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Air resistance - class-XII

Description: air resistance
Number of Questions: 25
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Tags: force and pressure physics friction mechanics dynamics - explaining motion force
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Fill in the blanks 
Dynamic Fluid friction is _______ the sliding friction of solids.

  1. heigher than

  2. lower than

  3. same as

  4. higher than or equal to


Correct Option: A
Explanation:

Friction is the force that opposes motion between any surfaces that are in contact. There are four types of friction: static, sliding, rolling, and fluid friction. Static, sliding, and rolling friction occur between solid surfaces. Fluid friction occurs in liquids and gases.

The maximum limit of static friction is called limiting friction . So,the coefficient of static friction is usually more than that of sliding or kinetic friction. ... Static friction is greater because it works when two surfaces in contact are just to start a relative motion or are in rest.

The frictional force exerted by fluids is called:

  1. lift

  2. drag

  3. rolling friction

  4. dynamic friction


Correct Option: B
Explanation:

Lift is the upward acting force which is against gravity. Drag is the resistance force or friction force in fluid. The rolling friction is a resistive friction when a body rolls on a surface. The dynamic friction is the friction that must be overcome to maintain steady motion of one object relative to another because they remain in contact.

Boats and aeroplanes are ................. so as to reduce fluid friction.fill in the blank.

  1. large

  2. broader

  3. streamlined

  4. bigger


Correct Option: C
Explanation:

Boats and aeroplanes are streamlined so as to reduce fluid friction.
Aeroplanes, boats, fishes and birds which move through fluids have bodies of special shape, called streamlined shape, so as to reduce the friction due to fluid and avoid energy loss. 
For example,if the boat isn't streamlined, then it also has to push the water out of its way, increasing the drag force.

Fluid friction is exerted by _____ .

  1. solids and liquids

  2. solids and gases

  3. liquids and gases

  4. all the above


Correct Option: C
Explanation:

Liquids and gases are together referred to as fluids and the frictional force exerted by them on various objects is called fluid friction.

Fluid friction depends on ______

  1. speed of the object

  2. shape of the object

  3. nature of fluid

  4. all of the above


Correct Option: D
Explanation:

The speed of the object moving through a fluid affects the amount of friction acting against the motion of the object. More the speed, more is the friction. If the object is broad, it gives more area for the friction to act on. Thus, a streamlined body structure helps to cut through the fluid and reduce friction. Also, the nature of the fluid affects friction.

What measures are taken to reduce fluid friction in aeroplanes?

  1. Optimum speed is maintained with respect to the air

  2. Metals are choosen in a way to make the aeroplane lighter with smoother surface

  3. Streamlined body inspired by birds

  4. All of the above


Correct Option: D
Explanation:

It is very important to minimize fluid friction for aeroplanes because more friction for aeroplanes may produce more heat, which may harm the plane. Thus, various measures are taken to reduce friction. An optimum speed is maintained and the body structure of the plane is more streamlined so that it can cut through air with minimum friction. Also, such metals are chosen which are light but strong and also make for smooth surface.

To move at same speed, a car with flat face and another with streamlined body, will require different amount of force from the engine. The engine of the streamline bodied car will:

  1. exert less force

  2. exert more force

  3. exert zero force

  4. exert no force


Correct Option: A
Explanation:

A car with streamline body suffers less air friction while moving. And the car with flat face will suffer higher air friction. As a result, the car with streamline body will require less force from the engine to maintain the same speed.

While the arrow moves towards its target, the force acting on it are due to gravity and due to ____ of air.

  1. gravitational force

  2. muscular force

  3. magnetic force

  4. frictional force


Correct Option: D
Explanation:

When the arrow moves toward target, the force acting on it, is due to gravity and resistive force of air. The resistance force of air is also called as frictional force.  

Driving a car on a highway with windows open reduce the speed. This is due to:

  1. air resistance

  2. mechanical defect

  3. bad driving

  4. none of these


Correct Option: A
Explanation:

With windows open, air comes in and resists the motion of the car. So, due to air resistance, speed of the car reduces.

________________ is a force that resists the movement of a solid object through a fluid.

  1. Thrust

  2. Magnetic force

  3. Fluid friction

  4. None of these


Correct Option: C
Explanation:

By definition, fluid friction is a force that resists the movement of a solid object through a fluid.

What is drag ?

  1. Frictional force offered by fluids

  2. Friction force offered by a solid surface

  3. Pulling a body over a surface

  4. None of these


Correct Option: A
Explanation:

The other name for fluid friction is drag. It opposes the relative motion of any object in liquids or gases.

If we drop a cup of water on a plate and a cup of oil on another plate, water is seen to spread faster along diameter. This is an example related to:

  1. fluid friction

  2. rolling friction

  3. sliding friction

  4. static friction


Correct Option: A
Explanation:

This example relates to fluid friction. Speed of spreading of a volume of liquid depends on the liquid friction.Friction offered by water is less as its density is less than oil.

 Fill in the blank.
Dynamic Fluid Friction is ______ the sliding friction of solids.
  1. higher than

  2. lower than

  3. same as

  4. higher than or equal to


Correct Option: B
Explanation:

Dynamic Fluid Friction is lower than sliding friction of solids.

The deeper we go inside a flowing river, fluid friction:

  1. decreases

  2. increases

  3. remains same

  4. becomes zero


Correct Option: B
Explanation:

Fluid friction increases with the depth of a flowing liquid.

Which of the following statements indicates advantage of fluid friction?

  1. Walking on road

  2. Rolling stone on carpet

  3. Lubricating machine parts

  4. None of the above


Correct Option: C
Explanation:

Lubricating machine parts indicates advantage of fluid friction. This helps in lowering the friction and thus helps to keep parts of machinery in good condition for some time. 

Fill in the blank. 
Water offers _______ fluid friction as compared to honey. 
  1. higher

  2. lower

  3. same

  4. zero


Correct Option: B
Explanation:

Water, having less density than honey, offers lower fluid friction as compared to honey.

Hailstones falling from atmosphere relates to:

  1. static friction

  2. rolling friction

  3. sliding friction

  4. fluid friction


Correct Option: D
Explanation:

Hailstones falling from atmosphere is related to Fluid friction. Fluid friction occurs when there is resistance between particles in fluids or in gases.

Aeroplanes are streamlined to reduce friction.

  1. True

  2. False


Correct Option: A
Explanation:

Fluid

Aeroplanes are streamlined to reduce:

  1. fluid friction.

  2. sliding friction.

  3. kinetic friction.

  4. limiting friction.


Correct Option: A
Explanation:

Fluid friction occurs when a solid object travels through a liquid or gas. Aeroplanes are streamlined to reduce the fluid friction.

A spherical body of radius R consists of a fluid of constant density and is in equilibrium under its own gravity. If$ P(r)$ is the pressure at $r(r < R)$, then the correct option(s) is(are)?

  1. $P(r=0)=0$

  2. $\dfrac{P(r=3R/4)}{P(r=2R/3)}=\dfrac{63}{80}$

  3. $\dfrac{P(r=3R/5)}{P(r=2R/5)}=\dfrac{16}{21}$

  4. $\dfrac{P(r=R/2)}{P(r=R/3)}=\dfrac{20}{27}$


Correct Option: B

Mr. Verma (50 kg) and Mr. mathur (60 kg) are sitting at the two extreames of 4 m long boat (40 kg) standing still in water. to discuss a mechanics problem, they come the middle of the boat . neglecting friction with water, how far does the boat move in the water during the process?

  1. $1.33 m$

  2. $2 m$

  3. $10 m$

  4. $1 m$


Correct Option: A

Smooth surface produces ----- friction than a rough surface.

  1. Less

  2. More

  3. Constant

  4. None of these above


Correct Option: A
Explanation:

[A]  less

As the coefficient of friction $( \mu)$ is less for smooth surfaces 
so they produce less friction than rough surface.

The arrow with a pointed tip can move faster in air than that of with a blunt tip.

  1. True

  2. False

  3. Ambiguous

  4. Data insufficient


Correct Option: A
Explanation:

A pointed tip is more sharper than a blunt tip. So the air resistance for the pointed tip is less than that of the blunt tip. Since the air resistance opposes the motion, so the pointed can tip move faster than that of blunt tip. Thus, the given statement is true.

State whether the given statement is true or false:

Fluid friction is more than solid friction.

  1. True

  2. False


Correct Option: B
Explanation:

Static friction is the force that is the greatest. This is because it opposes motion the strongest between the various types of friction. Friction is greatest in the absent of lubrication,and since there is presence of fluid lubricant in fluid friction it has less friction as compared to solid friction,and in solid friction there is no presence of any lubricant.Hence the given statement is false.

A man of mass $m$ walks from end $A$ to the other end $B$ of a boat of mass $M$ and length $l$. The coefficient of friction between the man and the boat is $ \mu $ and neglect any resistive force between the boat and the water.

  1. If the man runs at his maximum acceleration, the acceleration of the boat is $ (m/M)g $.

  2. The maximum time take by the men to reach the other end of the boat is $ \sqrt { \dfrac { 2Ml }{ (M+m)\mu g } } $.

  3. If the man runs at his maximum acceleration, the acceleration of the boat is $ \dfrac { m }{ m+M } \mu g $.

  4. The minimum time take by the men to reach the other end of the boat is $ \sqrt { \dfrac { 2ml }{ (M+m)\mu g } } $.


Correct Option: A,B
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