Dynamics - Physics For Class IX (Science Group) - Self Assessment and Test book Exercise

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Physics For Class IX (Science Group)
UNIT 3: DYNAMICS
Self Assessment and Test book Exercise

SELF ASSESSMENT QUESTIONS

Q.1: Why momentum is considered equal to zero when a body comes to rest?
Ans: We know that momentum is P = mv.
If a body comes to rest, it means its velocity, v = 0, therefore its momentum becomes:

P = mv

P = m(0)

P = 0

So, when a body comes to rest its momentum is considered equal to zero.

Q.2: Why do you pull your hands while catching a fast moving ball?
Ans: A fast-moving ball tends to keep moving due to inertia. According to Newton's second law, the force with which a ball is moving is equal to its mass multiplied by its acceleration. When we catch a ball, the momentum of the ball is transferred from ball to hand. If we keep our hands stationary, the force with which momentum is transferred might hurt our hands. But as soon as we pull our hands back, it increases the time during which the high velocity of the moving ball decrease to zero and net momentum is decreased, thus reducing the force with which the ball makes an impact with our hands.

Q.3: What is reason that you experience a jerk whenever the school bus stops all of sudden?
Ans: When the school bus stops suddenly, we tend to fall forward because due to our inertia we tend to remain in a state of motion even though the bus has come to rest.

Q.4: Why it is dangerous to jump from a moving bus?
Ans: A man jumping out from a moving bus holds the inertia of motion. As the man lands on the ground, his feet come to rest instantly while the upper part of the body continues to move due to inertia of motion. Therefore, the person may fall forward. So, it is very dangerous to jump out of a moving bus.

Q.5: What is the role of force according to Newton's second law of motion?
Ans: According to Newton's second law of motion,
"A force is a vector that causes an object with mass to accelerate. Newton's second law states that the acceleration of an object depends upon two variables:

• The net force acting on the object and
• The mass of the object"

The acceleration of the body is directly proportional to the net force acting on the body and inversely proportional to the mass of the body. This means that as the force acting upon an object is increased, the acceleration of the object is increased. Likewise, as the mass of an object is increased, the acceleration of the object is decreased.

Q.6(a): What happens according to Newton's third law, while you pull a catapult?
Ans: Newton's Third-law of motion states that:

"For every action force, there is a reaction force that is equal in strength and opposite in direction."

Newton's Third-law is without a doubt the law that is seen most in the launch of the catapult. It is displayed countless moments in the experiment. This law applies a pushing force as well as pulling force too.

• "Pulling back the lever is the action, and flinging forward the marshmallow is the reaction."
• Similarly as the rubber bands of catapult are pulling on the hook forcefully, the hooks are pulling on the rubber bands with just as much force in the opposite direction.
• Also, when the key of the catapult is being pulled out of the key post, the key is pulling the string with the same amount of force that the string is pulling the key. These forces are just going in the opposite direction.

Q.6(b): What happens according to Newton's first law, while you pull a catapult?
Ans: According to Newton's first law of motion:

"An object at rest stays at rest until a force is applied, and an object in motion stays in motion, at the same speed, until a force acts upon it".

An object at rest stays at rest- this means that the projectile will always sit in the cap if we don't apply a force to it. Until a force is applied- the force we applied was the arm of the catapult. When we pull back the arm it stores up a lot of energy, but when we let go of the arm it changed the form of energy and applied a force to the projectile. This change in the energy created a force that launched the projectile forward.

Q.7: Why mass does not differ, while weight differs from place to place?
Ans: Weight of a body is the gravitational force on it and mass is the amount of matter in the body. Thus, weight is dependent on gravitational acceleration, g but mass does not depend on the value of g. Hence, the weight of a body will change from one place to another place because the value of g is different in different places. For example, the value of g on the moon is ¹ /₆ times the value of g on earth. As mass is independent of g, so it will not change from place to place.

Q.8: Why do we feel pushed outward while a car turns on a curved road?
Ans: The force that pulls out from the center on a body in circular motion is called centrifugal force and it increases with acceleration. Centrifugal force results in a strong outward pull on our vehicle. That's why we feel pushed outward while a car turns on a curved road. So we need to slow down a car before entering a curve.

Q.9: Which force prevents a passenger from falling down a roller coaster while it turns the riders into upside-down position?
Ans: This force is inertia which prevents us from falling down or out in a a roller coaster because inertia is a resistance against a change in direction. It keeps us pressed against the bottom of the car with a force stronger than gravity.

Q.10: Why it is easier to walk wearing flat slippers than the high heel sandals?
Ans: Wearing flat slippers will likely be far more comfortable than high heel sandals. This is because the whole of the foot and arch are fully protected without putting too much pressure on sensitive areas such as the toes and heel. Flat slippers have a greater area in contact with the soft sand as compared to high heel sandals. Due to this, there is less pressure on the surface area and less reaction force and it is easy to walk on it.

Q.11: Why leather sheet is used in brake drums of motor bike?
Ans: A drum brake is a brake that uses friction caused by a set of shoes or pads that press outward against a rotating cylinder-shaped part called a brake drum. The leather sheet is used in brake drums because it provides high friction to stop the motorbike.

TEXT BOOK EXERCISE

Section (B) Structured Questions

Momentum

Q.1(a): 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

Q.2 (a): When a free falling object moves towards earth due to pull of earth on it. Does earth also move towards that object due to reaction? Explain.
Ans: Yes, when a free falling object moves towards earth due to pull of earth on it. Earth also move towards that object due to reaction, but the acceleration produced by the earth towards the object is negligible.
This can be easily explained by the equation of Newton's second law of motion F = ma.
Earth and any free falling object pull each other towards them. The force applied on both of them remaining the same, but the changes in the position of an object are determined by its mass and acceleration.
According to the universal law of gravitation, two bodies attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the distance between them. So, theoretically, a free-falling object also pulls the earth in the same way the earth does. But comparing to the mass of the earth, the mass of the free-falling object is negligible. Hence the motion of Earth in not noticed because the acceleration produced in Earth is negligible small, due to large mas of the Earth. So we can say the earth does not move toward the falling object.

Q.3 (a): Why a wire fence is designed in the helmet of batsman?
Ans: When a helmet breaks, it's absorbing that's called "impulse" - a secondary effect of an initial force. Impulse, which gives objects momentum, is what transmits kinetic energy through a system. It takes into account not just force, but also how long that force was applied.
A wire fence is designed in the helmet of batsman aims to reduce the risk of serious jaw and teeth injuries by reducing the impact of a force or collision to the face.

Q.3 (b): How does it (helmet) prevent from injuries?
Ans: When a bike helmet breaks, it's absorbing that's called "impulse" - a secondary effect of an initial force. Impulse, which gives objects momentum, is what transmits kinetic energy through a system. It takes into account not just force, but also how long that force was applied. A helmet aims to reduce the risk of serious head and brain injuries by reducing the impact of a force or collision on the head. A helmet works in three ways: It reduces the deceleration of the skull, and hence the brain movement, by managing the impact.

Laws of motion

Q.4 (a): State the Newton's first law of motion. Give some common examples?
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 state 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.4 (b): Enlist some common observations that are caused by the property of inertia?
Ans: Example To Show Property Of Inertia

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 as 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 slide 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.5 (a): Define Newton's second law of motion.
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:

F = ma

where,

• F = net force acts on a body
• a = acceleration in the direction of force
• m = mass of body

Q.6 (a): Show the relationship between applied force and the acceleration produced in the body?
Ans: According to Newton's second law of motion:

Q.8 (a): Enumerate at least three clear differences 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.

(Note: Write down any three differences as mentioned in above question.)

Circular motion

Q.9 (a): Define the forces acting on an object in circular motion?
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.

Q.9 (b): Draw a figure showing the direction of centripetal force, centrifugal force and velocity of an object along a circular path.
Ans: Diagram Showing The Direction OF Centripetal Fore And Centrifugal Force

Q.9 (d): List down some purposeful uses of centrifuge that human are benefiting everyday.
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.

Friction

Q.10 (a): What is force of friction? Explain with two examples from daily life.
Ans: FORCE OF FRICTION:

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

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.

Q.11 (a): How anti-lock braking system prevents the risk of sliding?
Ans: Anti-Lock Braking System:
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.

Q.11 (b): Enlist any four uses of rolling friction in everyday life?
Ans: Any ball or wheel has rolling friction when rolled on a surface. Some uses of rolling friction include:

1. Truck, car and skateboard tires
2. Rolling of Ball bearings
3. Bike wheels
4. Rolling pin
5. Roller skate wheels

Q.12: Explore the following phenomenon in relation with dynamics.
(a) When an air filled balloon is released.
Ans: When a hot air filled in a balloon, it released. Different forces acting in opposite direction to raised the hot air filled balloon. These forces are:

1. Buoyancy: Buoyancy or lift is created when the temperature in the balloon is increased, causing the density of the air to decrease. The less-dense (lighter) air inside the balloon tends to float on the more dense (heavier) air on the outside of the balloon That is why hot-air balloons are referred to as lighter than air vehicles. If the amount of lift is greater than the force of gravity acting on the mass of the balloon, then the balloon will rise.
2. Pressure: Warmer air inside the balloon will also cause the pressure inside, the balloon to increase. The pressure inside the balloon will be greater than that on the outside of the balloon. For the balloon to maintain its shape, this force has to be greater than the forces acting in the opposite directions (pushing inward on the balloon).
3. Drag And Force Of  Gravity: In hot-air balloons, drag is the fiction that occurs as the balloon rises and moves through the surrounding air. Friction occurs between the moving balloon and the molecules of air. It hits as it rises. Both drag and the force of gravity pulling on the mass of the balloon act in a downward force in opposition to the lift.
   If the lift is greater than the drag and force of gravity, then the balloon rises.
   If the lift is less than the drag and the force of gravity, then the balloon descends.
   If the lifting force is equal to the force of drag and gravity, then the balloon will neither rise nor fall.
4. Wind: However, wind can also act as a force on the balloon. The wind can come from nearly any direction and will tend to move the balloon in the direction it is blowing.

(b) Riding a bicycle needs continuous paddling.
Ans: Riding a bicycle needs continuous pedalling because When the rider stops pedalling the bicycle, the force of friction between the tyres of the bicycle and the road acting in the direction opposite to the direction of motion of the bicycle, opposes the motion of the bicycle and this force is now unbalanced, thus slowing down the bicycle.

(d) The biker ridding in the death well.
Ans: When a bike moves on the walls, there are a number of forces acting. These include:

• The gravitational force, which acts downward from the bike to the walls.
• The frictional force that the walls exert against the tires of the bike, and
• The normal reaction force, a perpendicular push back by the wall surface when it receives a force.
• There is also centripetal force, which is directed towards the center of the circular path that the bike traces.

For a bike moving in a horizontal circle on a vertical wall, the normal reaction (N) is the factor that supplies enough force to sustain motion in a circle. Also, the fact that the bike does not slide down the wall signifies that the forces of friction and gravitation balance each other out.
In short, the two forces, the gravitational force and the force of friction act in opposite directions and compensate each other, while the normal reaction from the wall keeps the bike moving. However, it's not that simple. The frictional force exerted on the tires of the bike depends on the speed of the bike as it moves along the circle. This means that there has to be a minimum velocity of the bike that produces the maximum frictional force, effectively balancing out the gravitational force. This is crucial because if the gravitational force is greater, then the bike will slide down and the rider will fall off. The friction becomes stronger as the speed increases, but with increasing speed, it becomes more and more difficult for the rider to steer the vehicle safely.
The frictional force is acting on the tires, but the gravitational force is acting through the center of mass of the system consisting of both the bike and the rider. Since the three forces are balanced but do not lie in the same line, the bike will tend to rotate, producing a turning effect that will eventually lead to it fall off. This anomaly has to be compensated for to keep these brave riders safe!
In order to counter this dangerous turning effect, the rider has to lean at an angle away from the vertical. This will make the normal reaction from the wall produce a tendency to rotate (a torque) in the opposite direction. If the rider bends at the correct angle, the torques will be perfectly balanced out; therefore, there will be no rotating or turning effect on the bike and the impressive display can continue. However, if the rider leans at an angle other than the correct one, then the unbalanced torques will cause the bike to rotate and fall. Therefore, the rider will have to push harder in the opposite direction to supply extra torque and maintain his balance.

(e) You always feel a pullback whenever you pull on your school bag or some heavier object.
Ans: By putting a heavyweight on our shoulders in the wrong way, the weight's force can pull us backwards. So people who carry heavy backpacks sometimes lean forward.


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