DAV Class 8 Science Solution Chapter 4 Force and Pressure
DAV School Books Class 8 Science Solution Chapter 4 Force and Pressure all Question Answer. DAV Class 8 4th Chapter Force and Pressure full Chapter explanation by expert teacher.
DAV School Books Class 8 Science Solution Chapter 4 Force and Pressure
A.) Fill in the blanks.
1.) For a force to come into play, the two concerned bodies must ___________ with each other.
2.) To draw water from a well, we have to ___________ the rope.
3.) An unbalanced force, acting on a moving object, may change its ___________ and/or its ___________.
4.) In the game of cricket, the ‘bails’ fall when the ball strikes them. This is an example of a/an ___________ force.
5.) The force, responsible for raising our body hair, when we try to take off a synthetic (terylene or polyster) shirt in dry weather conditions, is an ___________ force.
6.) The atmospheric pressure ___________ as we move towards higher altitudes.
Answer: (1) interact (2) pull (3) speed, direction (4) contact (5) electric (6) decreases
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, it is an example of contact forces because for frictional force to exist one object needs to be moving across another object hence contact is necessary.(4) False,the gas balloon moves up because its weight is lighter than that of air (5) True (6) False, the pressure is higher at the bottom.
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: She should 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
5.) An empty bottle has to be placed on a table so that it exerts the maximum pressure on the surface of table ?
Answer: position B as the bottle is placed upside down, the force of gravity also acts along with it due to its positioning.
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: presence of atmospheric pressure
7.) 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.
(b) A person diving into a swimming pool.
(c) A labourer moving a loaded cart.
(d) A car coming to rest once its engine is switched off.
(e) An athlete making a high jump to clear a bar kept at a certain height.
Answers –
Agents of Force | effects |
a. Fingers | change in shape. |
b. Muscles of the swimmer | Change in motion |
c. The labourer | Change in motion |
d. Frictional force from the road | Change in motion |
e. Muscles of the athlete | 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: It is not possible because the total or resultant force is zero. The force acting on the body is therefore non balanced thus it can be concluded that the speed is subjected to constant change and it wouldn’t follow an uniform motion in a straight line
3.) Distinguish between:
(a) balanced and unbalanced forces.
Balanced Forces |
Unbalanced Forces |
The forces are referred to as balanced forces when the sum of two or more forces acting on a body is zero. | Unbalanced forces are those produced when the sum of two or more forces acting on a body is not zero. |
An object’s resting or moving state is unaffected by balanced forces. They could alter the item’s dimensions and shape. | Unbalanced forces can alter an object’s size, shape, and state of motion in addition to its resting or moving position. |
(b) contact and non-contact forces.
Contact Forces |
Non-contact Forces |
.The forces that only operate when two objects come into direct physical contact | the forces at play when two objects are not physically touching one another. |
Examples-Frictional force, muscular force. | Examples – Gravitaty and 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: The speed of a ball when it is thrown upwards decreases because gravity is pulling the ball downward. Additionally, air resistance works against the ball’s motion.
5.) The figure shows a man with a parachute.
(a) Name the force which is responsible for his downward motion. Is it a contact force or a non-contact force?
Answer: Gravitational force causes him to move in a downward motion; It is non-contract force.
(b) Will the man come down with the same speed without the parachute? Explain.
Answer: No, because the parachute encounters a significant amount of air resistance, which reduces speed. Therefore, the man will arrive faster without a parachute.
6.) How would the pressure (exerted by a force on an area) change if-
(a) the area is doubled keeping the thrust constant?
Answer: If area is doubled then pressure is reduced by half.
(b) thrust is doubled keeping the area constant?
Answer: If thrust is doubled then pressure is then doubled.
7.) When will a diver ‘experience a greater pressure’-100 cm below the surface of water or 200 cm below the surface of water? Justify your answer.
Answer:
He will experience a greater pressure at 200 cm below the surface of water because pressure will increase within a liquid as the depth increases.
E.) Answer the following questions.
1.) Define force. Is it necessary for the two bodies to be in direct contact for a force to exist between them? Explain with examples.
Answer: Force is said to be a push or pull that an object experiences resulting from its contact with other objects. Even if two bodies are not in direct contact with each other they still experience force such an example would be and iron nail being repelled from a magnet even at a distance
2.) State three effects of a force. Give suitable examples.
Answer: Force causes a change in state of motion for exampleIf we apply a certain amount of force on the doorknob the door opens or it may close
force causes the size or shape of an object to get changed
if a Potter makes pots out of clay of various sizes and shapes this is a result of application of different amount of force.
Motion can be changed by applying a certain amount of force such as a football player can kick a football in different direction by using force
3.) Give reasons for the following statements-
(a) ‘Snow shoes’ are more effective than ordinary shoes for walking on snow.
Answer: Since pressure and area are inversely proportional snowshoes have larger width than regular shoes as a result it exerts less pressure on the snow and it becomes easier for people to walk in them without sinking their feet in the snow.
(b) Porters place a large round piece of cloth on their heads when they carry heavy loads.
Answer: wearing a large piece of cloth on their head increases the surface area for putting the luggage on top of it .As a result the pressure felt is lesser by the porter.
(c) A sharp knife cuts vegetables and fruits more effectively than a blunt knife.
Answer: a sharp knife has a smaller surface area hence less pressure has to be applied to cut vegetables using it but a blunt knife has larger surface area hence it requires more pressure to cut vegetables
(d) School bags are provided with wide straps to carry them.
Answer: The wider straps ensure a larger surface area. Even though there are heavy weights within the bag it becomes easier if the straps are wide, as the pressure exerted on the shoulders that carry them are lesser.
(e) It is much easier to burst an inflated balloon with a needle than (directly) with the finger.
Answer:.needle has a very small surface area as compared to that of finger as a result very small amount of pressure needs to be applied while using a needle to deflate balloon
(f) Mountaineers may suffer from nose bleeding at high altitudes.
Answer: there is a harmonious balance between our blood pressure and the atmospheric pressure outside our body however this balance is somewhat changed when we travel to higher altitude where the atmospheric pressure is considerably less as a result often nose bleeds occur since intricate blood vessels burst out due to this difference in pressure
(g) Airplanes have pressurised cabins for passenger safety while flying high in the atmosphere.
Answer:. at higher altitudes the atmospheric pressure is very low as a result people travelling in aeroplanes can feel uneasy hence the pressure cabin is created so that there isn’t much difference in pressure and people can travel comfortably.
4.) Two rods, ‘A’ and ‘B’, of the same weight and equal length, have different thicknesses. A boy, while playing with them on a beach, held them vertically on the surface of sand as shown in the figure. Which one of them will sink less? Explain.
Answer:According to what we already know, the pressure varies inversely with the area that the force is applied to and directly with the thrust. The rod “B” will sink less than the rod “A” because, for an equal amount of force, a smaller area will result in greater pressure. Rod “B” has a larger surface area, which is why.
5.) Three holes, A, B and C, are made in an empty can at different levels, one over the other, as shown in the figure. They are (temporarily) closed with an adhesive tape. The can is now filled with water. What will you observe, when the tape (on all of them) is pulled out? Justify your answer.
Answer: Soon, water will begin to emerge from these three holes. Water will travel the farthest from hole “A” because bottom pressure is greatest. Water from hole “C” will travel the furthest but at the least distance due to low pressure.
6.) Make two tiny holes, of the same size, at equal heights from the bottom in a plastic bottle. Cover them and fill the bottle with water. Let the ‘covers’ be now removed. State the ‘observations’, and the resulting ‘conclusions’, based on this activity.
Answer: Due to equal pressure on both of the plastic bottle’s walls and the holes being at the same height, water will flow out and travel a similar distance.
7.) Discuss, in brief, the variation in atmospheric pressure with altitude. Do changes in temperature also cause a change in atmospheric pressure?
Answer: as we move towards higher altitude the atmospheric pressure decreases while at lower altitude atmospheric pressure is considerably higher.
Yes, change in temperature can affect atmospheric pressure.
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