NCERT Solutions for Class 8 Science Chapter 11 - Force and Pressure
Chapter 11 - Force and Pressure Exercise 142
Two examples of push force are as follows:
(i) A heavy box at rest is pushed to move it from one room to another. This changes the state of motion of the box.
(ii) A player pushes a football using his foot. This changes the state of motion of the ball.
Two examples of pull force are as follows:
(i) Rope is pulled to draw water from a well. This changes the state of motion of the water bucket.
(ii) A drawer is pulled to open it. This changes the state of motion of the drawer.
Concept insight: Force is a push or pull.
Two examples of forces that cause a change in the shape of an object are as follows:
(i) Squeezing of a plastic bottle changes the shape of the bottle.
(ii) Deformation of clay by pressing it between the hands.
(iii) Making a chapati from a ball of dough.
Concept insight: Force may cause a change in the state of motion of a body or a change in the shape of the body.
(c) push or pull
Chapter 11 - Force and Pressure Exercise 143
(d) The rise of water in the dropper is due to atmospheric pressure.
Concept insight: When all the air escapes from the nozzle, the atmospheric pressure, which is acting on the water, forces the water to fill the nozzle of the dropper.
Concept insight: Force in which contact between to objects is necessary is called contact force.
(d) gravity, friction
Concept insight: Friction due to air is called air resistance; it is a non contact force. Gravity is also a non contact force.
(a) In squeezing a piece of lemon, we make use of muscular force to extract its juice. This muscular force is exerted on the lemon by our fingers. As a result, the shape of the lemon gets changed.
Concept insight: Muscular force is a type of contact force.
(b) We use our muscular force to take out paste from a toothpaste tube. The muscular force is exerted on the toothpaste tube by our fingers. As a result, the shape of the tube gets changed.
(c) Here, the suspended load exerts a force on the spring and pushes the spring downwards. As a result, the spring gets stretched. Hence, its shape gets changed.
(d) An athlete pushes the ground with his feet. His feet exert a muscular force on the ground. This allows him to jump over the bar. As a result, his state of motion gets changed.
Concept insight: Every action has an equal and opposite reaction.
While hammering a hot piece of iron blacksmith changes the shape of iron using his muscular force.
Concept insight: Force brings change in the shape of an object.
Electrostatic force is responsible for the attraction between the balloon and the wall. The balloon rubbed with a synthetic cloth gets electrically charged and sticks to the wall. This is because a charged object attracts uncharged object.
Concept insight: Electrical force is a type of non contact force. A charged object attracts an uncharged object.
A bucket filled with water held above the ground experiences two types of forces:
1. Muscular force (acting upwards)
2. Gravity (acting downwards)
These two forces are balanced because they are equal in magnitude but act in opposite directions. So, the net force on the bucket is zero and hence the state of motion of the bucket remains unchanged.
Concept insight: Non-zero net force is required to change the state of motion of a body.
The two forces acting on the rocket immediately after leaving the launching pad are:
(i) Upward force applied by the rocket engine.
(ii) Downward gravitational force applied by the Earth.
(Ignoring frictional force due to air resistance)
The upward force due to rocket engine exceeds the downward gravitational force which makes the rocket move upward.
Concept insight: Unbalanced forces can change the state of motion of a body.
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