CBSE Class 7 Science Advanced Physics Machines and Tools Short Answer Questions Set 01

Short Answer Questions

Question 1. What are the advantages and disadvantages in using a vice?
Answer:
Advantages
1. It holds the work piece firmly, so both hands of the worker are free.
2. A small piece of metal can be cut easily after it is gripped tightly between the jaws.
Disadvantages
1. Very thin and soft metals cannot be held in a vice.
2. A soft metal may change shape and show marks of the jaws.
3. A work piece that is too big cannot be held.
In simple words: A vice grips small, hard pieces well but is no good for thin, soft or large ones.

Teacher's Note:
1. Write advantages and disadvantages under separate headings.
2. Give at least two points for each.

Question 2. Between the maintenance of agriculture tool or workshop tool, which is difficult? Why?
Answer:
Maintenance of agricultural tools is more difficult.
1. Agricultural tools are in constant contact with soil, moisture and manure, so they rust and corrode faster.
2. They are bigger in size and are used outdoors, so they are harder to clean and oil.
3. Workshop tools are smaller and are kept indoors, so they stay in better condition.
In simple words: Farm tools get wet and muddy, so they need more care than tools kept in a workshop.

Teacher's Note:
1. Always give reasons when the question says 'why'.
2. Water and manure make iron rust quicker.

Question 3. What are the advantages of sowing seeds by using seed drill over that done by hand?
Answer:
1. The distance between rows of seeds remains constant.
2. Seeds fall at equal distances, so plants get equal space to grow.
3. Seeds are dropped into grooves at the same depth.
4. Fertilizer or manure can be placed near the roots.
5. Sowing is faster and needs less effort.
In simple words: A seed drill sows neatly and quickly, while hand sowing is uneven and slow.

Teacher's Note:
1. Compare machine work with hand work point by point.
2. Equal spacing is the key idea.

Question 4. Write the uses of a divider.
Answer:
1. A divider is used to measure distances.
2. It is used to mark equal lengths.
3. It is used to draw circles on wood and plastic.
In simple words: Measure, mark equal lengths and draw circles.

Teacher's Note:
1. Mention all three uses.
2. Dividers do not draw straight lines.

Question 5. Which part of the wooden plough is made of steel? What is the use of this part?
Answer:
The blade, fixed to the pointed end of the main plough, is made of steel (metal).
It helps the plough to go deep into the soil easily.
In simple words: The metal blade on the tip digs the soil.

Teacher's Note:
1. The rest of the wooden plough is wood.
2. Metal is harder than wood and cuts the soil better.

Question 6. What is the use of a harrow?
Answer:
A harrow is used to remove weeds from the field.
It is also used to level the wet field before transplantation.
In simple words: It pulls out weeds and levels wet soil.

Teacher's Note:
1. Write both uses for full marks.
2. The nails of the harrow do the work.

Question 7. Write the use of a Vice.
Answer:
A vice is used to hold a metal plate, pipe or rod firmly while it is cut, threaded or smoothed.
In simple words: A vice holds the work so you can work on it.

Teacher's Note:
1. The vice holds, but it does not cut.
2. It is fixed at the edge of a table.

Question 8. What are the three different parts of a wooden plough?
Answer:
1. The main plough (a bent wooden piece with a metal blade).
2. The long pole.
3. The handle.
In simple words: Main plough, long pole and handle.

Teacher's Note:
1. Name all three parts, in order.
2. The blade is fixed on the main plough.

Question 9. What is the use of a leveller?
Answer:
A leveller is used to level the surface of the soil.
In simple words: It makes the soil surface flat.

Teacher's Note:
1. It is pulled by bullocks or a tractor.
2. The farmer holds its handle.

Question 10. Why is the tip of the screw driver magnetized?
Answer:
The tip of the screw driver is magnetized so that the steel screw sticks to it. This makes it easy to hold the screw in place while it is being fixed, especially in places that are hard to reach.
In simple words: A magnet tip holds the screw so that it does not drop.

Teacher's Note:
1. Steel and iron objects are attracted by a magnet.
2. This is a useful design that makes the work easier.

Question 11. Why is the maintenance of tool very important?
Answer:
1. Proper maintenance stops metal tools from rusting.
2. Tools stay sharp and work well, so the job is done with less effort.
3. Tools last longer, which saves money.
4. Clean, well-kept tools are safer to use.
In simple words: Care keeps tools working and lasting.

Teacher's Note:
1. Mention rust and long life.
2. Cleaning and oiling are the two main steps.

Question 12. Why do we use a sharp continuous metal blade for cutting a mango and a blade with many small teeth for cutting metals?
Answer:
The sharp teeth of a saw blade concentrate the force on a very small area, so a large pressure is produced and the blade can enter a hard material such as metal.
A mango is soft, so only a small effort is needed to cut it. A continuous sharp edge of a knife is enough for it.
In simple words: Hard metal needs a toothed blade; a soft fruit needs only a sharp edge.

Teacher's Note:
1. Pressure is force divided by the area it acts on.
2. Teeth make the area small and the pressure large.

Question 13. Sriharsha had a doubt as to why the head of a screw driver is made of plastic material and not metal. Clarify his doubt.
Answer:
The handle (head) of a screw driver is made of plastic to avoid electric contact between the screw and the hand, so the person does not get an electric shock.
Plastic is an insulator, whereas metal conducts electricity.
In simple words: Plastic does not conduct electricity, so the handle keeps the user safe.

Teacher's Note:
1. Insulators protect us from electric shocks.
2. Electricians always use screw drivers with plastic handles.

Question 14. A thick metallic cylindrical rod which is to be cut when placed between the jaws of a 'vice', the handle rotated through 4 × 360°, if the distance moved by the movable jaw is 0.5 cm per rotation then calculate the radius of the rod. (Assume that initially the jaws are in contact with each other)
Answer:
The handle makes 4 rotations, so n = 4.
Distance moved by the jaw per rotation = 0.5 cm.
Total distance moved by the movable jaw = 4 × 0.5 cm = 2 cm.
The jaws started in contact, so this distance is the width of the rod, that is the diameter d = 2 cm.
Radius = d ÷ 2 = 2 ÷ 2 = 1 cm.
In simple words: Four turns of 0.5 cm each make a diameter of 2 cm, so the radius is 1 cm.

Teacher's Note:
1. The jaw's total shift equals the rod's diameter.
2. Do not forget to halve it to get the radius.

Question 15. A leveller with one metre long metal strip is used to level the surface of the soil. If the farmer takes the leveller from one end to another end of the field of length 25 m, then calculate the area of the field which is levelled.
Answer:
Width levelled = length of the metal strip = 1 m.
Length of the field = 25 m.
Area levelled = 25 m × 1 m = 25 m².
In simple words: The strip sweeps a 1 m wide path 25 m long.

Teacher's Note:
1. The area of a rectangle is length × breadth.
2. The strip's length is the breadth of the path.

Question 16. The movable jaw of a vice moves through a distance of 7 mm for one complete rotation of the handle. Through how many rotations should the handle be rotated so that it holds a metal plate of thickness 3.5 cm firmly.
Answer:
Thickness of the plate = 3.5 cm = 35 mm.
Distance moved per rotation = 7 mm.
Number of rotations = 35 ÷ 7 = 5.
In simple words: Divide 35 mm by 7 mm and you get 5 turns.

Teacher's Note:
1. Convert cm to mm before dividing.
2. The jaws must close through a distance equal to the plate thickness.

Question 17. A leveller with a 1.5 meters long metallic strip is used to level an area of 45 square metres. Then calculate the length of the field which is levelled.
Answer:
Area levelled = length of field × length of the strip.
45 m² = ℓ × 1.5 m.
ℓ = 45 ÷ 1.5 = 30 m.
In simple words: The field is 30 m long.

Teacher's Note:
1. Area = length × breadth.
2. The strip's width of 1.5 m is the breadth.

Question 18. Venkat went to a factory workshop where he saw hot metallic square plates. He measured the surface area of the hot plates. How did he do it?
Answer:
A hot plate cannot be touched, so he used a divider.
He adjusted the divider so that its pointed ends touched the two ends of a side of the plate.
Then he measured the gap between the two points with a metre scale and got the length ℓ of the side.
Area of the square plate = ℓ × ℓ = ℓ².
In simple words: A divider measures the side without touching the hot metal, and the area is side squared.

Teacher's Note:
1. A divider is a safe measuring tool for hot objects.
2. Area of a square = side × side.

Question 19. What is the use of spring in a tin cutter?
Answer:
The spring is fixed between the handle of one blade and the other blade.
When the handles are pressed, the spring is stretched and stores energy.
This stored energy opens the blades again after cutting, so it reduces the effort.
In simple words: The spring opens the blades after each cut, saving effort.

Teacher's Note:
1. Mention energy stored in the spring.
2. Without the spring you would have to open the blades by hand.

Question 20. Rahul, by using a hack saw blade, was cutting metal pipes to supply water to his house. What will happen if he starts moving the hack saw blade very fast over the metal, due to lack of time.
Answer:
Moving the blade very fast produces a lot of friction, so the blade becomes hot.
The heat spoils the sharpness of the teeth, so the blade cuts poorly.
A very fast stroke can also break the blade.
In simple words: A fast stroke heats and may break the blade, so he should saw at a steady pace.

Teacher's Note:
1. Friction produces heat.
2. Always saw at a steady, moderate speed.

Question 21. Why does an axe has a curved sharp edge?
Answer:
A curved sharp edge touches the wooden log on a small area, so the force of the blow is concentrated there and a large pressure is produced.
So the axe can enter the wooden log easily.
In simple words: A small cutting area gives a big pressure, so the axe goes into the wood.

Teacher's Note:
1. Pressure = force ÷ area.
2. A sharp edge has a very small area.