CBSE ADDA

Question:  A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 ms–1 .  Ans. (I) Wavelength of sound waves corresponding to 20 Hz, u = u = 344 ms–1, v = 20 Hz λ 1  =  v/n = 334/20  = 17.2 m  λ 2 =v/n = 334/20,000  = 0.0172 m Question:   The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute? Ans. Frequency of sound = 100 Hz = 100s –1   Number of vibrations in 1 s = 100 Number of vibrations in 1 minute, i.e. 60 seconds =100x60 =6000vibrations Question:   Explain how bats use ultrasound to reach a prey. Ans. Bats produce high pitched ultrasonic waves that are not heard by humans. These ultrasonic waves strike the prey like an insect and it is reflected back in the form of an echo, which is heard by the bat. This enables the bat to hover on the insect and catch it. Question:   A sonar device on a subma

1) Gravitation :   Gravitation is the force of attraction between two objects in the universe.  i) Gravitation may be the attraction of objects by the earth.     Eg :- If a body is dropped from a certain height, it falls downwards due to earth’s gravity.  If a body is thrown upwards, it reaches a certain height and then falls downwards due to the earth’s gravity. ii) Gravitation may be the attraction between objects in outer space.   Eg :-  Attraction between the earth and moon.           Attraction between the sun and planets. Centripetal force :-     When a body moves in a circular path, it changes its direction at every point. The force which keeps the body in the circular path acts towards the centre of the circle. This force is called centripetal force. If there is no centripetal force, the body will move in a straight line tangent to the circular path. 2) Universal law of gravitation :-  The universal law of gravitation states that,

Q. 1.  A toy of mass 0.1kg acquires a spped of 5 m/s when pushed forward. Ahat is the impulse given to the toy? Q. 2.  A projectile weighing 200kg is fired from a gun with a velocity of 500m/s. If the velocity of recoil of the gun is 12.5m/s calculate the mass of the gun. Q. 3.  A car of mass 2500kg accelerates at the rate of 4m/s2 and then at the rate of 16 m/s2. Calculate t6he ratio of the forces exerted by the engine in the two cases. Q. 4.  A driver accelerates his car first at the rate of 1.8m/s2 and then at the rate of 1.2m/s2 calculate the ration of the forces exerted by the engines. Q. 5.  A body of mass 5 kg undergoes a change in speed from 30 to 40m/s . calculate its increase in momentum. Q. 6.  A body of mass 100g is moving with a velocity of 15m/s Find the momentum associated with the ball. Q. 7.  body of mass 5 kg is moving with a uniform velocity of 10m/s It is acted upon by a force of 20N What will be its velocity after 1s. Q. 8.  In order to g

9tt Forces and Laws of motion Q. 9. What do you mean by a force? Q. 10. Give an example of a balanced force. Q. 11. Can force change the shape of an object? Give an example to prove your point. Q. 12. Do action and reaction force react in the same direction? Q. 13. Why a moving ship does takes longer time as compared to a car when equal breaks are applied? Q. 14. Why are passengers through in forward direction when a running bus stops suddenly? Q. 15. Why do passengers tend to fall sideways when the bus takes a sharp u turn? Q. 16. Why should a passenger hold on to prevent himself from swaying in a turning bus? Q. 17. Why do passengers tend to fall back when it starts suddenly? Q. 18. Why do passengers jumping out of a rapidly moving bus fall forward with his face downwards, if he does not run forward? Q. 19. Why does a passenger jumping out of a rapidly moving bus fall forward with his face downwards? Q. 20. Why can dust be removed by shaking it, or beating it by a carpet

CBSE ADDA Newton's Law of Motion (Objective Type Questions) (1) Friction force can be reduced to great extent by a. lubricating the two moving parts b. using ball bearing between two moving parts c. introducing a thin cushion of air maintained between two relatively moving surfaces d. all the above (2)  Force exerted on a body can change its a. kinetic energy b. direction of motion c. speed d. momentum (3)  A block is released from the top of a smooth inclined plane. Another block of same mass is allowed to fall freely from the top of the inclined plane. Both blocks reach the bottom of the plane a. with equal speed b. in equal interval of time c. with equal kinetic energy d. with equal momentum (4)  Which of the following expression does not represent the force acting on the body of mass m a. GM e m/(R e ) 2 b. m a c. μN d. 6ηπrv (5)  A body of mass m is resting on the floor. A minimum amount of force is applies on the body and the force continues to act

CBSE ADDA Calculate the average speed of a train, which covers 120 km in 2 hours.Distance covered by the train = 120 km .How far does a car travel in 2.5 h if its average speed is 60 km/h ? .A carriage moves 30 km in one hour. Calculate its speed in m/s. Is it average or uniform speed?  You are asked to run in a straight line. If you cover a distance of 5 m in each second, what is your (a) speed (b) velocity? Draw the velocity time graph of a body moving with uniform velocity of 5m/s. Write the equation of motion connecting S, u, v and a where the symbols have their usual meanings. What do you infer, if the distance-time graph is a straight line? The distance (S) in meters traveled by a particle is related to time(t) in seconds by the equation of motion . What is the initial velocity of the particle? .On a 60 km track, a train travels the first 30 km at a uniform speed of 30 km/h. How fast must the train travel the next 30 km so that the average speed is 40 km/h for the entire

CBSE ADDA 1. A car moves 100 m due east and then 25 m due west, (a) What is the distance covered by the car? (b) What is its displacement?  2. A person walks along the sides of a square field. Each side is 100 m long. What is the maximum magnitude of displacement of the person in any time interval?  3. In the hare-tortoise race, the hare ran for 2 min at a speed of 7.5 km/h, slept for 56 min and again ran for 2 min at a speed of 7.5 km/h. Find the average speed of the hare in the race.  4. A bus takes 8 hours to cover a distance of 320 km. What is the average speed of the bus?  5. The maximum speed of a train is 80 km/h. It takes 10 hours to cover a distance of 400 km. Find the ratio of its maximum speed to its average speed. 6. An object moves through 10 m in 2 minutes and next 10 m in 3 minutes. Calculate its average speed. 7. A car moves through 20 km at a speed of 40 km/h, and the next 20 km at a speed of 60 km/h. Calculate its average speed. 8. A boy leaves his house at

CBSE ADDA Q. 1. When the passenger m oving in a bus , the road side trees appear to b e moving ---------------------------------- (a) Back ward (b)For ward  Q. 2. In uniform motion the object covers ------------ (a) equel distance (b) un equal distance  Q. 3. Where objects cover un equal distances in equal interval of time then the motion is called ---------------- (a) uniform motion (b) non uniform motion  Q. 4. The average speed = =A car accelerate = (a) Total time taken /total distance traveled (b) Total distance traveled/total time taken  Q. 5. v = (a) t/s (b) s/t Q. 6. An object travels 16m in 4sec and another 16 m in 2 sec. what is the average speed of the object----------- (a) 5.33 ms-1 (b) 5. 34 ms-1  Q. 7. If we specify , its direction of motion along with its speed , the quantity that specifies with the aspects is called------------- (a) velocity (b) motion  Q. 8. Automobiles are fitted with a device that show the distance traveled . such a device is known ---------

Q. 1.  When the passenger m oving in a bus , the road side trees appear to b e moving ---------------------------------- (a) Back ward (b)For ward Q. 2.  In uniform   motion the object covers ------------ (a) equel distance (b) un equal distance Q. 3.  Where objects cover un equal distances in equal interval of time then the motion is called ---------------- (a) uniform motion (b) non uniform motion Q. 4.  The average speed = =A car accelerate = (a) Total time taken /total distance traveled (b) Total distance traveled/total time taken Q. 5.  v = (a) t/s (b) s/t Q. 6.  An object travels 16m in 4sec and another   16 m   in 2 sec. what is the average speed of the object----------- (a) 5.33 ms-1 (b) 5. 34 ms-1 Q. 7.  If we specify , its direction of motion along with its speed , the quantity that specifies with the aspects is called------------- (a) velocity (b) motion Q. 8.  A