Long Answer Questions - Force & Pressure Notes | EduRev

Science Class 8

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Class 8 : Long Answer Questions - Force & Pressure Notes | EduRev

The document Long Answer Questions - Force & Pressure Notes | EduRev is a part of the Class 8 Course Science Class 8.
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Q1: Explain contact and non-contact forces. Give two examples for each.
Ans: 
Contact forces: Forces which act only when there is physical contact between two interacting objects are known as Contact forces.

Example:
Muscular force: This is the force we can exert with our bodies by using our muscles, e.g.push, pull etc.
Frictional force: The force acting against the relative motion of surfaces in contact is called frictional force or friction.

Non-contact forces: Forces which can act without physical contact between objects, i.e. those that can act from a distance, are called non-contact forces or field forces.

Example:
Magnetic force: Magnets exert forces of attraction or repulsion on other magnets
Electrostatic force: The force exerted by a charged body on another charged or uncharged body is known as electrostatic force.

Q2: (a) How can friction be reduced?

(b) How can it be increased? Give examples.

Ans: Reducing Friction

1. By using wheels and ball bearings.Use of wheels between surface moving over each other reduces friction. Ball bearings have small balls of steel between steel surfaces. Because of the balls the steel surfaces can easily moves over each other.

2. By making the rubbing surfaces smooth by polishing them.

3. By using a suitable lubricant, like oil (for light machinery) or grease (for heavy machinery). This helps because fluid friction is less than solid friction.

4. Friction due to air (air resistance) or water is reduced by using streamlined shapes in aeroplanes or ships. A streamlined shape is narrow in front and broader at the back. Birds and aquatic animals have streamlined shapes which held them in flying or swimming.

Increasing Friction

1. Sand and gravel is strewn on slippery ground during the rainy season to increase friction. It is then easier to walk on the ground.

2. By making the moving surfaces rough, e.g. tyres have designs and patterns with grooves on the surface to increase resistance with the road. This prevents slipping of the tyres on a wet road.

3. To increase friction, spikes are provided in the soles of shoes used by players and athletes.
Q3: Do liquid and gases exert pressure on the walls of container in all direction? Give example to justify your statement. 

Ans: Liquid and gases exerts pressure on the walls of container. For example,If we take a plastic bottle and drill four holes around near the bottom of bottle at the same height. After filling the water in that bottle, we observe that water comes out of the holes and falls at the same distance. This shows that liquid exerts pressure on the walls of container in all directions.
Similarly, we are not able to inflate a balloon which has holes because air inside balloon exerts pressure in all direction.Hence we can say that gases exert pressure on the walls of container in all directions.
Q4: Why is it easy to push a nail into a wooden plank by the pointed end? 

Ans: It is easy to push a nail into a wooden plank by pointed end because the smaller the area, larger the pressure on a surface for the same force. The area of the pointed end of the nail is much smaller than that of its head. The same force therefore produces a pressure sufficient to push the pointed end of the nail into the wooden plank.

Q5: Read Table and try to identify the action as push or pull.
Ans:
Table Identifying Actions as Push or Pull

S. No.

Description of

the situation

Action: (Pushing/ pulling/ picking/ hitting/ lifting/lowering/ flying/ kicking/ throwing

Action can be grouped as a

 

 

 

shutting/flicking)

 

push

pull

1.

Moving a book placed on a table

Pushing

Pulling

Lifting

Yes

Yes

2.

Opening or shutting a door

Pulling

Pushing

Lifting

Lowering

Yes

Yes

3.

Drawing a bucket of water from a well

Pulling

Lifting

Lowering

Yes

4.

A football player taking a penalty kick

Kicking

Flying

Yes

5.

A cricket ball hit by a batsman

Hitting

Flying

Yes

6.

Moving a loaded cart

Pulling

Yes

7.

Opening a drawer

Pulling

Yes

Q6: Read Table and complete it.
Ans: 
Table  Studying the Effect of Force on Objects

Description of situation

How to apply force

Action of force

Change in state of motion

Change in shape

Yes

No

Yes

No

A lump of dough on a plate

Pressing it down with your hands

No

Yes

Spring fixed to the seat of bicycle

By sitting on the seat

-

No

Yes

-

A rubber band suspended from a hook/nail fixed on a wall

By hanging a weight or by pulling its free end

Yes

-

Yes

-

A plastic or metal scale placed between two bricks

By putting a weight at the centre of the scale

-

No

Yes

-

 

Q7: What are the various effects of force on different objects?
Ans:
The various effects of force are:
(i) A force can make an object move from rest.
(ii) It can change the speed of a moving object.
(iii) It can bring about a change in the shape of an object.
(iv) It can change the direction of motion of an object.
(v) It can cause some or all of these effects.

Q8: What are contact forces? State different types of contact forces.
Ans:
The forces which come into play only when two objects come in contact with each other are called contact forces.
Some contact forces are:
(i) Muscular force: The forces resulting due the action of muscles are known as muscular forces. Muscular force is a contact force because it comes into play when two object, come in contact with each other.
(ii) Force of friction: When a body is moving, then a force equal and opposite to the direction of motion is exerted on that moving body. This force is called force of friction. It is also a contact force because it is exerted when two surfaces come in contact with each other.

Q9: What are non-contact forces? Explain different types of non-contact forces.
Ans.
The forces which can be exerted from a distance, without establishing a contact are called non-contact forces. Some non-contact forces are:

(i) Magnetic force: The force exerted by a magnet on other magnet or some other magnetic substance like iron is called magnetic force. Like poles of a magnet repel each other and unlike poles of a magnet attract each other without contact. So it is called non-contact force.

(ii) Electrostatic force: The force exerted by a charged body on other charged or uncharged body is called electrostatic force. Electrostatic force also acts without making a direct contact with other charged or uncharged body. So it is also a non-contact force.

(iii) Force of gravity: Earth pulls every thing or body towards it. The force of attraction exerted by earth on any object is called force of gravity. This is also a non-contact force as it acts from a distance.

Q10: What is force? Name different types of forces.
Ans.
A push or pull on an object is called force. There are following types of forces:
(i) Muscular force
(ii) Force of friction
(iii) Magnetic force
(iv) Force of gravity
(v) Electrostatic force
Q11: Prove that the force of friction depends on the nature of the two surfaces in contact.
Ans.
Collect the following things: A thick book, nylon cloth, gunny cloth, plastic sheet, jute cloth and sand paper. Place the book on each of the materials and slide it on the floor one by one. If your book slides off the plastic sheet or nylon cloth, use adhesive tape to stick it firmly on the surface of the book.
You will observe that the different materials offer different amounts of resistances to sliding.
This activity shows that force of friction depends on the nature of the surface in contact. In general, smooth surface offers lesser friction than rough surface.

Q12: Prove that the pressure exerted by water at the bottom of the container depends on the height of its column.
Ans.
Take a transparent glass tube or plastic pipe. Also take piece of thin sheet of a good quality rubber. Stretch the rubber sheet tightly over one end of pipe. Hold the pipe at the middle, keeping it in a vertical position. Pour some water in the pipe. Note the height of the water column in the pipe. Pour some more water. Observe, the bulge in rubber sheet and height of water column in the pipe.
Repeat this process a few more times. You observe that as the height of water column increases the bulge in the rubber sheet also increases.

Long Answer Questions - Force & Pressure Notes | EduRev

Q13: Show that a liquid exerts pressure on the walls of the container.
Ans.
Take a plastic bottle. Fix a cylindrical glass tube a few cm long near its bottom. You can do so by slightly heating one end of the glass tube and then quickly inserting it near the bottom of the bottle. Make sure that water does not leak from the joint. If there is any leakage seal it with molten wax. Cover the mouth of the glass tube with a thin rubber sheet. Now fill the bottle up to half with water. We observe the bulge in the rubber sheet. Pour some more water in the bottle. We see more bulge in rubber sheet. This activity indicates that water exerts pressure on the walls of the container.

Long Answer Questions - Force & Pressure Notes | EduRev

Q14: Explain that liquids exert equal pressure at the same depth.
Ans.
Take an empty plastic bottle. Drill four holes all around near the bottom of the bottle. Make sure that all the holes are at the same height from the bottom. Now fill the bottle with water. We observe that different streams of water coming out of the holes fall at the same distance from the bottle. This observation indicates that liquids exert equal pressure at the same depth.

Long Answer Questions - Force & Pressure Notes | EduRev

Q 15: What experiment was performed to prove that air has pressure?
Ans
. Otto von Guericke, a German Scientist invented a pump in 17th century to extract air out of a vessel. He demonstrated the force of the air pressure. He joined two hollow metallic hemispheres of 51 cm diameter each and pumped air out of them. Then he employed eight horses on each hemisphere to pull them apart. So great is the force of air pressure that the hemispheres could not be pulled apart.

Long Answer Questions - Force & Pressure Notes | EduRev

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