HomeLearn Fast select Course
Change UnitChange SubjectLast Menu
CBSE-IX-Science

03: 2-Forces and Laws of Motion

 digibest Free NCERT Solutions with no signup needed page 2
 
  • Qstn #12
    According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
    Ans : According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force result is the two opposite and equal forces cancel each other but when one of these forces is bigger than inertia so the object moves in the direction of force applied. As this student explains the truck is massive so the force applied cannot overcome force caused by inertia. Therefore, the truck does not move.
  • Qstn #13
    A hockey ball of mass 200 g travelling at is struck by a hockey stick so as to return it along its original path with a velocity at. Calculate the change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.
    Ans : mass of hockey ball = 200g = 0.2 kg

    (return velocity)

    initial momentum of hockey ball = 2 kg m/s

    final momentum of hockey ball = -1 kg m/s

    change in momentum of hockey ball = 2 - (-1) = 2 + 1 = 3 kg m/s
  • Qstn #14
    A bullet of mass 10 g travelling horizontally with a velocity of strikes a stationary wooden block and comes to rest in 0.03 s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.
    Ans : v =u + at



    a = -150/0.03 =

    the distance of penetration of the bullet into the block



    = 4.5 - 2.25

    = 2.25 m

    the magnitude of the force exerted by the wooden block on the bullet

    m = 10g = 0.01kg

    = -50 N
  • Qstn #15
    An object of mass 1 kg travelling in a straight line with a velocity of collides with, and sticks to, a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.
    Ans : Wooden block is stationery (at rest) so its velocity

    mass of combined object is = 1 kg + 5 kg = 6 kg

    total momentum before the impact = 10 kg m/s

    law of conservation of momentum:

    total momentum just before the impact = total momentum after the impact= 10 kg m/s

    therefore, the velocity of the combined object:

    v = 10/6 = 1.67 m/s
  • Qstn #16
    An object of mass 100 kg is accelerated uniformly from a velocity of to in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.
    Ans : Initial momentum of the object = 500 kg m/s

    Final momentum of the object = = 800 kg m/s

    since v = u + at





    since = 50 N
  • Qstn #17
    Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.
    Ans : Since the mass of insect is negligible in comparison to the mass of motorcar therefore there will be no any change in the momentum of motorcar.
  • Qstn #18
    How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm? Take its downward acceleration to be.
    Ans : height from which dumb bell falls = 80 cm = 0.8 m

    since we know



    = 16

    v =

    v = 4 m/s

    momentum = mv = 40 kg m/s

    NCERT Solutions Class 9 Science
next page