and therefore the impact of catching the fast moving ball is also reduced. If the ball is stopped suddenly then its high velocity decreases to zero in a very short interval of time Thus, the rate of change of momentum of the ball will be large. Therefore, a large force would have to be applied for holding the catch that may hurt the palm of the fielder. Q.9. Define Inertia. Ans. Inertia is the natural tendency of an object to resist a change in its state of motion or of rest For more Downloads logon: www.homestudyportal.com (99 88 156 156) Q10 When a bus suddenly starts moving forward the passengers in the bus fall backward. Explain why? Ans This is because the lower part of the bodies of the passengers being in contact with the floor of the bus come in motion along with the bus. On the other hand the upper part of their bodies remains at rest due to inertia of rest. Hence the passengers fall backward. Q.11 Do all bodies have the same inertia? Ans. No, mass of an object is a measure of its inertia. Heavier or more massive objects offer larger inertia. Q12. Define Momentum. Ans. It is the product of mass and velocity of the body. Its units are kgm/s. It is a vector quantity. Q13 Show that Newton’s first law of motion is a special case of Newton’s second law of motion. Ans. The first law of motion can be mathematically stated from the mathematical expression for the second law of motion. F = ma Or F =m (v- u)/t That is, when F = 0, v = u for whatever time, t is taken. This means that the object will continue moving with uniform velocity, u throughout the time, t. If u is zero then v will also be zero. That is, the object will remain at rest. For more Downloads logon: www.homestudyportal.com (99 88 156 156)