A. Multiple choices
1. (a) Gravitational force
3.(d)at the same depth, pressure is the same in all directions.
5. (b) A B acting in the direction of A
6. (d) Mass
7. (a) Gravitational force
8. (d) The pressure doubles. Pressure (P) = Force (F)/Area (A) = 10 N / 0.1 m2 = 100 N/m2 = 100 Pa
Now, according to the question,
New area, A' = A/2 and F' = F ∴ P' = F'/A' = 2F/A = 2P = 2 × 100 = 200 Pa
Thus, the pressure gets doubled.
B. V. Short types:
1. True. [Push or pull acting on a body are called force. The direction in which an object is pushed or pulled is called the direction of the force.]
2. False. [When we apply the brakes to slow down a bicycle it will bicycle will move forward. Thus, the bicycle will move in the opposite direction of the applied force.]
3. The net force acting on the object is A + B. [If two forces act on a body in the same direction, then the net force acting on object will be sum of the two forces.]
4. The net force acting on the object is A B.[If two forces are applied on an object in opposite directions, then the net force acting on the object is the difference between the two forces. ]
5. No. The net force of all forces determines the acceleration of the body
6. False. [Every object in the universe exerts gravitational force on other matter.]
7. Weight [An object’s weight is a measure of the Earth's gravitational pull acting on it. Weight = mg]
8. Magnetic force 9. Electrostatic force 10. Friction force
11. The larger the area over which a force acts, the lesser the pressure is.
12. Pressure exerted by a liquid increases with depth and acts in all directions.
13. True. [Due to the atmospheric pressure of air, we experience a force equal to that exerted by a mass of 1 kg on every square centimetre of our body.]
14. False. Atmospheric pressure is caused by the weight of air molecules above the atmosphere. As we go to higher altitudes, the air becomes thinner and the atmospheric pressure decreases
C. Short type (i)
1. Force can be described by two things: its magnitude and the direction in which it acts. Force is a vector quantity.
2. If several forces act on a body in different directions, the effect on the body will be due to the magnitude and direction of the net force acting on it. So, the body will move in the direction of the resultant force
3. Gravitational force is the force with which the Earth pulls everything towards itself. So, if an object is thrown up, it slows down and then comes down because of the gravitational force of the Earth.
The mass of a body is the quantity of matter contained in it
It is a scalar quantity that has only magnitude, no direction
The SI unit of mass is the kilogram (kg).
The weight of a body is the force with which it is attracted towards the centre of the Earth
Weight is a vector quantity having magnitude as well as direction
The SI unit of weight is the Newton (N).
5. Every magnetic substance is surrounded by its own invisible magnetic field, depending on its pole strength. A magnet can act from a distance because of the field of magnetic force.
6. Pressure is the trust acting on a unit area of an object. Pressure = Thrust/Area
Pressure depends on two factors: (i) Force applied (ii) Area over which the force acts
7. Atmospheric pressure is the force per unit area exerted on the surface of the Earth by the atmosphere. Atmospheric pressure is caused by the weight of air molecules in the atmosphere.
8. It is difficult to cut vegetables with a blunt knife because it has more area compared to a sharp knife, therefore the pressure exerted by the blunt knife is less than that of the sharp knife.
D. Short type (ii)
1. Force: Force is an agent that change or try to change the state of an object. The SI unit of force is kg m/s² or Newton (N).
Effects of force: 1. Change the state of rest and motion. 2. It can change the speed of a moving object. 3. It can change the direction of a moving object. 4. It can change the shape or size of an object.
2. Contact Force: The force which acts by touching objects is called Contact Force. For example muscular force, friction force
Non Contact Force: The force which act without touching objects is called non contact Force .. For example: Gravitational force, Electrostatic force
3. Friction is a kind force that always opposes motion of body. For example, when you stop pedalling a bicycle, it slows down and stops after some time because of the frictional force.
4. Caterpillar tracks are used in battle tanks instead because it distributes force over larger area and reduce the pressure over surface. This prevents tracks from sinking in areas where wheeled vehicles of the same weight can easily sink.
5 The pressure exerted by the liquid is given by the following expression:
P = ρgh
Here, ρ = Density of the liquid g = Acceleration due to gravity h = Height/depth of the liquid column
Here, pressure is directly proportional to the height of the liquid column. If we go down into the liquid, the depth (h) of the liquid column increases. Thus, the pressure in the liquid increases with increasing depth.
6. When we press a rubber sucker on a smooth surface, we remove some of the air between the sucker and the smooth surface. This deficiency of air lets the air outside to exert pressure on the surface of the sucker. Force due to the pressure keeps the sucker stuck to the surface.
1. False. If no external force is applied on the body, then there must be two equal and opposite forces acting on the body. Both these forces add up to zero.
2. False. Contact force is the force in which a direct contact is required between two bodies. The contact may also be with the help of a stick or a piece of rope. If we use a stick to push an object, we make the contact with the help of stick. This push or force is caused by the action of the muscles in our body. This muscular force is a contact force.
3. The magnitude of gravitational force depends on the masses of two objects and the distance between them.
Gravitational force generally felt between objects of greater masses. Since mass of building and our body smaller as compared to the earth, we are not pulled towards the building.
4. It is necessary to define pressure as a separate quantity to find the impact of force on a surface.
As we know that the impact of force depends on both the amount of force applied and the area over which it is applied. So, only force is not enough for this purpose.
5. If one of the windows in a jet plane breaks due to an accident, the air pressure inside plane decreases. At such low pressure, we can have problems like nose bleeding, because the pressure exerted by the blood in our body will be much higher than the pressure outside. This can cause blood vessels to burst.
6. If we press a rubber sucker on a rough surface, it will not stick to the surface because an airtight seal cannot be achieved between the surfaces. It is so because air rushes into the space between the rubber sucker and the rough surface.