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Q1. What is the most basic and correct definition of a force?
Force is defined as an interaction that can change the state of motion or shape of an object. It can be a push, pull, or hit. It does not change chemical composition, temperature, or measure the amount of matter (which is mass).
Q2. The ancient belief that ‘rest’ is the natural state of objects was primarily based on what everyday observation?
Ancient thinkers observed that when we stop pushing or pulling an object, it eventually comes to rest. This led them to believe that rest is the natural state of all objects and that a continuous force is needed to keep an object moving.
Q3. Which two scientists are credited with overturning the ancient belief about rest and motion?
Galileo Galilei was the first to challenge the ancient belief through experiments with inclined planes. Isaac Newton later built upon Galileo’s work and formulated the Three Laws of Motion, which correctly described the relationship between force and motion.
Q4. Which of the following is a direct effect that a force can have on an object?
A force can change the shape or size of an object. For example, squeezing a rubber ball changes its shape. Force does not change chemical identity, colour, or mass of an object.
Q5. What does the application of an unbalanced force on an object necessarily cause?
An unbalanced force always causes a change in the state of motion of an object. This means it can make a stationary object move, change the speed of a moving object, or change its direction.
Q6. A force can change the magnitude of which specific physical quantity?
Force can change the velocity of an object by changing its speed or direction. Force does not change mass, density, or volume of an object (unless it causes deformation, but velocity is the primary quantity affected).
Q7. What is the net result when balanced forces act on an object?
Balanced forces are equal in magnitude and opposite in direction. They cancel each other out, resulting in no change in the object’s state of motion. If the object is at rest, it remains at rest; if it is moving, it continues with the same velocity.
Q8. An object remains stationary when equal and opposite forces act on it. This is an example of:
When equal and opposite forces act on an object and it remains stationary, the forces are balanced. The net force is zero, so there is no change in motion.
Q9. The force of friction always acts in a direction that is:
Friction always opposes motion or the tendency of motion. If an object is moving, friction acts opposite to its direction of motion. If an object is at rest and we try to move it, friction acts opposite to the intended direction of motion.
Q10. A box on a rough floor does not move when pushed with a small force because:
Static friction is the force that prevents an object from moving when a force is applied. It increases up to a maximum value. If the applied force is less than this maximum static friction, the box will not move.
Q11. Newton’s First Law of Motion is also commonly known as the Law of:
Newton’s First Law is called the Law of Inertia because it describes the property of objects to resist changes in their state of motion. Inertia is the tendency of an object to remain at rest or in uniform motion.
Q12. According to Newton’s First Law, an object will maintain uniform motion only if:
Newton’s First Law states that an object will continue in its state of rest or uniform motion unless acted upon by a net external force. So uniform motion is maintained when the net external force is zero.
Q13. To change the velocity of an object (i.e., to accelerate it), what is required?
According to Newton’s Second Law, a net unbalanced force is required to produce acceleration, which is a change in velocity. Balanced forces produce no change in velocity.
Q14. If a moving object experiences a sudden net force of zero, what will happen to its motion?
According to Newton’s First Law, if the net force on a moving object becomes zero, it will continue to move with the same velocity (same speed and direction). It will not stop or change direction.
Q15. The property of an object that quantifies its inertia is its:
Mass is the measure of inertia of an object. Greater mass means greater inertia, meaning it is harder to change the state of motion of the object. Weight is the force of gravity on an object, not a measure of inertia.
Q16. What is the SI unit of force?
The SI unit of force is the Newton, named after Sir Isaac Newton. One Newton is the force required to give a mass of 1 kilogram an acceleration of 1 metre per second squared.
Q17. Newton’s Second Law of Motion establishes a relationship between:
Newton’s Second Law states that force is equal to mass times acceleration (F = ma). It establishes a direct relationship between the net force applied to an object, its mass, and the acceleration produced.
Q18. According to Newton’s Second Law (F=ma), for a given net force, the acceleration of an object is:
From F = ma, we can write a = F/m. For a constant force, acceleration is inversely proportional to mass. This means heavier objects accelerate less than lighter objects when the same force is applied.
Q19. Newton’s Third Law of Motion states that for every action, there is:
Newton’s Third Law states that for every action force, there is an equal and opposite reaction force. These forces act on different objects and are always equal in magnitude and opposite in direction.
Q20. When you push against a wall, you feel the wall pushing back on you. This is an example of:
When you push against a wall, your hand applies a force on the wall (action). The wall applies an equal and opposite force back on your hand (reaction). This is a perfect example of Newton’s Third Law.
Q21. The force that opposes the relative motion between two surfaces in contact is called:
Friction is the force that opposes the relative motion or tendency of relative motion between two surfaces in contact. It acts parallel to the surfaces in contact.
Q22. Which of the following is NOT a type of friction?
The types of friction are static friction (between surfaces at rest), kinetic or sliding friction (between moving surfaces), rolling friction, and fluid friction (in liquids and gases). Gravitational friction is not a type of friction; gravity is a different force.
Q23. Which factor does NOT affect the force of friction between two solid surfaces?
For a given normal force, the area of contact does not significantly affect the force of friction. Friction depends on the nature of surfaces, the normal force, and for kinetic friction, the relative speed of sliding.
Q24. A passenger in a moving bus is thrown forward when the bus stops suddenly. This is due to:
When the bus stops suddenly, the passenger’s body tends to continue moving forward due to inertia. The passenger was in motion with the bus, and their body resists the change in motion, causing them to lurch forward.
Q25. When a carpet is beaten with a stick, dust particles are removed. This demonstrates:
When the carpet is beaten, the carpet moves but the dust particles tend to remain at rest due to their inertia. As the carpet moves away, the dust particles fall off. This demonstrates the inertia of rest of the dust particles.
Q26. The momentum of an object is defined as the product of its:
Momentum (p) is defined as the product of mass (m) and velocity (v). So p = m × v. It is a vector quantity and has both magnitude and direction.
Q27. What is the SI unit of momentum?
Momentum is mass × velocity. The SI unit of mass is kg and velocity is m/s, so the unit of momentum is kg m/s. It can also be written as N s (Newton-second).
Q28. According to the law of conservation of momentum, the total momentum of a system of objects remains constant if:
The law of conservation of momentum states that the total momentum of a system remains constant if no external unbalanced force acts on it. This means the system is isolated, and only internal forces act between the objects.
Q29. In a collision between two objects where no external forces are involved, if one object gains momentum, the other object must:
According to the law of conservation of momentum, total momentum before collision equals total momentum after collision. So if one object gains momentum, the other must lose an equal amount of momentum.
Q30. A gun recoils when fired. The speed of the recoil of the gun is less than the speed of the bullet because:
According to conservation of momentum, momentum of gun = momentum of bullet. Since the gun has much larger mass, its recoil velocity must be much smaller than the bullet’s velocity to have equal momentum.
Q31. What is the relationship between force and the rate of change of momentum?
Newton’s Second Law can be stated as: Force is equal to the rate of change of momentum. F = (mv – mu)/t = m(v-u)/t = ma. This is the more general form of the law.
Q32. The impact of a force depends on both its magnitude and the:
The impact or effect of a force depends on its magnitude and the time for which it acts. This is why the concept of impulse (force × time) is important.
Q33. Catching a cricket ball involves moving the hands backwards with the ball. This is done to:
By moving hands backwards, the time of impact is increased. Since impulse = force × time = change in momentum, increasing the time reduces the force experienced by the hands, making it less painful.
Q34. Seat belts in cars are designed to prevent injuries by:
Seat belts increase the time over which a passenger’s momentum changes during a collision. By increasing the stopping time, the force on the passenger is reduced, preventing serious injuries.
Q35. An object is moving with uniform velocity. What can be concluded about the forces acting on it?
Uniform velocity means both speed and direction are constant. According to Newton’s First Law, this means the net force acting on the object is zero. Forces may be acting, but they are balanced.
Q36. The tendency of an object to remain in its state of rest or uniform linear motion is a definition of:
Inertia is the property of an object to resist any change in its state of rest or uniform motion. It is the fundamental concept behind Newton’s First Law of Motion.
Q37. Which of the following has the greatest inertia?
Inertia depends on mass. The greater the mass, the greater the inertia. Among the options, a truck has the largest mass, so it has the greatest inertia.
Q38. When a force of 1 Newton acts on a mass of 1 kg, the acceleration produced is:
According to Newton’s Second Law, F = ma. So a = F/m = 1 N / 1 kg = 1 m/s². This is the definition of a Newton.
Q39. A constant net force ‘F’ acting on an object of mass ‘m’ produces an acceleration ‘a’. If the mass is doubled and the force is halved, the new acceleration will be:
Original: a = F/m. New: a’ = (F/2)/(2m) = F/(4m) = (1/4)(F/m) = a/4. So the new acceleration is one-fourth of the original acceleration.
Q40. The force exerted by the earth to pull an object downwards is called:
The Earth exerts a gravitational force on all objects, pulling them towards its centre. This force is called gravity and is responsible for objects falling downwards.
Q41. The force with which an object is pulled towards the centre of the Earth is its:
Weight is the force with which an object is pulled towards the centre of the Earth. It is calculated as mass × acceleration due to gravity (W = mg). Weight is a force and is measured in Newtons.
Q42. A force of 10 N is applied to two objects A and B. The mass of A is 5 kg and the mass of B is 10 kg. Which object will have greater acceleration?
Using F = ma, acceleration a = F/m. For object A: a = 10/5 = 2 m/s². For object B: a = 10/10 = 1 m/s². So object A has greater acceleration because it has smaller mass.
Q43. The total momentum of a system of two identical balls moving with equal speeds towards each other is:
If two identical balls have equal speeds moving towards each other, their momenta are equal in magnitude but opposite in direction. So they cancel out, giving total momentum of zero.
Q44. A rocket works on the principle of:
A rocket works on the principle of conservation of momentum and Newton’s Third Law. The rocket expels exhaust gases backwards, and the rocket itself moves forward with an equal and opposite momentum.
Q45. Which law of motion explains why it is difficult for a firefighter to hold a hose that ejects large amounts of water at high speed?
When water is ejected from the hose with high speed, the hose experiences an equal and opposite reaction force. This causes the hose to push back on the firefighter, making it difficult to hold.
Q46. A book lying on a table is an example of:
A book lying on a table is at rest because the forces acting on it are balanced. The downward force of gravity is balanced by the upward normal force from the table.
Q47. The force of friction is considered a necessary evil because:
Friction is called a necessary evil because it is essential for many everyday activities like walking and braking vehicles. However, it also causes wear and tear of machinery and wastes energy.
Q48. If the mass of a body is halved and its velocity is doubled, its momentum will:
Momentum p = m × v. New momentum p’ = (m/2) × (2v) = m × v = p. So the momentum remains the same.
Q49. The force required to keep a body moving with uniform velocity on a frictionless horizontal surface is:
On a frictionless horizontal surface, no force is needed to keep a body moving with uniform velocity. According to Newton’s First Law, the body will continue with its uniform motion unless acted upon by a net force.
Q50. The statement “an object at rest stays at rest” is a part of Newton’s First Law. This is true because of the object’s:
An object at rest stays at rest because of its inertia, which is the tendency to resist any change in its state of motion. This is the fundamental concept behind Newton’s First Law.