9th Class (CBSE) Science: Force and Laws of Motion

Question: How does a karate player breaks a slab of ice with a single blow?

Answer: A karate player applied the blow with large velocity in a very short interval of time on the ice slab which therefore exerts large amount of force on it and suddenly breaks the ice slab.

Question: What is law of conservation of momentum?

Answer: Momentum of two bodies before collision is equal to the momentum after collision. In an isolated system, the total momentum remain conserved.

Question: Why are roads on mountains inclined inwards at turns?

Answer: A vehicle moving on mountains is in the inertia of motion. At a sudden turn there is a tendency of vehicle to fall off the road due to sudden change in the line of motion hence the roads are inclined inwards so that the vehicle does not fall down the mountain.

Question: For an athletic races why do athletes have a special posture with their right foot resting on a solid supporter?

Answer: Athletes have to run the heats and they rest their foot on a solid supports before start so that during the start of the race the athlete pushes the support with lot of force and this support gives him equal and opposite push to start the race and get a good start to compete for the race.

Question:Why do you think it is necessary to fasten your seat belts while travelling in your vehicle?
Or
How are safety belts helpful in preventing any accidents?

Answer: While we are travelling in a moving car, our body remains in the state of rest with respect to the seat. But when driver applies sudden breaks or stops the car our body tends to continue in the same state of motion because of its inertia. Therefore, this sudden break may cause injury to us by impact or collision. Hence, safety belt exerts a force on our body to make the forward motion slower.

Question: Explain how momentum gets conserved in collision of two bodies.

Answer: Consider two bodies i.e., balls A and B, the mass and initial velocities are m_Au_A and m_Bu_B respectively before collision. The two bodies collide and force is exerted by each body. There is change in their velocities due to collision.

∴ The total momentum of the two balls remains unchanged or conserved provided no other external force acts.