class 8 ch 4 question answer of dav answer of E Q/A of science
Answers
Answer:
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Explanation:
B. State True or False for the following statements.
1. The effect of a force may change by changing the direction of the applied force.
2. A force has to be specified both in terms of its magnitude as well as its direction.
3. Frictional force is an example of a non-contact force.
4. A gas filled balloon moves up. The upward force acting on it is smaller than the force of gravity, i.e. (the downward acting force).
5. We can reduce the pressure of a given force by increasing the area over which the force acts.
6. Inside a bottle filled with water, pressure is least at the bottom and the greatest at the surface of water.
Answer: (1) True (2) True (3) False (4) False (5) True (6) False
C. Tick the correct option.
1. When two forces, applied on an object, are equal and opposite, then these forces-
Answer: do not move the object but may cause a change in its shape.
2. The following are the names of some of the well-known forces A. gravitational force B. muscular force C. magnetic force D. frictional force Out of these, the ‘contact forces’ are the forces labelled as-
Answer: B and D
3. Rohini is pushing a box towards the west direction with a force of magnitude F. To help her move the box faster in the same direction, her friend should-
Answer: pull the box in the west direction with a force of magnitude ‘F’.
4. A container is filled with water as shown in the figure below. Which of the following statement is correct about the pressure of water in this container?
Answer: pressure at C > pressure at B > pressure at A
DAV Solutions for Class 8 Science Chapter 4
Science 8 Chapter 4
Answer: position B
6. When we press the bulb of a dropper, with its nozzle kept under water, air in the dropper is seen to escape in the form of bubbles. If we then release the pressure on the bulb, water gets filled in the dropper. The rise of water in the dropper is due to-
Answer: atmospheric pressure7. The length, breadth and height, of a given rectangular box, are 100 cm, 50 cm and 20 cm, respectively. This box is kept on ground in three different ways, one by one. Which of the following statements is the correct statement about the pressure exerted, by this box, on the ground?
Answer: the pressure exerted is maximum when the ‘breadth’ and ‘height’ sides form the base.
D. Answer the following questions in brief.
1. In the following situations, identify the agent exerting the force. Also, state the effect of the force in each case.
(a) Squeezing a piece of lemon between the fingers to extract its juice.
Answer: Agent of force is fingers & effect is change in shape.
(b) A person diving into a swimming pool.
Answer: Agent of force is person muscles & effect is change in motion.
(c) A labourer moving a loaded cart.
Answer: Agent of force is labourer & effect is change in motion of cart.
(d) A car coming to rest once its engine is switched off.
Answer: Agent of force is friction from road & effect is change in motion.
(e) An athlete making a high jump to clear a bar kept at a certain height.
Answer: Agent of force is muscles & effect is change in motion.
2. An object experiences a net non-zero force. Is it possible for the object to move with a constant speed along a straight line?
Answer: No, because a net non-zero force i.e., Unbalanced force is acting on the body, so speed must be changing.
3. Distinguish between:
(a) balanced and unbalanced forces.
Answer:
Balanced Forces Unbalanced Forces
When resultant of two or more forces acting on a body is zero then these forces are called balanced forces. When resultant of two or more forces acting on a body is not zero, then these forces are called unbalanced forces.
Balanced forces do not change the state of rest or motion of an object. They may change in the size and shape of the object. Unbalanced forces may change in state of rest or motion as well as in size and shape of an object.
(b) contact and non-contact forces.
Answer:
Contact Forces Non-contact Forces
The forces which act only when objects are in direct physical contact with each other. The forces which act when objects are not in direct physical contact with each other.
Examples-Frictional force, muscular force. Examples – Gravitational force, Electric Force, Magnetic Force.
4. When we throw a ball upwards, what happens to its speed while it is going up? Give reason for your answer.
Answer: When we throw a ball upwards, it’s speed decreases because gravitational force is acting on the ball in downward direction. Also, air resistance opposes the motion of the ball.