Motion — Science STD 9 — Question

Is it possible that a force is acting on a body but still the work done is zero? Explain giving one example.

In an experiment to measure his power, a student records the time taken by him in running in a flight of steps on a staircase. Use the following data to calculate the power of the student:
Number of steps = 28
Height of each step = 20cm
Time taken = 5.4s
Mass of student = 55kg
Acceleration = 9.8ms^-2
Due to gravity.

Separate the following into physical and chemical changes:
Sublimation of a solid, Decomposition of water into hydrogen and oxygen by passing electric current, Formation of clouds, Making a fruit salad from raw fruits, Dissolving carbon dioxide in water.

Why is it difficult to hold a school bag having a strap made of a thin and strong string?

During the downpour in a village, the rain water carried away excess of nitrogenous and other compounds present in the soil to a pond. How will they affect the growth of algae and phytoplankton in the pond?

Calculate:
The mass of 1.0505 × 10²³ molecules of carbon dioxide (CO₂),

A man is instructed to carry a package from the base camp at B to submit A of a hill at a height of 1200 metres. The man weighs 800N and the package weighs 200N. If g = 10m/s²,

1. How much work does man do against gravity?
2. What is the potential energy of the package at A if it is assumed to be 

Define average speed, write any one point of difference between average speed and average velocity.

In which type of change a new substance is formed?