class 8th science long questions please tell me
1 Two forces act on an object in the same direction, and then in opposite directions. What will be the net force on the object in each of the two cases ?
2 What are the effects that a force can have on an object?
3 Explain contact and non-contact forces, with the help of two examples for each .
4 The mass of an object is the same all over the universe but the weight varies from place to place . Explain why .
5 Why is it better to use a school bag that has broad straps ? What problem might you face if the straps are narrow ?
6 What is a manometer ? Explain its working with the help of a labelled diagram.
7 Describe an experiment to show that atmospheric pressure acts in all directions.
8 Why is the pressure inside an aeroplane maintained at the normal ground-level atmospheric pressure?
Please tell me all long questions and answers of class 8th
Answers
Answer:
1) if two forces act on an object in the same direction then the net force is equal to the difference between the two forces.if two forces act on an object in the opposite direction then the net force is equal to their addition of forces.
2) A force can produce the following effects:
A force can move a stationary object.
A force can stop a moving object.
A force can change the speed of a moving object.
A force can change the direction of a moving object.
A force can change the shape and size of an object.
3) Contact Forces : The forces which are applied on bodies by making a physical contact with them, are called contact forces. e.g. the force of friction (frictional force), muscular force
Non-contact Forces : The force experienced by the bodies even without being physically touched, are called non-contact forces. e.g. Gravitational force, Magnetic force.
4) As the amount of matter does not change, the mass does not change. Weight is a measure of the gravitational force acting on a body. The gravitational force can change at different places . So, weight will be different in different places.
5) Bags should be provided with broad straps because it decreases the pressure by increasing surface area which makes it easier to carry them. On the other hand, thin straps decrease surface area which results in more pressure and it becomes hard for us to carry them.
6) A manometer may be any device that measures pressure. However, unless otherwise qualified, the term “manometer” most often refers specifically to a U-shaped tube partly filled with fluid. One end of the tube is connected with a gas-tight seal to a test pressure source. The other end of the tube is left open to the atmosphere and will therefore be subjected to a pressure of approximately 1 atmosphere (atm). If the test pressure is greater than the reference pressure of 1 atm, the liquid in the test column is forced down the column. This causes the fluid in the reference column to rise by an equal amount.
(image is attached)
7)Air pressure or atmospheric pressure is the weight of air acting on a unit are of surface.Air exerts pressure in all directions.
Experiment to demonstrate Air pressure
Apparatus Required:Glass tumbler, water and card board.
Procedure :
Fill the glass tumbler completely with water.
Cover it with card board.
Note : There should not be air between water and card board.
Support the card board with you hand and invert the tumbler.Holding the glass tumbler gently, remove the hand.
Observation:
Cardboard does not fall because the air pressure is acting from all the direction..
8)The atmospheric pressure is maximum at sea level. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases. At the height which the airplane flies, the air pressure is very low. ... That is why pressure inside an aeroplane is maintained at the normal ground atmospheric pressure
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Explanation:
A force can move a stationary object. If force is applied to a stationary object, does it always move?
ANSWER:
No, it is not necessary that the stationary object will move always when a force is applied. If there is a friction force between the object and the ground then the friction force will oppose the motion of the object. Hence, the stationary object will move only if the applied force is greater than the friction force between the object and the ground.
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Question 2:
Two equal forces act on an object. Is this information enough to say which way the object will move? Give reasons.
ANSWER:
No, this information is not enough. Two equal forces may or may not cause an object to move. If both the forces are acting on the same side of the object and are large enough to overcome the friction force between the object and the ground, then the object will move. If both the forces are acting on the object from the opposite directions then the object will not move as the net force on the object will become zero.
Hence, the direction of the force and value of the friction force must be defined in order to determine the movement of the object.
Question 1:
A force acts perpendicular to a given surface. What is the quantity 'force per unit area' known as?
ANSWER:
The quantity 'force per unit area' is known as 'pressure'.
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Question 2:
How is pressure related to the area over which a force acts?
ANSWER:
The pressure is defined as the force per unit area. So, the pressure is indirectly proportional to the area over which the force acts.
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Question 3:
What is the force of 1 N acting over an area of 1 m2 called?
ANSWER:
Force per unit area is defined as the pressure. If 1 N of force is acting over an area of 1 m2, then it would be called unit pressure or pressure of 1 Pa.
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Question 4:
How does the pressure exerted by a liquid vary with a. direction? b. depth?
ANSWER:
(a) The pressure exerted by a liquid is the same in all the directions at the same height.
(b) But it is directly proportional to the depth in the liquid. As the depth increases, the pressure exerted by the liquid increases too.
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Question 5:
You are put into a room where the air pressure is very low. What effect will this have on your body?
ANSWER:
The air present in the atmosphere exerts pressure on our body, but that pressure is compensated by our blood pressure which acts from inside out. If the air pressure of the surroundings will become very low then our blood vessels will burst out due to excess pressure of the blood inside our body.
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Question 1:
There is one force which is exerted by all matter on all other matter. Which force is this?
(a) gravitational force
(b) magnetic force
(c) electrostatic force
(d) frictional force
ANSWER:
(a) Gravitational force
According to Newton, every object in this universe attracts the other object with gravitational force.
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Question 2:
Which of these is a contact force?
(a) friction
(b) magnetic force
(c) gravitational force
(d) electrostatic force
ANSWER:
(a) Friction
Frictional force is a contact force which resists the relative motion between two surfaces.
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Question 3:
Which of the following is true for the pressure exerted by a liquid?
(a) pressure does not depend on depth.
(b) pressure is only exerted in the downward direction.
(c) Pressure is exerted both downwards and sideways but downwards pressure is greater than sideways pressure.
(d) At the same depth, pressure is same in all directions.
ANSWER:
(d) At the same depth, pressure is the same in all directions.
Pressure exerted by a liquid increases with depth and acts in all directions, but it is the same at all points in a horizontal plane at a given depth in a stationary liquid.
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Question 4:
The SI unit of pressure is
(a) kg/m3.
(b) kg/m2.
(c) pascal.
(d) newton.
ANSWER:
(c) Pascal
The SI unit of pressure is the pascal (Pa).
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Question 5:
Two forces A and B act on an object in opposite. A is bigger than B. The net force on the object is
(a) A + B acting in the direction of A.
(b) A − B acting in the direction of A.
(c) A + B acting in the direction of B.
(d) A − B acting in the direction of B.
ANSWER:
(b) A - B acting in the direction of A
When two forces are applied on an object in opposite directions, the net force on the object is the difference between the forces; this net force acts in the direction of the greater force.
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Question 6:
Which of these can a force acting on an object not change?
(a) direction of motion
(b) state of rest
(c) shape
(d) mass
ANSWER:
(d) Mass
Mass is the amount of matter in an object; it cannot be changed by the application of a force.