Structure of the Atom — Science STD 9 — Question

Joseph jogs from one end A to the other end B of a straight 300m road in 2 minutes 30 seconds and then turns around and jogs 100m back to point C in another 1 minute. What are Joseph’s average speeds and velocities in jogging (a) from A to B and (b) from A to C?

What are infrasonic waves? Name two animals which produce infrasonic waves.

State the equations of free fall.

A boy tries to push a truck parked on the roadside.The truck does not move at all. Another boy pushes a bicycle moves through a certain distance. In which case was the work done more: on the truck or on the bicycle? Give a reason to support your answer.

Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?

A girl weighing 25kg stands on the floor. She exerts a downward force of 250N on the floor. What force does the floor exert on her?

A car is moving along a straight road at a steady speed. It travels 150m in 5 seconds:
What is its average speed?

 Explain why, wooden (or concrete) sleepers are kept below the railway line.

Which of the following conditions will give the most benefits? Why?

1. Farmers use high-quality seeds, do not adopt irrigation or use fertilizers.
2. Farmers use ordinary seeds, adopt irrigation and use fertilizer.
3. Farmers use quality seeds, adopt irrigation, use fertilizer and use crop protection measures.

When sound is reflected from a distant object, an echo is produced and heard in a winter night. Now, if the distance between the reflecting surface and the source of sound production remains the same, will you hear the echo of the same sound on a summer day? Explain.