PHYSICS 9TH - Short / Detailed Question Answers Chapter #05: FORCES AND MATTER
Q.1: What can a force do? Give some examples.
Ans:
- 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 hands on the dough changes its shape.
- A spring can be stretched or compressed by applying force.
Q.2: Define force.
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.
Q.3: What are the effects of a force on solids?
Ans:
Force Acting on Solids:
Solids have definite shapes and sizes; however, it is possible to change their shapes and sizes by applying external forces. Solids can be stretched, squashed, bent, or twisted. A sufficiently large force will permanently deform or break an object. The effect of a force depends on the materials involved. Soft materials, such as rubber, can bend and flex very easily. A spring returns to its original shape when we remove the force. Materials like this are called elastic.
Q.4: What material is called elastic?
Ans:
Elastic Material:
When the external force is removed, the object tends to return to its original shape and size. This behavior is called elastic behavior. For example, a spring comes back to its original shape when we remove the force. Materials like this are called elastic.
Q.5: What are springs?
Ans:
Springs:
As springs are designed to stretch a long way when force is applied, therefore it is easy to measure a change in their lengths.
Q.6: How solids are deformed?
Ans:
To explain how solids are deformed, let us experiment with the spring. We consider a spring hung from rigid support so that its top is fixed as shown in figure (a) below. Weights are hung on the 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, as shown in figure (b). If the load is increased greatly, the spring will change its shape permanently.
Q.7: Explain the extension of spring.
Ans:
Extension of a Spring:
The length of the spring increases as the force (load) increases. This increase in the length of the spring is known as extension. Hence,
Length of stretched spring = Original length + Extension
Let us experiment to stretch a spring of the original length of 20 cm. The given table shows the recorded result of this experiment. The first column shows the increase in load in regular steps.
The second column shows the increase in the length of the stretched spring. The third column shows the value of extension, due to the change in length in each step.
| 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 | 10.0 |
The above table shows how the spring stretches as the load on it increases. The dependence of the extension on the load is shown in the given graph.
We can see that the graph has two parts:
(i) At first, the graph slopes up steadily. This shows that the extension increases in equal steps as the load increases. This behavior can also be observed in the above table.
(ii) Then the graph bends. This happens when the load is great enough to damage the spring permanently. As a result, the spring will not return to its original length.
Q.8: Define the term elasticity.
Ans:
Elasticity:
Elasticity is the property of a body to regain its original shape and size when deforming forces are removed.
Q.9: 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 the elastic limit, the displacement produced in the spring is directly proportional to the force applied.
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:
or
where is the spring constant (stiffness of the spring).
Hooke's law is applicable to all kinds of deformation and all types of matter, i.e., solids, liquids, or gases within a certain 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, it is a limit within which a body recovers its original length, volume, or shape after the deforming force is removed. Beyond this limit, the spring deforms permanently (see the graph).
Q.10: Write two properties of a spring.
Ans:
Properties of a Spring:
- An ideal spring material has high strength properties, a high elastic limit, and a low modulus. Because springs are resilient structures designed to undergo large deflections, spring materials must have properties of extensive elastic range.
Q.11: Define elastic limit.
Ans:
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, it is a limit within which a body recovers its original length, volume, or shape after the deforming force is removed. Beyond this limit, the spring deforms permanently.
Q.12: Define pressure.
Ans:
Pressure:
The force acting normally per unit area on the surface of a body is called pressure.
Explanation:
If we 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 with our thumb. It is because the force we apply on the drawing pin is confined just to 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.
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 the pencil or that of the nail is very small and hence increases the effectiveness of the force. The quantity that depends upon the force and increases with a decrease in the area on which force is acting is called pressure.
Thus,
Q.13: How do we calculate pressure? Write units of pressure.
Ans:
How to Calculate Pressure:
If is the magnitude of a force exerted perpendicular to a given surface of area , then the pressure equals the force divided by the area:
Unit of Pressure:
As the force is measured in Newton (N) and area in square meters (). Therefore in the S.I system, the unit of pressure is Newton per square meter (). It is also known as Pascal (Pa).
Q.14: Define 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.
Q.15: Describe pressure in fluids.
Ans:
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.
