The ozone layer in the upper atmosphere is crucial for protecting life on Earth. It absorbs harmful ultraviolet (UV) radiation from the Sun, which could otherwise cause skin cancer, cataracts, and other health problems in humans and animals. So, ozone in the upper atmosphere is not harmful but rather protective.
The other statements are correct:
A: Most of the forests have been lost in tropical areas. This is true, as deforestation has been a significant issue in tropical rainforests.
B: Greenhouse effect is a natural phenomenon. This is correct; the greenhouse effect is a natural process that warms the Earth.
C: Eutrophication is a natural phenomenon in freshwater lakes. This is also correct; eutrophication, often caused by excess nutrients, is a natural process, though it can be exacerbated by human activity.
(A) True - Tropical forests have faced highest deforestation rates
(B) True - Greenhouse effect naturally maintains Earth's temperature
(C) True - Natural eutrophication occurs over geological time
(D) False - Stratospheric ozone (upper atmosphere) is protective, not harmful (it shields from UV rays)
The correct answer is (D) Ozone in upper part of the atmosphere is harmful to animals.
A block of certain mass is placed on a rough floor. The coefficients of static and kinetic friction between the block and the floor are 0.4 and 0.25 respectively. A constant horizontal force \( F = 20 \, \text{N} \) acts on it so that the velocity of the block varies with time according to the following graph. The mass of the block is nearly (Take \( g = 10 \, \text{m/s}^2 \)):
A wooden block of mass M lies on a rough floor. Another wooden block of the same mass is hanging from the point O through strings as shown in the figure. To achieve equilibrium, the coefficient of static friction between the block on the floor and the floor itself is