# Class 9 Physics Dynamics 3.1 Encircle the correct answer from the given choices:i. Newtons first law of motion is valid only in the absence of:(a) force(b) net force(c) friction(d) momentum

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##### 3.1 Encircle the correct answer from the given choices:i. Newtons first law of motion is valid only in the absence of:(a) force(b) net force(c) friction(d) momentum

A thermos flask containing milk as a system is shaken rapidly. Does the temperature of milk rise?

Show that ratio of the root mean square speeds of molecules of two different gases at a certain temperature is equal to the square root of the inverse ratio of their masses.

3.5 A body has weight 20 \mathrm{~N} . How much force is required to move it vertically upward with an acceleration of 2 \mathrm{~ms}^{-2} ? (24 N)

ii. Inertia depends upon(a) force(b) net force(c) mass(d) velocity

A heat engine performs 100 \mathrm{~J} of work and at the same time rejects 400 \mathrm{~J} of heat energy to the cold reservoirs. What is the efficiency of the engine?

3.2 The weight of a body is 147 \mathrm{~N} . What is its mass? (Take the value of g as 10 \mathrm{~ms}^{-2} )(14.7kg)

EXAMPLE 11.4The turbine in a steam power plant takes steam from a boiler at 427{ }^{\circ} \mathrm{C} and exhausts into a low temperature reservoir at 77^{\circ} \mathrm{C} . What is the maximum possible efficiency.

EXAMPLE 3.6 A body of mass 5 \mathrm{~kg} is moving with a velocity of 10 \mathrm{~ms}^{-1} . Find the force required to stop it in 2 seconds.

3.1 A force of 20 \mathrm{~N} moves a body with an acceleration of 2 \mathrm{~ms}^{-2} . What is its mass?(10 kg)

3.3 What is the difference between:(ii) Action and reaction

Explain the working of a carburetor of a motorcar using Bernoullis principle

Identify the correct answer. What do you infer from Bernoullis theorem?(i) Where the speed of the fluid is high the pressure will be low.(ii) Where the speed of the fluid is high the pressure is also high.(iii) This theorem is valid only for turbulent flow of the liquid.

iv. A string is stretched by two equal and opposite forces 10 \mathrm{~N} each. The tension in the string is(a) zero(b) 5 \mathrm{~N} (c) 10 \mathrm{~N} (d) 20 \mathrm{~N}

An adiabatic change is the one in which.(a) No heat is added to or taken out of system.(b) No change of temperature takes place.(c) Boyles law is applicable.(d) Pressure and volume remains constant.

3.19 What would happen if all friction suddenly disappears?

Give an example of a natural process that involves an increases in entropy.

EXAMPLE 3.5 Two masses 4 \mathrm{~kg} and 6 \mathrm{~kg} are attached to the ends of an inextensible string which passes over a frictionless pulley such that mass 6 \mathrm{~kg} is moving over a frictionless horizontal surface and the mass 4 \mathrm{~kg} is moving vertically downwards. Find the acceleration in the system and the tension in the string.

Specific heat of a gas at constant pressure is greater than specific heat at constant volume. Why?

An airplane wing is designed so that when the speed of the air across the top of the wing is 450 \mathrm{ms}^{-1} the speed of air below the wing is 410 \mathrm{~ms}^{-1} . What is the pressure difference between the top and bottom of the wings? (Density of air =1.29 \mathrm{kgm}^{-3} ).

Is it possible to convert internal energy into mechanical energy? Explain with an example.

Explain how the swing is produced in a fast moving cricket ball.

Estimate the average speed of nitrogen molecules in air under standard conditions of pressure and temperature.

Worked Example 3Find the force that can stop a body to rest in 4 seconds from its initial velocity of 16 \mathrm{~ms}^{-1} . The mass of body is 3 \mathrm{~kg} .

A reversible engine works between two temperatures whose difference is 100^{\circ} \mathrm{C} . If it absorbs 746J of heat from the source and rejects 546 \mathrm{~J} to the sink calculate the temperature of the source and the sink.

EXAMPLE 3.8 A stone of mass 100 \mathrm{~g} is attached to a string 1 \mathrm{~m} long. The stone is rotating in a circle with a speed of 5 \mathrm{~ms}^{-1} . Find the tension in the string.

3.10 How much centripetal force is needed to make a body of mass 0.5 \mathrm{~kg} to move in a circle of radius 50 \mathrm{~cm} with a speed 3 \mathrm{~ms}^{-1} ? (9 \mathrm{~N})

3.12 Why is the law of conservation of momentum important?