8th class physics force lesson question and answers
Answers
Answer:
11.9 Pressure Exerted by Liquids and Gases
11.10 Atmospheric Pressure
Force and Pressure Class 8 Science NCERT Textbook Questions
Question 1.
Give two examples each of the situations in which you push or pull to change the state of motion of objects.
Answer:
(i) Push: We close drawer by pushing.
We move a wooden box by pushing.
(ii) Pull: We draw water from a well by pulling the rope.
A horse pulls a cart.
Question 2.
Give two examples of situations in which applied force causes a change in the shape of an object.
Answer:
When we apply force on a rubber band to stretch it and on clay to change its shape.Question 3.
Fill in the blanks in the following statements.
(a) To draw water from a well we have to ______ at the rope.
(b) A charged body ______ an uncharged body towards it.
(c) To move a loaded trolley we have to ______ it.
(d) The north pole of a magnet _______ the north pole of another magnet.
Answer:
(a) pull
(b) attracts
(c) push
(d) repelsQuestion 3.
Fill in the blanks in the following statements.
(a) To draw water from a well we have to ______ at the rope.
(b) A charged body ______ an uncharged body towards it.
(c) To move a loaded trolley we have to ______ it.
(d) The north pole of a magnet _______ the north pole of another magnet.
Answer:
(a) pull
(b) attracts
(c) push
(d) repels Name the forces acting on a plastic bucket containing water held above ground level in your hand. Discuss why the forces acting on the bucket do not bring a change in its state of motion.
Answer:
Forces acting on bucket are as follows:
(i) Muscular force of arms acting upward.
(ii) Force of gravity acting downward.
Both the forces do not bring any change in the state of motion because both of them are acting in equal and opposite directions and thus they cancel each other’s effect.
Question 9.
A rocket has been fired upwards to launch a satellite in its orbit. Name the two forces acting on the rocket immediately after leaving the launching pad.
Answer:
The forces that act when a rocket leaves launching pad are as follows:
(i) Gravitational force of the earth (downward)
(ii) Frictional force of air (in opposite direction)
Question 10.
When we press the bulb of a dropper with its nozzle kept in water, air in the dropper is seen to escape in the form of bubbles. Once we release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to
(a) pressure of water
(b) gravity of the earth
(c) shape of rubber bulb
(d) atmospheric pressure
Answer:
(d) atmospheric pressure
Force and state of motion of any object is described through laws of motion given by Newton.
First law of motion:
- It states "Any object in motion will remain in motion and any object at rest will stay at rest until and unless an external push or pull is applied on it".
- This law introduces the concept of inertia.
Second law of motion:
- It states "Force is defined as the rate of change in momentum with time".
- It gives the definition of force and its expression.
- Force = mass * acceleration
Third law of motion:
- It states "Every action of force will have an equal and opposite reaction".
- According to this law, force always come in pairs i.e. action-reaction pair.