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Q1. What is the difference between Thrust & Pressure? 

Solution

Thrust:

• Thrust is the force that a body exerts perpendicularly on a surface.

        Thrust = Pressure × Area

• Its SI unit is newton (N).
• It is numerically equal to the weight of the object.

  Pressure:

• The thrust on a unit area of surface is called pressure.

         Pressure = Thrust / Area

• Its SI unit is newton per metre squared or pascal (Pa).

 

Q2. In each of the instances given below, identify the action as 'push' or 'pull'. i. A cricket ball hit by a batsman ii. Planting a pin on the pin board iii. Plucking the nail off the wall iv. Picking up a shopping bag

Solution

i. A cricket ball hit by a batsman: Push ii. Planting a pin on the pin board: Push iii. Plucking the nail off the wall : Pull iv. Picking up a shopping bag: Pull

Q3. How does atmospheric pressure play a role while drinking through a straw?

Solution

A drinking straw is used by creating suction with the mouth. This causes a decrease in air pressure inside the straw. Since pressure acting on the surface of the drink is equal to atmospheric pressure, so this greater pressure pushes the soft drink up the straw into the mouth.

Q4. (i) Mention any three points to give the importance of gravitational force.(ii) Is muscular force a type of contact force?

Solution

(i) (a) The gravitational force of the earth keeps us bound to the earth. (b) Gravitational force between earth and sun makes the earth move around the sun. (c) Gravitational force between moon and earth makes the moon go around the earth.(ii) Yes, muscular force needs to be in contact with the body on which it acts, so it is a contact force.

Q5. Make a list of at least four effects of force that can be produced, when an net force acting on an object is non-zero.

Solution

i. The force may make the object move from rest. ii. The force may change the speed of the object if it is moving. iii. The force may change the direction of motion of the object. iv. The force may bring about a change in the shape of the object

Q6. What is gravitational force? Give two examples from your daily life.

Solution

The attraction of objects towards the earth is known as force of gravity or gravity.  Every object in the universe whether a small or large exerts a force on every other object, thus this force is known as gravitational force. For example, the force acting between any two books or between two students sitting next to each other etc.

Q7. Define: 1. Muscular force  2. Friction force

Solution

1. The force resulting from the action of human/animal muscles is called as muscular force. 2. The force which is responsible for changing the state of motion of an object is called as friction.

Q8. State the characteristics of liquid pressure.

Solution

The characteristics of liquid pressure are: 1) Liquid pressure increases with depth. 2) Liquid pressure remains the same in all directions at a given depth. 3) Liquid pressure depends upon the density of the liquid. 4) Liquid exerts pressure on the sides of the container. 5) A liquid seeks its own level.

Q9. 1. When does force of friction come into play? 2. If  a force is applied towards East, in order to move a body, in which direction does the force of friction act?

Solution

1. The force of friction come into play when one body moves upon another body. 2. When  a force is applied towards East, then the force of friction will act towards West i.e. opposite to the direction of force applied.

Q10. Why do soccer shoes have studs on the outsole?

Solution

The studs on the outsole of a soccer shoe have only a small area of contact with the ground. The pressure under the studs is high enough for them to sink into the ground, which gives extra grip.

Q11. How a moving body is made to slow down or is completely stopped?

Solution

In order to stop or slow down a moving body, a force in the direction opposite to the direction of motion of the body is applied.

Q12. What are contact forces?

Solution

Contact forces are those types of forces which result when the two interacting objects are perceived to be physically contacting each other.

Q13. Is there a gravitational force of attraction between two books lying on the table? If yes, why do we not see the books moving towards each other?

Solution

Yes, there is gravitational attraction between each and every object in this Universe.   When two books are placed on the table, they are gravitationally attracted by each other.   We do not see these two books moving towards each other because their masses are very small.   According to Newton's law of gravitation, we have    Now, the value of G is very small G = 6.67 × 10^-11 Nm²kg^-2   So, even if the books are massive, the force will be less because of the value of G.   Gravitational force is visible only when masses are huge of the order of 10¹⁰ or more.

Q14. Why the weight of any object on the moon is one-sixth of its weight on earth?

Solution

This is because the gravitational force of the moon is about one-sixth of what it is on earth.

Q15. Which force is involved in the bending of our body? Mention any two day-to-day activities that can be performed by exerting such a force.

Solution

Muscular force is involved in the bending of our body. Activities like jumping and climbing require muscular force.

Q16. Explain usage of lubricants with respect to effects of frictional force 

Solution

Frictional force is reduced by using a lubricant, such as oil, water, or grease, which is placed between the two contacting surfaces, reducing the coefficient of friction. The science of friction and lubrication is called tribology.

Q17. What do we call such forces that do not require any physical contact with the object on which they act? Give two examples of such forces.

Solution

The forces that do not require any physical contact with the object on which they act are called non-contact forces. For e.g. gravitational force and electrostatic force.

Q18. Name two effects a force can have on a stationary body.

Solution

The two effects a force can have on a stationary body are 1. a force can cause a motion in stationary objects. 2. a force can change the shape of stationary object.

Q19. Once the rowing is stopped, what brings the boat to rest?

Solution

The force responsible for changing the state of motion of objects is the force of friction. The, friction between water and the boat brings it to a stop once you stop rowing it.

Q20. Give an example of a non contact force which is always of attractive nature.

Solution

 A non-contact force is the one which acts on a body without any physical contact. An example of the same is gravitational force. It is always attractive in nature.  

Q21. (a) What do you understand by the term atmospheric pressure? (b) What is the cause of atmospheric pressure?

Solution

(a) The thrust per unit area exerted by air on the surface of the earth is called atmospheric pressure. (b) It is the weight of the air (thrust), which is responsible for the atmospheric pressure.

Q22. The atmospheric pressure on the earth's surface at sea level is about one hundred thousand Pascal. If such an enormous amount of pressure is acting on us, why we do not feel it?

Solution

This is because the pressure of the blood in our blood vessels balances out the atmospheric pressure.

Q23. Why does a balloon expand when you blow air into it? Why does it make a noise when it's popped?

Solution

When a balloon is blown up, the air pressure inside the balloon slowly becomes greater than the air pressure outside the balloon. Since the balloon is made of rubber and is expandable, it grows larger and larger. When the balloon is popped, the air escapes instantly. The sound heard is due to the molecules of air inside the balloon coming into sudden contact with the molecules of air outside the balloon.

Q24. (i) What happens when we rub a plastic object like a pen or a comb with hair? Which type of force is operating in this case? (ii) Why are skis used to glide over snow?

Solution

(i) When we rub a plastic object like a pen or a comb with hair, it gets electrically charged. When such an object is brought close to tiny bits of paper, the bits of paper get attracted to the object. This is due to electrostatic force.(ii) Skis have a large surface area to reduce the pressure on snow. This ensures that the skis do not sink in too far in the snow.

Q25. If a force of 2N is applied over an area of 2 cm², calculate the pressure produced.

Solution

Area = 2 cm²= 2/10000 = 0.0002 m² We know that, Pressure = Force/Area = 2N/0.0002 m² = 10000 Pa

Q26. Can air pressure tell us about the kind of weather to expect?

Solution

Yes, air pressure can tell us about what kind of weather to expect. If a high pressure system is on its way, often we can expect cooler temperatures and clear skies. If a low pressure system is coming, then we look for warmer weather, storms and rain.

Q27. Why force of friction is considered as contact force?

Solution

The force of friction always acts on all the moving objects. It arises only when the surfacs of two objects are in touch with each other, therefore, it is considered as contact force.

Q28. A force is applied on a rigid and non rigid body what is the difference between the effects of force in the two cases.

Solution

When force is applied on a rigid body, there will be no change in the shape or size of the rigid body. In case of a non rigid body the force will distort shape and/or size of the body.

Q29. Why are walls of dams made thicker at the bottom?

Solution

The water pressure increases with depth. Hence, as we go towards the bottom of the water source, the pressure increases. To withstand this large pressure the walls of the dam are made thicker than at the top.

Q30. Give two examples, where force of friction is a hindrance.

Solution

1. It wears out the sole of shoes and  2. It wears out the moving parts of machinery.

Q31. Why astronauts have to wear special suits?

Solution

Spacesuits are specially designed to protect astronauts from the cold, dangerous radiations and low pressure in space and to provide oxygen to breathe. Thus, wearing a spacesuit allows an astronaut to survive and work in space.

Q32. Which materials are considered brittle? Give two examples of brittle and non-brittle substances.

Solution

The materials that break easily when force is applied on them are considered as brittle. Examples of brittle materials are ceramic and glass. Objects made up of plastic and wood are non-brittle in nature.

Q33.  In what ways atmospheric pressure is similar to liquid pressure?

Solution

Atmospheric pressure is similar to liquid pressure in the following ways: 1. Both of these pressures act in all directions. 2. Both of these pressures become less as we rise up through air or liquid. Atmospheric pressure is similar to liquid pressure in the following ways: 1. Both these pressures act in all directions. 2. Both these pressures become less as we rise up through the liquid or air.

Q34. Give an example where the application of force can change the shape of the object.

Solution

When we squeeze a rubber ball between the palms of our hands, forces act thereby changing the shape of the rubber ball.

Q35. When a small rubber cup is pressed hard on a smooth plane surface, it sticks to it. Give reason.

Solution

The small rubber cup sticks to the surface because most the air between the cup and the surface escapes out ,hence the pressure inside decreases while the outer pressure which is more, acts on it.

Q36. While holding your hand on your ribs, take a deep breath and observe what happens to your chest. How would you explain the observation?

Solution

The chest expands because the number of air molecules inside the lungs increases which causes the lungs to expand in order to provide space for the increased number of air molecules.

Q37. Why does an object always fall downwards and not upwards?

Solution

This happens because the earth pulls all objects towards itself due to the attractive force called the force of gravity.

Q38. In your day to day life where do you use air pressure? Think and write.

Solution

We use air pressure in the following activities in our daily activities:

We use a straw to drink cold drinks.

We use water gun or pichkari to spray water by filling it with water.

We use a cycle pump to fill air in the bicycle tube.We use an ink filler to fill ink in a pen.

We use a siphon to transfer water from two vessels kept at different levels.

Q39. Do contact forces applied on an object in the opposite directions add to one another?

Solution

No, contact forces acting in same direction add and those acting in the opposite directions subtract.

Q40. Give three examples of muscular force.

Solution

(i) Lifting a bucket full of water. (ii) Kicking a football. (iii) Pushing an object.

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