Dynamics - Physics For Class IX (Science Group) - Question Answers

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Physics For Class IX (Science Group)
UNIT 3: DYNAMICS
Questions Answers

Q.1: Define dynamics?
Ans: DYNAMICS:

"Dynamics is the branch of Mechanics which deals with the study of cause of motion."

In common, force is cause of motion. Several other factors like mass of the object and frictional force also affect the motion of an object. These factors are also studied under dynamics.
Example:
Newton's laws of motion.

Q.2: Define force and its unit?
Ans: FORCE:
An object at rest needs a force to get moving; a moving object needs a force to come in rest or change its velocity or direction.
Thus force can be defines as

"Force is the agent that changes the state of rest or uniform motion of a body."

In short,

• Force is required to change the position, state or shape of an object.
• Force can act as pull or push agent.
• Force produces acceleration.
• It can produce distortion.

Nature:
Force is a vector quantity. It is denoted by "F".
Formula:

F = ma

Where,
F is force applied on a body
m is mass of a body and
a is acceleration of a body
Unit:
In SI system, unit of force is Newton (N) or kg-ms^-2.
Newton:
One Newton (1 N) is the amount of force that can produce 1 ms^-2 acceleration in 1 kg mass.

OR

"Force acting on a body is said to be of one Newton if it produces an acceleration of 1 m/s² in the body of mass 1 kg in the direction of the applied force."
Instrument Use to measure force:
The magnitude of a force can be measured using a spring balance.

(Force moves or tends to move a body from state of rest)

Q.3: Define momentum with SI unit?
Ans: MOMENTUM:
The momentum depends upon the quantity of mass and velocity of the object. Greater the mass greater will be momentum. Similarly faster the speed greater will be momentum.
Definition:

"Momentum is defined as the quantity of motion contained in a body."

In terms of an equation,

The momentum of an object is equal to the mass multiplied by the velocity of the object
OR
 Momentum is the product of the mass and the velocity of a moving object.

Momentum = mass x velocity

Symbolically, the momentum is represented by p.
Formula:
Thus, the above equation can be written as:

p = mv

where,

• p is the momentum of a body
•  m is the mass and
• v is the velocity.

Nature:
The momentum is vector quantity. Its direction is same as that of velocity.

SI Unit of Momentum
The SI unit of momentum is kilogram meter per second (kg-ms^-1) or Newton second (Ns).
A mass unit is multiplied by a velocity unit to provide a momentum unit. This is consistent with the equation for momentum. The SI unit of momentum is describe below as:

Momentum = mass x velocity
Momentum = kg-ms^-1
Momentum = kg ms^-2 x s (multiply and divide by second 's')
or Momentum= Ns (Newton second)

OR

Example:

• If a cricket ball and a car are moving with same speed, we can not stop the car with hands but we can stop the ball, because the mas of the car is greater than the mass of a ball.
• It is not possible for a person to stop even a slow moving truck by pulling from backside, because the mass of  truck is greater than us.

Q.4: Describe momentum in terms of force? OR Derive the equation of momentum in terms of force?
Ans: MOMENTUM IN TERMS OF FORCE:

"The change in momentum is equal to the force multiplied by the time interval for which it was applied."

Consider a body of mass m, moving with initial velocity v_i. A force F acts on the body to produce acceleration a, therefore the final velocity after time t will become v_f.

We know that,

p = mv

if m is constant, then the change in velocity changes the momentum of body.

p_i = mv_i

p_f = mv_f

and p_f - p_i = (mv_f - mv_i) change in momentum

p_f - p_i = m (v_f -v_i)

divide both sides by t:

Q.5: Write a short note on safety devices?
Ans: SAFETY DEVICES:

"Safety devices are designed to decrease momentum of a body and provide extended time to remain safer."

Importance OF Safety Devices
The equation for change in moment Δp = Ft is important when it comes to consider a number of safety features in our lives.

If we are moving, we have momentum. To stop moving, a force must be applied. According to the equation Δp = Ft if we take longer time to stop, smaller force will be used to slow us down.

Some Safety Devices:
In Car:
Observe a car to identify the safety measures taken to reduce the risk of injuries in case of road accident. The car bumpers and grills are designed to provide extra time to reduce speed before any collision.
We also find some crumple zones or bumpers on front and backside. Seat belts are provided to hold the passengers from moving suddenly. There are extra cushions and air bags as well. These measures provide extra time to change momentum of the passenger inside it. This means that force acting on the passenger is less to prevents from risk of fatal injuries.

Safety Bags And Styrofoam:
Fragile objects, glassware and sensitive electronic components are packed in safety bags and Styrofoam packing to reduce the effect of sudden shock.

Helmets:
The helmets protect from direct strike on head and provide extra time to reduce speed before something strikes to it. Different safety helmets are used by workers, riders and sportsmen.

Q.6: Define law of conservation of momentum. Also use the principle of conservation of momentum in the case of collision of two objects. OR Explain the law of conservation of momentum.
Ans: LAW OF CONSERVATION OF MOMENTUM:
Statement:
The law of conservation of momentum states that

“The total momentum of an isolated system always remains constant or conserved.”

Explanation:
The concept of momentum is important particularly in situations when two or more bodies are interacting with each other. It is very useful quantity when it comes to calculate what happens in collision or explosion. It is always conserved when the colliding bodies are in an isolated system. This means that when bodies collide no external forces act on the bodies.

Collision Of Two Objects:
For simplicity consider a system of two billiard balls of mass m₁ and m₂ moving in straight line in the same direction with velocities u₁ and u₂ respectively, Where u₁ is greater than u₂.

Total momentum of the system before collision

= m₁u₁ + m₂u₂ ....... (i)

As u₁ > u₂, then after time "t", they will collide with each other.

After collision the velocities become v₁ and v₂ respectively,
therefore,
Total momentum after collision

= m₁v₁ + m₂v₂ ....... (ii)

According to law of conservation of momentum:

[Total momentum of the system before collision] = [Total momentum of the system after collision ]

thus, m₁u₁ + m₂u₂ = m₁v₁ + m₂v₂.

Conclusion:
So, we conclude that the total momentum of the system before and after the collision will remain the same, if there is no external force acting on the bodies. This is known as the Law Of Conservation Of Momentum.

Q.7: State and explain Newton's first law of motion.
Ans: NEWTON’S FIRST LAW OF MOTION:
Newton's Laws of motion were published in Latin language in 1687 by Issac Newton (1642-1727).
The first law of motion was written in latin language as:
Statement In Latin Language:

“Lex I: Corpus omne perseverare in statu suo quiescendi vel movendi uniformiter in directum, nisi quatenus a viribus impressis cogitur statum illum mutare.”

So we can define Newton’s first law of motion as:
Statement in English Language:

"A body continues its state of rest or of uniform motion in a straight line unless an external force acts on it." The Newton's first law is also called law of inertia."

Examples Of Newton's First Law of Motion:

1. The table placed in classroom, always remains at the same place until some external force apply to move it.
2. A book placed on the table remains at its place unless someone picks it back.
3. Similarly, a satellite in the space continuously moves with constant speed because there is no air or force of friction in the space.
4. A ball rolling on the ground however stops after some time because friction of ground and air resistance exert force on it and change its state of motion or direction of motion.

Q.8: Define inertia? Why Newton's first law of motion is also called the law of inertia? Give examples of Inertia.
Ans: INERTIA:
Inertia is the property of an object due to which it tends to continue its state of rest or motion. Inertia is resistance to change the state.
That's why Newton's first law of motion is also called the law of inertia.

Example:

1. If we put our bag on the seat next to us in a bus. Whenever the bus stops suddenly, the bag slides forward off the seat. It happens because the bag was initially moving forward because it was on a moving bus. When the bus stopped, the bag continued moving forward, which was its initial state of motion, and therefore it slid forward off the seat.
   
   
2. When a bus starts moving the passengers feel a backward jerk, because their lower part of body moves along the motion of bus but the upper part of the body tends to stay at its initial position of rest.
   
   
3. When we stop paddling our bicycle it does not stop at once. The bicycle continues moving. However the road's friction and air resistance act against its motion and bring it to rest after some time.

Q.9: State Newton's second law of motion and derive its expression?
Ans: NEWTON'S SECOND LAW OF MOTION:
Newton's second law of motion describes the relation between force and acceleration. Newton's second law of motion states that:

”when a net force acts on a body it produces acceleration in the direction of force. The acceleration is directly proportional to force and inversely proportional to mass of body”.

Expression Mathematically:
Therefore,

Example Of Newton's Second Law Of Motion:
Take few marbles of different size. Select one marble of very small size and another one larger about double the mass of first marble. Now try hitting the marbles one by one with third marble with same force. We observed that the smaller marble catches almost double the acceleration as compared to bigger marble. which proves the statement of Newton's second law of motion.

Q.10: Define mass and weight?
Ans: MASS:

"Mass is the quantity of matter possessed by a body."

OR

"Mass is the actual amount of material contained in a body."

• It is denoted by m.
• It is independent of everything.
• It is an intrinsic property of the body and remains the same wherever the body might be.
• Nature:
  It is a scalar quantity.
• Unit:
  Its S.I. unit is kilogram (kg).
• Formula:
  It can be find out by the following formulas:
  
  m = F / a
  m = W / g

WEIGHT:

"Weight is the force with which the earth attracts a body towards its center."

OR

"Weight is the force exerted by the gravity on that object."

OR

"Weight is a measure of how strongly gravity pulls on that matter."

• It is denoted by W.
• The weight of the body differs from place to place. It is different on the earth, moon, and other places due to difference of gravitational pull.
• For example
  Objects weigh lesser on the moon where gravity is lower as compared to that on the Earth.
• Nature:
  It is a vector quantity.
• Unit:
  Its S.I. unit is Newton (N).
• Formula:
  The weight of an object is the mass times the acceleration due to gravity. So weight is a force.
  (Force = mass x acceleration). Therefore
  
  W = mg.

Q.11: Differentiate between Mass and Weight.
Ans: Difference Between Mass and Weight

Comparison Chart	MASS	WEIGHT
Definition	Mass is the quantity of matter in a body regardless of its volume or of any forces acting on it.	Weight is a measurement of the gravitational force acting on an object.
Effect of gravity	Mass is always constant at any place.	The weight of an object depends on the gravity at that place.
Unit of Measurement	Mass is measured in kilogram (kg).	Weight is measured in Newton (N).
Nature	Mass has no direction so it is scalar quantity.	Weight always directed downward towards the center of the earth so it is vector quantity.
Formula	Its formula is m = F / a.	Its formula is W = mg.
Balance used for measurement	Mass is measured using a pan balance, a triple beam balance, lever balance or electronic balance.	Weight is measured using a spring balance.

Q.12: State and explain Newton's third law of motion?
Ans: NEWTON'S THIRD LAW MOTION:
Newton’s third law of motion can be defined as:

"To every action, there is an equal and opposite reaction."

Example. 1: Many times we throw a ball towards wall and it bounces back. If it is thrown with greater force the ball is returned back with greater push. It is because the wall reacts against the action of ball.
Example.2: While walking on ground we push the ground with feet the ground pushes us back thus we move.

Action And Reaction Forces:
(i) As these forces always occur in pairs, so when one body pushes against another, the second body pushes back just as hard.
Example. 3: When we put a book on table the book pushes the table downward, the table pushes back the book upward.

(ii) The action and reaction are forces that occur together as a pair. They are always equal in quantity but opposite in direction.
Example.4: While standing on ground the gravity pulls us down against the ground, the ground pushes up against our feet.
Example.5: When a rocket ignites its fuel behind it, the expanding exhaust gas pushes on the rocket causing it to accelerate.

Q.No.13: What are different forces required for uniform circular motion? Define each and give example of uniform circular motions also?
OR
Explain the forces acting on a body moving on a curved path.?
Ans: FORCES REQUIRE FOR UNIFORM CIRCULAR MOTION:
There are two types of force required for uniform circular motion:

• Centripetal force
• Centrifugal force

Centripetal Force:
"The force required to move a body along a circular path is called Centripetal force."
It is denoted by F.
The centripetal force is always directed towards enter of the circular path. It depends on three factors:

1. The velocity of the object v
2. The object's distance from the center “r” and
3. The mass of the object “m”.

It is given by relation

Where m = mass of body moving in circle.
v = velocity of body.
r = radius of circle
The velocity of the object is constant and perpendicular to a line running from the object to the center of the circle.

Centrifugal Force:
Centrifugal force is the tendency of an object to leave the circular path and fly off in a straight line. Thus it is defined as:
"A force that acts outward on a body which moves along a curved path is called centrifugal force."

OR

"Centrifugal force is tendency of an object to move away from circular path." 

• It is always directed away from center of curvature.
• The magnitude of centrifugal force is equal but opposite in direction to centripetal force.

Example Of Uniform Circular Motions:
Take a smaller bucket, tie a piece of string to its handle. Hold the other end of string and rotate the bucket in vertical circle. We may feel some pull on our arm. Now we put few coins in the bucket, and again rotate it. It is amazing the coins do not fall even the bucket goes bottom up.
More interesting will be the experimenting with some water. Pour about a cup of water in the bucket. Now try rotating the bucket around and up. The water stuck to the bottom of bucket.
Forces acting:
The force that keeps it stuck is known as centrifugal force and
The force we apply against the pull on our arm is known as centripetal force.

Q.14: Write down few applications of centrifuge?
Ans: Application of Centrifuge:
Centrifuge appliances are used to separate heavier particles from lighter particles in liquids e.g.

• Sugar crystals are separated from molasses.
• Blood analysis is carried out through a centrifuge process in laboratory.
• Cream separator is used to separate the cream from skimmed milk.
• An ultra centrifuge is used for separating small particle from large molecules.
• Gas centrifuge is used for separation of isotopes etc.

Some of them are:

1. Road Banking:

• The outer edge or bank of the road is raised to a certain height at the curved part of roads.
• This provides the centripetal force against the tyres of vehicle hence prevents from skidding.

2. Cream Separator:

• The milk plants in country are using high speed spinners to separate cream from milk.
• The skimmed milk is heavier whereas the cream is lighter.
• When the milk is spun at high speed the heavy particles are pushed towards the walls of the spinner.
• These particles push the lighter particles of cream to the center where from it is collected through a tube.

3. Dryer:

• Now a days built-in dryer is available in most of washing machines.
• It spins the wet clothes hence the water droplets are thrown away from the perforated walls of the dryer and clothes get dry instantly.

Q.No.15: Define friction with examples? Write down its expression and on what factors it depends?
Ans: FRICTION:

"The force that resists relative motion between two surfaces is called friction."

Cause:
Friction is a contact force caused by the roughness or deformation of the materials in contact.
Frictional forces are always parallel to the plane of contact between two surfaces and opposite to the direction of the applied force.
Example:

• The frictional force between a wooden block and cemented floor caused by the roughness of both the surfaces is projected.
• When we through a ball, it comes to rest after covering some distance.
• When we kick a ball and a box with same force, the ball covers more distance than a box.
• Friction helps us walk easily, it prevents from sliding.

Expression:
Friction is self adjusting. It can increase to a certain value known as limiting force (F_δ).
It (F_δ) is proportional to normal force R.

F_δ ∝ R

The ratio between limiting force and normal reaction R is constant that is represented by coefficient of friction μ.
Thus,

F_δ = μR ........(i)

or μ = ^F_δ / _R

when a body is placed on a surface its weight w acts downward then according to Newton's third law of motion R = W
Here w = mg
By putting the value R = mg in eq. (i) we get

F_δ = μmg ....... (ii)

The coefficient of friction has different values for different surfaces. It can be define as:
CO-EFFICIENT OF FRICTION:
Coefficient of friction (µ) has no unit because it is the ratio of frictional force and normal reaction. Its value depends upon the nature of the surface in contact but it is independent of the area of surface in contact.

FACTORS ON WHICH FRICTION DEPENDS:
There are two factors on which the force of friction depends:

• It depends upon the nature of the surface.
• It is directly proportional to the normal reaction.

Q.No.16: Write down different types of frictions?
Ans: TYPES OF FRICTIONS:
1. Static friction:
It is force acting on an object at rest that resists its ability to start moving. The maximum static friction is known as “limiting friction”.

2. Kinetic friction:
It is the force that resists the motion of a moving object. It is interesting to know that in almost all situations, static friction is greater than kinetic friction.

3. Sliding friction:
When one body slides over the other body the friction between two surfaces is said to be sliding friction.

4. Rolling friction:
When a body moves on wheels the friction is said to be rolling friction. Rolling friction is much lesser than the sliding friction.

Q.No.17: Write down the advantages and disadvantages of friction.
Ans: ADVANTAGES OF FRICTION:

• Friction enables us to walk on ground.
• Friction protects from sliding, as sand is thrown to maintain friction on inclined railway tracks during rain.
• The car brakes slow down the car to stop safely.
• Threads and grooves are designed on tyres to increase the friction and improve grip between road and wheel.
• Now vehicles are equipped with Anti-lock Braking System (ABS). ABS is designed to maintain steering stability, improve vehicle control, avoid skidding and decreases stopping distances on dry and slippery surfaces. The ABS maintains the static friction as the wheel starts slipping it releases the brake automatically for a fraction of a second then holds wheel again to create static friction between road and tyres.

DISADVANTAGES OF FRICTION:

• A large amount of energy is wasted in the machines due to friction.
• Friction leads to wear and tear of parts hence increases the service cost.
• Failure of oil pump in car engine results contact between dry metals which yields high temperature hence the car engine is seized.
• Due to friction, surface destroys.
• Excess friction makes difficult to move object.

Q.No18: Describe some methods or ways to reducing friction.
Ans: METHOD OR WAYS TO REDUCING FRICTION:

• Wheels, pullies, ball bearings, lubricants and graphite are used to overcome the friction.
• Lubricating the motor axle, sewing machine and bicycle chain reduces friction and prevents wear and tear.
• The shape of vehicle is also designed to reduce air resistance.
• Reduce the force acting on the surface.
• A thick layer of oil is used between sliding surfaces to reduce friction.

Short Questions Answers

Q.1: Mention the required force for the following actions?
Ans: Required force For Different Actions:

S.NO.	Action	Required Force
1.	The pull of gravity on a fly	0.001 N
2.	The pull of gravity on an apple	1 N
3.	The frictional force slowing a rolling football	2 N
4.	The force required to squash an egg	50 N
5.	The tension in a rope towing a car	1000 N (1 kN)
6.	The fractional force exerted by the brakes of a car	5000 N (5 kN)
7.	The push from the engines of a space rocket	1000000 N (1 MN)

Q.2: Define the term limiting friction and self-adjust friction.
Ans: LIMITING FRICTION:
The maximum value of the resisting force between the two surfaces before the motion starts (static friction) is called as limiting friction.

SELF-ADJUSTING FRICTION:
The force of friction has ability to increase its value with the increase of applied force till it reaches a maximum value is called self-adjusting friction. Due to this strange nature of frictional force, we call it self-adjusting force.

Q.3: Mention the value of coefficient of friction for the following surfaces.?
Ans: Value of Coefficient Of Friction For Different Surfaces:

S.NO.	Different Surfaces	Coefficient Of Friction
1.	Tyre and Road	1.0
2.	Iron and Iron	1.0
3.	Glass and Glass	0.9
4.	Wood and Cemented floor	0.6
5.	Wood and Marble	0.4
6.	Wood and Leather	0.4
7.	Wood and Wood	0.3
8.	Metal and Metal	0.3

Q.4: Why do we use ball bearings in vehicles and other things?
Ans: In the case of rolling friction the contact area between two surfaces is lesser than the contact area in the case of sliding bodies. Therefore ball bearings are used in vehicles that reduce the contact area as compared to the contact area of axle and bush.
Similarly a pedestal fan with a ball bearing saves a lot of electricity therefore the customers always select a fan with a ball bearing rather than one with a bush and axle.

Q.5: Why safety ramps are constructed on a long roadside?
Ans: Safety ramps are constructed on a long roadside where failure of brakes is feared due to sharp inclination of road.

Q.6: What is the difference between Rolling Friction and Sliding Friction?
Ans: Differences Between Rolling Friction And Sliding Friction:

S.NO.	Rolling Friction	Sliding Friction
1.	Rolling friction takes place when an object rolls on the surface.	Sliding friction takes place when two surfaces are rubbed against each other.
2.	Rolling friction takes place due to the deformation of surfaces.	Sliding friction takes place due to interlocking between microscopic surfaces.
3.	The coefficient of rolling friction is dependent on the radius of the rolling object, the depth to which the object can sink, and the toughness of the surface.	The coefficient of sliding friction depends on the texture of the surface and temperature to a certain extent. It is independent of external factors.
4.	Coefficient of rolling friction: Fr = μrN	Coefficient of sliding friction: Fk = μkN