Question

Mass of one of the objects, m1 = 1.5 kg Mass of the other object, m2 = 1.5 kg Velocity of m1 before collision, v1 = 2.5 m/s Velocity of m2, moving in opposite direction before collision, v2 = -2.5 m/s (Negative sign arises because mass m2 is moving in an opposite direction) After collision, the two objects stick together. Total mass of the combined object = m1 + m2 Velocity of the combined object = v According to the law of conservation of momentum: Total momentum before collision = Total momentum after collision m1v1 + m2 v2 = (m1 + m2) v 1.5(2.5) + 1.5 (-2.5) = (1.5 + 1.5) v 3.75 – 3.75 = 3 v v = 0 Hence, the velocity of the combined object after collision is 0 m/s.

  • force and laws of motion CBSE Class 9
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Answer

The truck has a large mass. Therefore, the static friction between the truck and the road is also very high. To move the car, one has to apply a force more than the static friction. Therefore, when someone pushes the truck and the truck does not move, then it can be said that the applied force in one direction is cancelled out by the frictional force of equal amount acting in the opposite direction.

Therefore, the student is right in justifying that the two opposite and equal cancel each other.

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