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Laws of Motion Important Questions are given in this article as per the latest CBSE Exam Pattern. The important questions cover the concepts of laws of motion, force, and acceleration. Laws of Motion are the fundamental laws that are used to identify the relationship between an object’s motion and the forces acting upon it.
Basically. there are basically three laws of motion given by Newton that were employed by him to describe and study the motion of a variety of physical objects and systems, laying the groundwork for Newtonian Mechanics. Newton’s first law of motion implies that a body at rest or uniform motion will continue to be at rest or uniform motion until and unless an external force acts on it or is implied on it. The second law of motion that is given by Newton states that the force acting on the body is equal to the product of its mass and acceleration. Lastly, the third law of motion states that there is an equal and opposite reaction for every action.
Very Short Answer Questions (1 Mark Questions)
Ques. When a stone is thrown against a glass window, it shatters the pane. However, a bullet shot from a gun will pierce it and leave a hole. Why?
Ans. The velocity of stone is very less compared to that of the bullet fired from a gun. The stone remains in contact with the windowpane for a long time due to its low speed and hence motion is shared by the whole of the window pane whereas the bullet fired from the gun remains in contact with the window pane for such a small time and shares its motion only with a portion of the window pane. That is the reason why a bullet makes a clean hole in the windowpane.
Ques. Friction is a force that adjusts on its own. Justify.
Ans. Friction is a self-adjusting force that fluctuates in magnitude from zero to maximum in order to reduce friction.
Ques. The action and reaction forces are unbalanced. Why?
Ans. Action and reaction do not balance each other since a force of action and response always operate on two different bodies.
Ques. What gives a car turning on a level road centripetal force?
Ans. Centripetal Force is generated by friction between the tyres and the road.
Ques. Which principle of conservation governs the operation of the rocket?
Ans. The law of conservation of linear momentum governs the operation of the rocket.
Ques. Why are the wheels of vehicles are provided with mudguards?
Ans. Owing to the inertia of direction, dirt stuck to the wheel flies off tangentially whenever the wheel spins at a fast speed. Mudguards are installed on the wheels to keep flying dirt from ruining the clothes of passers-by.
Ques. Why is it advised to hold a gun tight to one's shoulder when it is being fired?
Ans. The gun must be held tight to the shoulder as the gun recoils after shooting. In this case, the gun and the shoulder are one mass system, due to which the back kick will be reduced.
Read More: NCERT Solutions for Class 11 Physics Chapter 5 Laws of Motion
Newton’s Laws of Motion
Short Answer Questions (2 Marks Questions)
Ques. In an upward direction, a lift is accelerated. Will the apparent weight of a person inside the lift rise, fall, or remain constant in relation to its true weight? So, if the lift is moving at a constant speed?
Ans. The perceived weight will increase i.e rise accordingly. If the lift moves at a constant speed, the apparent weight will remain the same as the true weight.
Ques. A force is applied to a body however no acceleration is produced. What explanations could be considered for the observation?
Ans. The possibilities that may be considered to explain the observation are as follows:
- No acceleration is produced if the force is a deforming force.
- Internal force cannot cause acceleration.
Read More: Average Acceleration Formula
Ques. What is the factor that influences the coefficient of friction?
Ans. The coefficient of friction is primarily determined by two factors, which are as follows:
- The surfaces in contact’s materials.
- The properties of the surfaces
Ques. Calculate the amount and direction of the net force exerted on the object.
- A single drop of rain falling at a consistent rate.
- A kite held perfectly still in the sky.
Ans. (a) The raindrop’s acceleration would be zero because it is falling at a constant pace. The net force exerted on the raindrop will be 0 since the force acting upon an object is given by.
(b) So because the kite is confined and stationary, the algebraic total of forces operating on it is zero, according to Newton’s first law of motion.
Read More: Types of Friction
Ques. A force of 10kg wt is applied to each end of the spring balance. What will the balance read?
Ans. Weight is gauged by spring balancing, which is dependent on the tension in the spring. If a 10kg weight is applied to both ends, the tension will be 10kg, and the reading will be 10kg.
Ques. A man lies still in the middle of a pond on absolutely sleek ice. How will he get to the pond’s edge?
Ans. If he throws his shirt or anything else he owns in the opposite direction of travel, or if he spits in the forward direction, he can get to the shore.
Ques. Give a reason why passengers are thrown forward from their seats when a speeding bus stops suddenly.
Ans. When a fast-moving bus stops suddenly, the bottom half of the body in touch with the seat comes to a complete halt, whereas the upper section of the body prefers to retain its uniform motion. Thus, the passengers are pushed forward.
Read More: Motion in a Straight Line
Long Answer Questions (3 Marks Questions)
Ques. Why does a swimmer push backwards in the water?
Ans. According to Newton's third law of motion, "when one body exerts a force on another body, the first body undergoes a force similar in magnitude in the opposite direction of the force exercised." As an outcome, the swimmer will push the water backward with his hands in order to swim forward.
Ques. A thief jumps from a house's roof while carrying a box weighing W on his head. What is the weight of the box as felt by the thief during the jump?
Ans. Ans. During the jump, the thief is in free fall. During that time, he/she and the box will both be weightless. As a result, the weight of the box experienced by the thief during the jump will be zero.
So, mathematically, it is as follows: Weight of the box, W = m(g - a) = m(g - g) = 0.
Ques. Which of the following is a scalar quantity? Linear momentum, inertia, and force
Ans. The resistance of a body to its own acceleration is quantified by its inertia. As a consequence, the mass becomes a qualitative indicator of inertia. Since mass is a scalar quantity, linear inertia is also a scalar quantity. As an outcome, inertia will be the scalar quantity among them.
Read More: Difference Between Momentum and Inertia
Ques. A number of passengers are standing in a moving bus, which is considered risky. What evidence do you have to back up your claim?
Ans. Such scenarios are risky for three reasons –
- Standing passengers shift the bus's centre of gravity, putting it in an unsteady balance. This is risky.
- Passengers will fall forward one on top of the other if the driver applies hard braking. As a result, they could be hurt or stamped.
- In a congested traffic condition, if the bus comes to an abrupt stop, the passengers' forward momentum may force it forward, causing an accident.
Equations of Motion
Very Long Answer Questions (5 Marks Questions)
Ques. A stream of water is flowing horizontally with a speed of 15 ms−1. It gushes out of a tube of cross-sectional area 10−2 m2, and hits a vertical wall nearby. Calculate what is the force exerted on the wall by the impact of water, assuming it does not rebound.
Ans. The speed of the water stream is given as v = 15 ms−1
The cross-sectional area of the tube (A) = 10−2 m2
Thus, the volume of water coming out from the pipe per second will be
V = A v = 15 × 10−2 m3 s−1
We know that the density of water is p = 103 kgm−3
Flow Rate of water through the pipe in gallons per second will be
p × V = 150 kgs−1
The water does not bounce back when it hits the wall. Here, as a result, Newton's second law of motion gives the force exerted by the water on the wall as
F = Rate of change of momentum = ΔP/Δt = mv/t
= 150 × 15 = 2250 N
Read More: Mass and Momentum: Definition, Law and Difference
Ques. Define the concepts "momentum" and "impulse" in your own words. In the SI system, what are their units?
Ans. The overall amount of motion held by the body is referred to as momentum. It is equivalent to the product of the body's mass and velocity when calculated mathematically. This is known as linear momentum P in linear motion. It's a quantity with a vector.
mv = p
kg ms-1 or NS in SI units are the units of linear momentum.
The action or impact of force is referred to as the force impulse. The product of the force F operating on the body and the time for which the force acts is equal to the impulse J. Thus.
J = F × t = Ft
J is a vector quantity with units of Ns or kg ms-1.
When a force or impulse acts for a shorter period of time, its activity is intensified.
Read More: Difference Between Scalar and Vector Quantities
Ques. Suresh saw a large Granite Rock in his neighbourhood. He believed that if they worked on it, they would be able to make money. He obtained approval from the government and completed all of the necessary paperwork. He used a bomb to destroy the Rock. Slices were cut out of the rock. They started a Granite business. Many people in the area began to earn money and live nicely.
(a) Which of Suresh's values do you admire?
(b) A bomb is hurled horizontally at a speed of 50 metres per second. It detonates into two halves, each weighing 6 kg and 3 kg. The heavier piece continues to move horizontally at an average speed of 80 m/s. Calculate the lighter fragment’s velocity.
Ans. (a) Suresh understands how to make use of natural resources and is concerned about others. He also understands how to complete all legal requirements prior to beginning any work.
(b) In accordance with the law of conservation of momentum,
Total momentum of fragments = Momentum of the Bomb
m1v1 + m2v2 = MV
⇒ 6 × 80 + 3 × V2 = 9 × 50
⇒ V2 = -10 m/s.
Ques. A railway car with a mass of 20 tonnes moves with an initial speed of 54 km/hr. When the brakes are applied, a constant negative acceleration of 0.3 m/s2 is produced.
- Calculate what is the breaking force acting on the car?
- At what time will the car stop?
- Calculate the distance that will be covered by the car before it finally stops?
Ans. m = 20 tonnes = 20 × 1000 kg
u = 54 km hr−1 = 15 ms−1
a = −0.3 ms-2 and ϑ = 0
(i) F = ma
F = 20000 × (−0.3)
F = −6000 N
(ii) v = u + at
v − u = at
t = (v − u)/a = (0−15)/(−0.3)
t = 50 s
(iii) v2 − u2 = 2as
(0)2 − (15)2 = 2 (−0.3) s
s = 375 m
Read More: Equations of Motion
Ques. Are there different ways to reduce friction?
Ans. Friction reduction saves a significant amount of energy. The following are some of the strategies for minimising friction:
- Smoothing the surfaces using a variety of techniques, such as:
- scraping the surfaces and polishing them
- Using smooth materials such as metal foils or solar mica to cover the surface.
- Using lubricating fluids and other materials, such as granite, to lubricate the surfaces.
- The irregularities, grooves, and other faults in the surface are filled with these methods, and the surface becomes smooth. Grease, for instance, is used to reduce friction in many devices.
- Streamlining the shape of bodies: Streamlining the shape of bodies reduces friction. To reduce fluid friction, high-speed vehicles, railway engines, ships, boats, and aeroplanes, for example, are designed with streamlines. It not only decreases friction but also aids in the application of pushing power.
- Making modifications to convert sliding to rolling friction: Because rolling friction is much less than sliding friction, several machine parts are designed with rolling friction. Ball bearings are utilised in bicycle and machine shafts, for example. For the same reason, the vehicles use wheels rather than sliding arrangements.
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