Class 8 Science · 23 questions · 23 with answers
During dry weather, while combing hair, sometimes we experience hair flying apart. The force responsible for this is
(b) electrostatic force.
Fig.11.3 shows a container filled with water. Which of the following statements is correct about pressure of water? Fig.11.3
This question refers to a figure in the original PDF.
(d) Pressure at A< Pressure at B<Pressure at C
Two objects repel each other. This repulsion could be due to
(d) either a magnetic or an electrostatic force
Which one of the following forces is a contact force?
This question refers to a figure in the original PDF.
(b) force of friction
A ball of dough is rolled into a flat chapatti. Name the force exerted to change the shape of the dough.
Muscular force.
Where do we apply a force while walking? 64 E XEMPLAR PROBLEMS
While walking we apply force on the ground.
A girl is pushing a box towards east direction. In which direction should her friend push the box so that it moves faster in the same direction? 12. Fig. 11.6 In the circuit shown in Fig.11.6, when the key is closed, the compass needle placed in the match box deflects. Name the force which causes this deflection.
This question refers to a figure in the original PDF.
Towards east.
During dry weather, clothes made of synthetic fibre often stick to the skin. Which type of force is responsible for this phenomenon?
Electrostatic force.
While sieving grains, small pieces fall down. Which force pulls them down?
Force of gravity.
Does force of gravity act on dust particles?
Yes
A gas filled balloon moves up. Is the upward force acting on it larger or smaller than the force of gravity?
Upward force is larger than the force of gravity.
Does the force of gravitation exist between two astronauts in space?
Yes SHORT ANSWER QUESTIONS
A chapati maker is a machine which converts balls of dough into chapati’s. What effect of force comes into play in this process? FORCE 65 19. Fig. 11.7 Fig.11.7 shows a man with a parachute. Name the force which is responsible for his downward motion. Will he come down with the same speed without the parachute?
This question refers to a figure in the original PDF.
The force changes the shape of the dough.
Two persons are applying forces on two opposite sides of a moving cart. The cart still moves with the same speed in the same direction. What do you infer about the magnitudes and direction of the forces applied.
Both the forces are of equal magnitudes and applied in the opposite directions.
Two thermocol balls held close to each other move away from each other. When they are released, name the force which might be responsible for this phenomenon. Explain.
Electrostatic force. The balls have similar charges. They move away due to repulsion between similar charges.
Fruits detached from a tree fall down due to force of gravity. We know that a force arises due to interaction between two objects. Name the objects interacting in this case.
Earth and fruits. 140 E XEMPLAR PROBLEMS
A man is pushing a cart down a slope. Suddenly the cart starts moving faster and he wants to slow it down. What should he do? 66 E XEMPLAR PROBLEMS 24. Fig. 11.8 Fig. 11.8 shows a car sticking to an electromagnet. Name the forces acting on the car? Which one of them is larger?
This question refers to a figure in the original PDF.
He should apply a force to pull the cart up the slope.
An archer shoots an arrow in the air horizontally. However, after moving some distance, the arrow falls to the ground. Name the initial force that sets the arrow in motion. Explain why the arrow ultimately falls down.
The archer stretches the string of the bow by applying muscular force. In the process the shape of the bow changes. When the string is released, it regains its original position that provides the initial force to set the arrow in motion. The force of gravity that acts on the arrow in the downward direction brings it to the ground.
It is difficult to cut cloth using a pair of scissors with blunt blades. Explain.
Blunt blades have larger area compared to the sharp-edged blades. Thus, the applied force produces a lower pressure in case of blunt blades, which makes it difficult to cut the cloth.
Two rods of the same weight and equal length have different thickness. They are held vertically on the surface of sand as shown in Fig.11.9. Which one of them will sink move? Why? Fig. 11.9 FORCE 67
This question refers to a figure in the original PDF.
Rod B will go deeper as it has a smaller area of contact, therefore the same force (weight of the rod) produces more pressure. In case of rod A the same force produces less pressure.
Two women are of the same weight. One wears sandals with pointed heels while the other wears sandals with flat soles. Which one would feel more comfortable while walking on a sandy beach? Give reasons for your answer.
The woman wearing sandals with flat soles will feel more comfortable while walking on the sandy beach. The flat soles have larger area compared to the sandals with pointed heels. Since the two women are of the same weight, they will apply same force on the ground. Therefore, the pressure exerted by the pointed heels will be more compared to that with sandals having flat soles. As a result the pointed heel sandals will sink more in the sand than the flat sole sandals. Hence, walking with flat sole sandals will be more comfortable.
It is much easier to burst an inflated balloon with a needle than by a finger. Explain.
When we prick the surface of an inflated balloon with a needle it exerts a larger pressure because it has a smaller area of contact compared to the finger. The large pressure pierces the surface of the balloon easily.
Observe the vessels A, B, C and D shown in Fig.11.10 carefully. Fig. 11.10 Volume of water taken in each vessel is as shown. Arrange them in the order of decreasing pressure at the base of each vessel. Explain.
This question refers to a figure in the original PDF.
B, D, A, C. Because pressure of a liquid column depends upon the height of the liquid column and not on volume of the liquid. ANSWERS 141