Forces And Matter - Physics For Class IX (Science Group) - Question Answers

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Physics For Class IX (Science Group)
UNIT 5: FORCES AND MATTER
Questions Answers

Q.1: Define Force and its type? What can a force do. Give some examples?
Ans: FORCE:
Force can be defined as:

"A push or a pull that changes or tends to change the state of rest or uniform motion of an object or changes the direction or shape of an object."

OR

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,

• Forces on an object can cause tensile deformation (stretching) and compressive deformation (squashing).
• 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.

TYPES OF FORCE:
There are two type of forces:

1. Force (action at distance):
   e.g. Gravitational force, Electrical force, Magnetic forces, strong nuclear force.
   
   
2. Force (contact forces)
   e.g. Frictional force; Tension force; Normal Force; Air Resistance Force, Applied Force, Spring Force.

Examples Of Force:

• Force is needed to move a car.
• Force causes the spring to stretch.
• We need force to move some luggage.
• If we bend our plastic ruler we will change its shape.
• Force applied by hands on the dough changes its shape.
• A spring can be stretched or compressed by applying force.

Q.2: What are the effects of a force on solids?
Ans: FORCES ACTING ON SOLIDS:

• Solids have definite shapes and sizes, however it is possible to change their shapes and sizes by applying external forces.
• When the external force is removed, the object tends to return to its original shape and size. This behavior is called elastic behaviour (elasticity).
• The effect of a force has, depends on the materials involved. Soft materials, such as rubber can bend and flex very easily.
• A spring comes back to its original shape when we remove the force. Materials like this are called elastic.
• Solids can be stretched, squashed, bend or twisted. A sufficiently large force will permanently deform or break an object.

Examples:

• A Force can twist an object, like twisting a wet cloth.
• A Force can bend an object, like bending a iron rod.
• A force can deform an object, like Crushing a tin can changes its shape and size.

Q.3: Define elasticity, elastic force and elastic limit? what are elastic materials. Give example also?
Ans: ELASTICITY:

"Elasticity is the property of a body to regain its original shape and size when deforming forces are removed."

In other words,
External forces are applied to change shapes and sizes of an object. When this external force is removed, the object tends to return to its original shape and size. This behavior is called elastic behaviour or elasticity.

ELASTIC FORCE:
The force applied to deform the shape of an object is called the elastic force. Or An elastic change occurs when an object returns to its original shape and size after the load is removed.
For example:
When we pull on the band, it stretches but doesn't break. The resistance we feel when we pull on it is elastic force. The farther we stretch the band, the greater the elastic force. After we stop pulling the band, it returns to its original shape.

ELASTIC LIMIT:
Hooke's law is applicable to all kinds of deformation and all types of matter i.e., solids, liquids or gases within certain limit. Such limit is called elastic limit. It can be define as:

"Elastic limit tells the maximum force or stress that can be safely applied on a body without causing permanent deformation in its length, volume or shape."

In other words,

"Elastic limit is a limit within which a body recovers its original length, volume or shape after the deforming force is removed. Beyond this limit spring deforms permanently."

ELASTIC MATERIALS:
Soft materials are flex very easily and return to its original position when we removed external force, Such materials are called elastic materials. Such as:

• Rubber can bend and return to its original shape very easily when we removed external force.
• A spring comes back to its original shape when we remove the force.

Q.4: How solids are deformed?
Ans: DEFORMING OF SOLIDS:
To explain that solids are deformed, let us perform an experiment with the spring.

• Consider a spring hung from a rigid support, so that its top end is fixed.
• Weights are hung on other end of the spring. These are called load.
• As load is increased, the spring is stretched and its length increases.
• When the load is removed, the spring returns to its original length. This is called elastic change.
• When the load is increased in regular steps the length of the spring also increases simultaneously.
• If the load is increased greatly, the spring will change its shape permanently.

Q.5: Define spring? Explain the extension of spring?
Ans: SPRING:
Springs are designed to stretch a long way when force is applied, therefore it is easy to measure changes in their lengths.

EXTENSION OF SPRINGS:

"The length of spring increases as the force (load) increases. This increase in length of spring is known as extension."

Hence,

Length of stretched spring = Original length + Extension

EXPERIMENT FOR STRETCHING SPRINGS OR EXTENSION OF SPRINGS:

• Let’s carry out an experiment to stretch a spring of original length 20 cm.
• The given table shows recorded result of this experiment.
• The first column shows the increase in load in regular steps.
• Second column shows the increase in length of stretched spring.
• Third column shows the value of extension, due to change in length in each step.
  
  

Table for load and extension

Load (N)	Length (cm)	Extension (cm)
0.0	20	0.0
2.0	21	1.0
4.0	22	2.0
6.0	23	3.0
8.0	24	4.0
10	25	5.0
12	26	6.0
14	28	8.0
16	30	10

The above table shows that the spring stretches as the load on it increases.

Graph:

Dependence of extension on the load is shown in the given graph. We can see that the graph has two parts.

• At first, the graph slopes up steadily. This shows that the extension increases in equal steps as the load increases. This behaviour can also be observed in the above table.
• Then the graph bends. This happens when the load is greater enough to damage the spring permanently. As a result the spring will not return to its original length.

Q.6: State and explain Hooke's law?
Ans: HOOKE'S LAW:
Robert Hooke, an English scientist first described the mathematical pattern of stretching a spring. He observed the dependence of displacement or size of the deformation upon the deforming force or load.
Hooke's law states that:

"Within elastic limit, the displacement produced in the spring is directly proportional to the force applied."

In other words,
Hooke’s Law state that the extension in spring is proportional to the load applied to it, provided the limit of elasticity is not exceeded.

Mathematically:
If ‘F’ is the applied force and ‘x’ is the displacement (extension) in the spring then the equation for Hooke's law may be written as:

F ∝ x

or F = kx ....... (i)

Where, k is spring constant (stiffness of spring).

Explanation:

• Hooke's law is applicable to all kinds of deformation and all types of matter i.e., solids, liquids or gases within certain limit. Such limit is called elastic limit. This limit tells the maximum force or stress that can be safely applied on a body without causing permanent deformation in its length, volume or shape.
• In other words,  elastic limit is a limit within which a body recovers its original length, volume or shape after the deforming force is removed. Beyond this limit spring deforms permanently.

Graph:
An extension against load graph is a straight line up to the limit of proportionality.

Q.7: Why was Robert Hooke interested in springs for his experiment?
Ans: Robert Hooke a scientist and inventor was interested in springs for two reasons.

1. Spring are useful in making balances. Hooke wanted to make a very sensitive and accurate weight machine or balance.
2. He also realized that a spiral spring could be used to control a clock or wrist watch.

Q.8: Give young's modulus of the following materials: Brass, Copper, Mild steel, Plastic and Rubber?
Ans: Young’s Modulus Of Given Materials:

Material	Young’s Modulus (in GPa)
Brass	91
Copper	120
Mild steel	210
Plastic	2
Rubber	0.02
Where 1 GPa = 109 Pa

Q.9: Define and explain pressure? Calculate the pressure using formula P = F/A.
Ans: PRESSURE:
Pressure is defined as:

"The force acting normally per unit area on the surface of a body is called pressure."

OR

"The quantity that depends upon the force and increases with decrease in the area on which force is acting is called pressure."

Examples:

• Press a pencil from its ends between the palms. The palm pressing the tip feels much more pain than the palm pressing its blunt end.
• We can push a drawing pin into a wooden board by pressing it by our thumb. It is because the force we apply on the drawing pin is confined just at a very small area under its sharp tip. A drawing pin with a blunt tip would be very difficult to push into the board due to the large area of its tip.
• When force is exerted on area, this is known as pressure, e.g. A hammer hits a nail, driving the nail downwards into a piece of wood.

Explanation:
In these examples, we find that the effectiveness of a small force is increased if the effective area of the force is reduced.
The area of the tip of pencil or that of the nail is very small and hence increases the effectiveness of the force. Thus, Pressure is greater when a large force acts on small area.

Calculation Of Pressure:
Formula:
If F is the magnitude of a force exerted perpendicular to a given surface of area A, then the pressure P equals to the force divided by the area:
Thus,

Nature:
It is a scalar quantity, because pressure acts in all directions. It has magnitude only.

Unit:
PASCAL (S.I Unit):
As the force is measured in newtons (N) and area in square meters (m²).
Therefore in SI system unit of pressure is newton per square meter (Nm^-2). It is also known as Pascal (Pa).

1 Pa = 1 Nm^-2

ATMOSPHERE:
The pressure exerted by the air molecule at sea level is measured in atmosphere.
Where,

1 atmosphere = 1.013 x 10^-2) Pascal

1 atm = 1.013 x 10^-2) Pa

Q.10: Give Reasons:
a. Why an acrobat does not hurt when he lied down on the bed of nails?
Ans: There is no miracle in this trick. We know that pressure is defined as force per unit area. If we step up on a nail, the entire body weight exerts more pressure because the area of nail tip is very small. In case of bed nails, the pressure exerted by weight of body is distributed on the hundreds or thousands of nails lying close to each other. Thus, net pressure on a nail is very small. Hence, an acrobat does not hurt when he lied down on the bed of nails.

b. Explain why foot ball shoes have spikes or studs?
Ans: The pressure under the studs on the soles of football shoes is high enough to sink into the ground, which gives extra grip. Therefore, studs prevent a player from slipping on the grass and help to run faster and change direction quickly without slipping.

c. Why do ice skates have blades?
Ans: Ice skates have blades in the part that is in contact with ice. Therefore the skater's weight is concentrated on a small area. The effect of this high pressure melts ice just below the blades. This gives the thin film of water, which provide lubrication for the skate to skim over the ice. As the skate moves on, the water re-freezes. On very cold days, the pressure may not be enough to melt the ice, and skating is impossible.

d. Why do elephants have broad soles?
Ans: An elephant has broad soles to reduce the pressure exerted on the ground. They have broad feet in proportion to their size, to distribute the weight better.

e. Why does the school bag have broad shoulder straps/pads?
Ans: Wide shoulder pads of school bag reduce pressure on the student's shoulder. These broad shoulder straps are more helpful to distribute the weight of the bag across all over the shoulders. It avoids the weight of concentrating on a particular area.

Q.11: Define fluid? Also describe pressure in fluids?
Ans: FLUIDS:

"A fluid is a collection of molecules that are randomly arranged and held together by weak cohesive forces and by forces exerted by the walls of a container. Both liquids and gases are fluids."

PRESSURE IN FLUIDS:

"The pressure exerted by fluids is known as fluid pressure. It acts in all directions."

This is because the molecules of fluids move around in all directions, causing pressure on every surface they collide with.
Example:
A swimmer in swimming pool experience the pressure by water which pushes the swimmer from all sides. The arrows represent the direction and magnitude of the forces exerted at various points on the swimmer. It shows that the net underneath force is larger due to greater depth, giving a net upward or buoyant force that is balanced by the weight of the swimmer.

Q.12: Describe the factors that affects the pressure?
Ans: FACTORS AFFECTING PRESSURE:
i) Depth:
Pressure P is proportional to the depth, the deeper one dives into water, greater will be the pressure. Twice the depth means twice the pressure.

ii) Density Of Material:
Similarly, pressure also depends upon the density of the material. If a material is five times denser than water, the pressure will be five times greater.

Relation Between Pressure, Depth And Density of Material:
Mathematically,
At a depth 'd' in a fluid of density 'ρ', the pressure 'p' can be written as:

Pressure = depth x density x acceleration due to gravity

p = dρg ..............(iii)

Thus, The pressure in a fluid is greater with large depth, and high density.

Q.13: What is Density?
Ans: DENSITY:
The term density of a substance is defined as mass of substance (m) per unit volume (V). It is denoted by Greek letter ρ (rho).

ρ = m / v

Density is characteristic property of any pure substance. Density tells us that the matter is packed tightly together. If something is packed very tightly together it is considered to be “dense”. More dense sinks and pushes up less dense. 
Objects made of a particular pure substance such as pure Gold can have any size or mass but its density will be same for each. In accordance with the above equation mass of a substance can be expressed as

m = ρV

S.I Unit Of Density:
The S.I unit for density is kg/m³ or kgm^-3. Sometimes density of substances is given in gm/cm³.

Example:
The density of Aluminum is 2.70 gm/cm³ which is equal to 2700 Kg/m³.

Q.14: What are hydraulic machines?
Ans: HYDRAULIC MACHINES:
The machine in which force is transmitted by liquids under pressure is known as hydraulic machine. It also work on the principals of fluid pressure. By the application of relatively small force they produce a greater force.
Example:
Hydraulic breaks are used almost in all vehicles. They cause a relatively small force from the driver's foot to be multiplied to produce a greater force, which acts equally on all four brake pads. Thus a hydraulic machine works on Pascal's law.

Q.15: State and explain Pascal's Law?
Ans: PASCAL'S LAW:
This law may be stated as:

"The pressure applied externally at any point of a liquid enclosed in a container is transmitted equally to all parts of the liquid in container."

Explanation:
The liquid pressure at the surface of the liquid increases when an external force is applied on the surface of the liquid. The increase in the liquid’s pressure is transmitted equally in all directions, in similar way it is transmitted equally to the walls of the container in which it is filled. This result leads to a law known as Pascal's law.

Example (Demonstration Of Pascal's Law):
It can be demonstrated with the help of a water filled glass vessel having holes around its surface. When we apply force through the piston the water rushes out of the holes with the same pressure.
The force applied on the piston exerts pressure on water. This pressure is transmitted equally throughout the liquid in all directions. In general, this law holds good for fluids both for liquids as well as gases.

Uses Of Pascal's law:

• A hydraulic machine works on this principle
• Hydraulic brakes
• Car lifts
• Hydraulic jacks
• Forklifts and other machines make use of this principle

Q.16: Describe the working of a hydraulic press?
Ans: HYDRAULIC PRESS:
A hydraulic press is made of two pistons connected by a liquid-filled pipe.
A force of magnitude F₁ is applied to a small piston of surface area A₁. The pressure is transmitted through an incompressible liquid to a larger piston of surface area A₂. Because the pressure must be the same on both sides.

Therefore, the force F₂ is greater than the force F₁ by a factor ^A₂/_A1.
By designing a hydraulic press with appropriate areas A₁ and A₂, a large output force can be applied by means of a small input force.
Each side of this equation is the work done by the force. Thus, the work done by F₁ on the input piston equals to the work done by F₂ on the output piston. Thus the principle of conservation of energy applies in the hydraulic press.