List any three reasons why you would thi | Class 9 Science Chapter Why Do We Fall ill, Why Do We Fall ill NCERT Solutions

Abdul, while driving to school, computes the average speed for his trip to be 20 km h^-1. On his return trip along the same route, there is less traffic and the average speed is 40 km h^-1. What is the average speed for Abdul’s trip?

An object of mass 40 kg is raised to a height of 5 m above the ground. What is its potential energy? If the object is allowed to fall, find its kinetic energy when it is half-way down.

A driver of a car travelling at 52 km h^-1 applies the brakes and accelerates uniformly in the opposite direction. The car stops in 5 s. Another driver going at 3 km h^-1 in another car applies his brakes slowly and stops in 10 s. On the same graph paper, plot the speed versus time graphs for the two cars. Which of the two cars travelled farther after the brakes were applied?

Fig 8.11 shows the distance-time graph of three objects A,B and C. Study the graph and answer the following questions:

          Fig. 8.11

(a) Which of the three is travelling the fastest?
(b) Are all three ever at the same point on the road?
(c) How far has C travelled when B passes A?
(d) How far has B travelled by the time it passes C?

Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 m s^-1 before the collision during which they stick together. What will be the velocity of the combined object after collision?

Soni says that the acceleration in an object could be zero even when several forces are acting on it. Do you agree with her? Why?

Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 m s^-1 and 1 m s^-1, respectively. They collide and after the collision, the first object moves at a velocity of 1.67 m s^-1. Determine the velocity of the second object.

How do poriferan animals differ from coelenterate animals?

The speed-time graph for a car is shown is Fig. 8.12.

          Fig. 8.12

(a) Find how far does the car travel in the first 4 seconds. Shade the area on the graph that represents the distance travelled by the car during the period.
(b) Which part of the graph represents uniform motion of the car?

How are sol, solution and suspension different from each other?

A 0.24 g sample of compound of oxygen and boron was found by analysis to contain 0.096 g of boron and 0.144 g of oxygen. Calculate the percentage composition of the compound by weight.

An Italian bee variety A. mellifera has been introduced in India for honey production. Write about its merits over other varieties.

A stone is released from the top of a tower of height 19.6 m. Calculate its final velocity.

Establish the relationship between speed of sound, its wavelength and frequency. If velocity of sound in air is 340 m s^–1, calculate

(i) wavelength when frequency is 256 Hz.
(ii) frequency when wavelength is 0.85 m.

Give two reasons to justify—

(a) water at room temperature is a liquid.
(b) an iron almirah is a solid at room temperature.

On the earth, a stone is thrown from a height in a direction parallel to the earth’s surface while another stone is simultaneously dropped from the same height. Which stone would reach the ground first and why?

State the number of atoms present in each of the following chemical species
(a) CO₃^2–
(b) PO₄^3–
(c) P₂O₅
(d) CO

Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the friction force that will be exerted on the cabinet?

How are sol, solution and suspension different from each other?

How do substances like CO2 and water move in and out of the cell? Discuss.