A. Zero, because the acceleration of his fall is equal to the acceleration due to gravity of the earth.
A. Chalk consists of pores forming capillaries. When it is immersed in water, the water begins to rise in the capillaries and air present there is expelled in the form of bubbles.
A. The presence of air, a poor conductor of heat, between the double glass wall of a thermos flask, keeps the liquid hot or cold inside a flask for a long time.
A. When a ball falls, it is temporarily deformed. Because of elasticity, the ball tends to regain its original shape for which it presses the ground and bounces up (Newton's Third Law of Motion).
A. On tilting the centre of gravity of the boat or bus is lowered and it is likely to overturn.
A. By addition of salt the boiling point of water gets raised which helps in cooking.
A. Sea water contains salt, and other impurities with different boiling points, which jointly raise its boiling point.
A. Addition of soap decreases the surface tension of water. The energy for spraying is directly proportional to surface tension.
A. Steel is more elastic for the same stress produced compared with rubber.
A. Violet and blue light have short waves which are scattered more than red light waves. While red light goes almost straight through the atmosphere, blue and violet light are scattered by particles in the atmosphere. Thus, we see a blue sky.
A. As we go up, the pressure and density of air goes on decreasing. A partially filled pen leaks when taken to a higher altitude because the pressure of air acting on the ink inside the tube of the pen is greater than the pressure of the air outside.
A. The gravity of the moon is one-sixth that of the earth; hence the weight of a person on the surface of the moon will be one-sixth of his actual weight on earth.
A. A liquid burns if its molecules can combine with oxygen in the air with the production of heat. Hence, oil burns but water does not.
A. We see objects when light rays from them reach our eyes. As mirrors have a shiny surface, the light rays are reflected back to us and enter our eyes.
A. Because the density of iron is more than that of water but less than that of mercury.
A. As the pressure inside the cooker increases, the boiling point of water is raised, hence, the cooking process is quicker.
A. Wood contains a complex mixture of gases and tar forming vapours trapped under its surface. These gases and tar vapours escape, making a crackling sound.
A. The light from a star reaches us after refraction as it passes through various layers of air. When the light passes through the earth's atmosphere, it is made to flicker by the hot and cold ripples of air and appears as if the stars are twinkling.
A. Because the rolling force of friction is less than the dynamic force of sliding friction.
A. All will fall at the same speed in vacuum because there will be no air resistance and the earth's gravity will exert a similar gravitational pull on all.
A. As the bullet leaves the nozzle of the gun's barrel with momentum in a forward direction, as per Newton's Third Law of Motion, the ejection imparts to the gun an equal momentum in a backward direction.
A. Both wood and wool are bad conductors of heat. They do not permit heat rays to reach the ice easily.
A. When the body temperature rises, the sweat glands are stimulated to secrete perspiration. It is nature's way to keep the body cool. During the process of evaporation of sweat, body heat is taken away, thus, giving a sense of coolness.
A. Because ice is lighter than water it floats on it. However, ice is heavier than alcohol and therefore it sinks in alcohol.
A. By keeping hands close to mouth the sound is not allowed to spread (phenomenon of diffraction of sound) in all directions but is directed to a particular direction and becomes louder.
A. Because of low temperature the water inside the bottle freezes. On freezing it expands, thereby its volume increases and pressure is exerted on the walls.
A. To permit expansion of rails due to heat generated by friction of a moving train.
A. Copper melts at I08.3°C and forms a black powder on reacting with atmospheric oxygen. For heater elements a metal should have more resistance to produce heat.
A. The surface of a liquid is the seat of a special force as a result of which molecules on the surface are bound together to form something like a stretched membrane. They tend to compress the molecules below to the smallest possible volume which causes the drop to take a round shape as for a given mass the sphere has minimum volume.
A. Weight of hydrogen is less than the weight of air displaced by it. In balloons hydrogen is normally filled because it is lighter than air.
A. In order to keep the vertical line passing through our centre of gravity always between our feet, which is essential to attain equilibrium or stability.