Class 9 Science : Practice Questions and Answers

The speed of sound in steel is approximately 6000 m/s.

Close particle packing, rigidity, and elasticity make sound travel faster in solids.

Earthquake tremors produce infrasonic waves with frequencies below 20 Hz.

SONAR uses the reflection of sound waves to detect underwater objects.

Sound waves reflect from hard surfaces.

Reverberation is caused by repeated reflections of sound.

An echo is heard when reflected sound reaches the listener after a noticeable time gap.

Sound travels fastest in solids because particles are closely packed.

Radio waves are electromagnetic waves and can travel through a vacuum.

Light waves are transverse electromagnetic waves.

A fan converts electrical energy into mechanical motion.

Gravitational potential energy depends on the height of an object.

Work done = mgh = 75 × 10 × 4 = 3000 J.

One horsepower is approximately equal to 746 watts.

Total energy remains constant in a closed system according to the law of conservation of energy.

Electric irons consume a large amount of electrical power.

Power is the rate at which work is done or energy is used.

Power = Work ÷ Time = 100 ÷ 5 = 20 W.

As the ball rises, kinetic energy changes into potential energy.

Mechanical energy is the sum of kinetic and potential energy.