Nature of Matter: Elements, Compounds, and Mixtures — Science STD 8 — Question

State the uses of coke.

Name two contact forces.

Name two things which may develop on the face of adolescent boy s and girls during puberty due to the increased activity of sebaceous glands and sweat glands in the skin.

Vibration is the cause of producing sound. The fact is that sounds of frequencies less than about 20 vibrations per second cannot be detected by the human ear. Here is the table of data of some animals with their frequency range of sound waves.

Animal	Frequency range of sound waves
Elephant	15-12000 Hz
Human	20-22000 Hz
Dog	65-45000 Hz
Chicken	125-2000 Hz

Q.1. A cat made a purring sound that had a frequency of 35 Hz. Which of the animals mentioned in the table can hear the purring?
(a) Only human
(b) Only elephant
(c) Dog and chicken
(d) Elephant and human
Q.2. Which of these will reduce noise pollution?
(a) Regulating the use of loudspeakers
(b) Setting up more thermal power plants
(c) Using more electric vehicles
(d) (a) and (c) both

Prakhar came back from school and ate the cooked rice that was kept outside on the dining table. After few hours, he complained of diarrhoea and abdominal pain. His mother took him to the doctor who explained why this happened and asked them to preserve their food properly.
(i) What must have happened to Prakhar ?
(ii) Give the probable cause for his condition.

State two uses of a convex lens.

Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.