Below i am providing you all with comprehensive notes on Forces and laws of motion . The question and answers have been asked in cbse exams a frequently hence are very important in ssc and railway exam point of view. Hope this helps.
Complete notes on Force and Laws of Motion (Important questions from Ncert Books)
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| Newtons' Craddle Find out which Law is demonstrated here? credits:wikipedia |
Q1: Define inertia.
Answer: Inertia is the resistance of a body to change in its state of rest or motion. It is an inherent property of the body. The property of inertia is due to the mass of the body. Greater the mass, higher will be the inertia.
There are three kinds of intertia:
- Intertia of rest
- Interia of motion
- Interia of direction.
(Some physicists consider inertia of direction is part of inertia of motion).
Q2(NCERT): Which of the following has more inertia:
(a) a rubber ball and a stone of the same size?
(b) A bicycle and a train?
(c) A five-rupee coin and a one-rupee coin?
Answer: The property of inertia is due to the mass of the body. Greater the mass, higher will be the inertia.
(a) The stone will have more inertia because it has more mass than the ball.
(b) Mass of a train is more than the mass of a bicycle, thus the train will have more inertia.
(c) Mass of a five rupee coin is more than that of a one-rupee coin. Hence, the five rupee coin has greater inertia than the one-rupee coin.
Q3: State Newton's First Law of motion.
Answer: Newton's First Law of motion states if a body is in state of rest, it will remain in the state of rest and if it is in the state of motion it will remain in the state of the motion with same velocity and same direction unless an external force is applied on it.
Q4: State why Newton's first law of motion is called law of inertia.
Answer: Inertia is a tendency of the object to resist change in its state. Newton's first law of motion also states similar i.e. the object will remain its present state unless an external force is applied. That's why Newton's first law is called Law of inertia.
Q5: Justify Newton's first law gives notion (or definition) about force.
Answer: According to Newton's first law of motion, an object tends to continue it is present state unless and external force is applied to change its state. For example, when a ball is rolled over a glass surface, it stops after covering a distance. It is because the frictional forces between the ball and the glass are being applied here. If there is frictionless surface in a vacuum chamber, the ball will continue to run and cover a longer distance. It implies the external force (frictional force in this case) is almost absent and the ball continues to move its present state.
Thus Newton's first law provides a notion about force.
Q6(NCERT): In the following example, try to identify the number of times the velocity of the ball changes:
“A football player kicks a football to another player of his team who kicks the football towards the
goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of
his own team”.
Also identify the agent supplying the force in each case.
Answer: We need to apply first law of motion and identify what force is applied which changes the velocity (state i.e. rest, direction, motion) of the ball. A force when applied can produce acceleration in body.
| Event | Agent | force and Action |
|---|---|---|
| A football player kicks a football | football player | push force, ball moves from rest. |
| to another player of his team who kicks the football towards the goal. | another player | force, changes direction of ball. |
| The goalkeeper of the opposite team collects the football | goal keeper | force to stop ball (ball velocity is zero now) |
| and kicks it towards a player of his own team | goalkeeper | force moves ball from rest to motion in opposite direction. |
Q7: What a force can do?
Answer: A force is a push or pull which produces acceleration in the body on which it acts. A force acting on a body can cause:
- it can alter the speed (or velocity) of the moving object.
- it can change the direction of motion of a body.
- it can change the shape of an object
Q8(NCERT): Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.
Answer: Some leaves of a tree get detached when we shake its branches vigorously isdue to inertia of rest. When the tree moves to and fro, leaves tend to remain in the rest state and resist the change. Due to this, some leaves gets detached and fall down.
Q9: Study the following scenarios and identify what type of inertia tends to resist the change.
(a) When a bus starts suddenly, the bus passengers standing in the bus tend to fall backwards.
(b) When a passenger jumps out of moving bus, he falls down.
(c) A cyclist on a levelled road does not come to rest immediately even he stops pedalling.
(d) When the playing card is flicked with the finger the coin placed over it falls in the tumbler.
(e) An athlete often jumps before taking a long jump.
(f) When a car enters a curved path, the car passengers tend to tilt outwards.
Answer:
(a) Inertia of rest
(b) Inertia of motion.
(c) Inertia of motion.
(d) Inertia of rest.
(e) Inertia of motion.
(f) Inertia of direction.
Q10(NCERT): Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?
Answer: Inertia tend to resist the change in state. When a moving bus brakes to stop, our body is in inertia of motion state. It tends to oppose the change of state of rest and we fall forward.
Similarly when the bus is at rest and starts, our body is in inertia of rest state. It opposes the forward motion of the bus and hence we fall backwards.
Q11: Is force required to keep a moving object in motion (yes/no)?
Answer: No.
Q12: Define momentum.
Answer: Momentum is defined as the product of its mass and velocity.
i.e. Momentum (P) = mass(m) × velocity(v).
Momentum is a measure of the quantity of motion of a body. It is a vector quantity. It has both direction and magnitude. and its direction is same as the direction of velocity. The SI unit of momentum is kg-m/s.
Q13: A body of mass 25 kg has momentum of 125 kg m/s. What is the velocity of the body?
Answer: Since P = m × v
⇒ v = P / m = 125 / 25 = 5m/s
Q14(CBSE 2010): Why is it easier to stop a tennis ball in comparison to a cricket ball moving with the same speed ?
Answer: Tennis ball is lighter than (less mass) than a cricket ball. Tennis ball moving with same speed has less momentum ( mass × velocity) than a cricket ball. ∴ It is easier to stop tennis ball having less momentum.
Q16: The term 'mass' is analogous to physical quantity (a) Weight
(b) Intertia
(c) Force
(d) Acceleration
Answer: (b) Intertia
Q17: What are balanced and unbalanced forces?
Answer: If a set of forces acting on a body does not change the state of rest or of motion of an object such forces are called balanced forces. In this case object is said to be inequilibrium state. The vector sum of all forces is zero i.e. ∑Fx = 0.
If set of forces acting on body results in change in state i.e. either changes the speed or direction, such forces are called unbalanced forces. In this case the object is in non-equilibrium state and the resultant force is non-zero. i.e. ∑Fx ≠ 0.
Q18: The object shown below moves with a constant velocity. Two forces are acting on the object. Considering negligible friction, the resultant force will be:
(b) 10N leftwards
(c) 3N leftwards
(d) 7N rightwards
Answer: (c) 3N leftwards.
Q19: If the set of forces acting on an object are balanced, then object
(a) must be at rest.
(b) must be moving
(c) must not be accelerating.
(d) none of these.
Answer: (c) must not be accelerating. If the forces are balanced, the object will remain in its present state i.e. if it is moving, it will keep on moving with uniform speed. If it is at rest, it will remain at rest.
Q20(NCERT): An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.
Answer: According to Newto's first law, in order to change the state of motion of a body, an external unbalanced force must be applied.
If an object is moving with a uniform speed and the object experiences a net zero external unbalanced force. It implies the object is in equilibrium state and it will remain in its present state i.e. it will keeps on moving with the same speed and direction.
Q21(NCERT): When a carpet is beaten with a stick, dust comes out of it. Explain.
Answer: Due to inertia of rest. When a carpet is beaten with a stick, dust particles tend to remain in inertia of rest and resist the change. Hence, the dust particles come out of the carpet.
Q22(NCERT): Why is it advised to tie any luggage kept on the roof of a bus with a rope?
Answer: While the bus is moving, luggage tends to remain in inertia of motion state. When the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall off. That's why it is advised to tie any luggage kept on the roof of a bus with a rope.
Q23: State Newtons second law of motion.
Answer: The second law of motion states that the rate of change of momentum of an object is
proportional to the applied unbalanced force in the direction of force.
Mathematically, the law tells that force is the product of mass and acceleration.
i.e. F = ma
where m = mass of the object, a = acceleration and
F is the net force (i.e. F = ∑Fx = F1 + F2 + ... + Fn)
Q24: Which of the following graph represents unbalanced force.
Answer: An unbalanced force results in acceleration. Figure (b) represents this condition.
In fig. (a), v-t graph represents uniform motion ⇒ acceleration (a) = 0.
Fig. b, v-t graph shows the velocity is not uniform, it is decreasing ⇒ acceleration is non-zero.
Fig. c is a Distance-time graph which shows uniform motion ( a straight line).
Q25: How Newton's Second law of motion is different from First law?
Answer: Newton’s first law of motion deals with the behaviour of objects on which all existing forces are balanced. It gives us notion of external force required to change the inertia state of the object.
While the second law of motion deals with the behaviour of objects on which all existing forces are not balanced. It gives us a measure of the force.
Q26: Name the categories of forces based on interaction.
Answer: Force exists only as a result of an interaction of two objects. Based on it, forces are categorised as:
- contact forces and
- non-contact-forces
Answer: Contact forces are forces in which the two interacting objects are physically in contact with each other. Examples of contact forces are:
- Tension force
- Normal force
- Air resistance
- Buoyant force
- Friction force
Q28: What are non-contact forces? Give examples.
Answer: Non-contact forces are forces in which the two interacting objects are not in physical contact which each other, but are able to exert a push or pull. These are also called Action-at-a-distance forces. Examples are:
- Gravitational force
- Magnetic Forces
- Electric Force
- Nuclear ForcE
Q30: Define 1N (Newton).
Answer: One Newton is the force acts on a body of mass 1 kg and it produces an acceleration of
1 ms-2.
⇒ 1N = 1kg × 1 ms-2
Q31: What is the unit of force in c.g.s system?
Answer: dyne (g cm s-2)
Q32: What's the relation between 1N and 1 dyne?
Answer: 1 N = 1kg × 1 ms-2
and 1 dyne = 1g × 1 cms-2
1 N = 1kg × 1 ms-2 = 1000g × 100 cms-2
⇒ 1N = 105 dyne
Q33: Is Newton's second law of motion is consistent with the first law? How?
Answer: Yes Newton's second law is consistent with first law. The first law of motion can be
mathematically stated from the mathematical expression for the second law of motion.
Since F = ma = m(v - u)/t
or Ft = mv - mu.
⇒ F = 0 when v = u for any given time t.
⇒ The object will continue moving with uniform velocity, u throughout the time, t.
Similarly when object is at rest, u = v = 0. ⇒ F = 0. No external force is being applied, the object will remain at rest.
Q34(NCERT): A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because
(a) the batsman did not hit the ball hard enough.
(b) velocity is proportional to the force exerted on the ball.
(c) there is a force on the ball opposing the motion.
(d) there is no unbalanced force on the ball, so the ball would want to come to rest.
Answer: (c) there is a force on the ball opposing the motion.
Q37(NCERT): What is the momentum of an object of mass m, moving with a velocity v?(a) (mv)2
(b) mv2
(c) ½ mv2
(d) mv
Answer: (d) mv
Q40(CBSE 2010): While catching a fast moving ball, fielder gradually pulls his hand backwards. Give reasons.
Answer: By doing so, the fielder increases the time during which the high velocity of the moving ball
decreases to zero. The acceleration of the ball is decreased and therefore the impact of catching the fast moving ball is also reduced.
Q43: State Newton's third law of motion.
Answer: It states that to every action there is always an equal and opposite reaction.
When two objects A and B act on each other, the force exerted by A on B (FAB) is equal to the force exerted by object B on A (FBA) in magnitude but are in opposite directions.
i.e. FAB = -FBA
or FAB + FBA = 0
One of the force (say FAB) can be said action force then other one is reaction force (i.e. FBA). Action-reaction forces always act on different bodies and their line of action is the same.
Q44: When two objects act on each other, does action-reaction force pair cancel out each other?
Answer: No. The action-reaction forces act on different bodies.
Q45: While driving on a highway, an insect strikes on the car windshield and splatters. Which experiences greater impact (force): an insect or the windshield?
Answer: It is an example of third law of motion. Both insect and the car windshield experiences same amount of force but in opposite direction. But the force is too great for the insect that its body splatters while the windshield is able to withstand the impact.
Q46(NCERT): If action is always equal to the reaction, explain how a horse can pull a cart.
Answer: According to Newton's third law of motion, action force is equal to reaction but acts on two different bodies and in opposite directions. When a horse pushes the ground, the ground reacts and exerts a force on the horse in the forward direction. The force is able to overcome friction force of the cart and it moves.
(Note: A detailed explanation on house cart problem can be read here)
Q47: Identify the action and reaction forces in the following cases:
(a) A television (TV) lying on a table
(b) Pushing a wall with your hand
(c) Firing a bullet from a gun.(d) Walking of a person on a ground.
Answer:
(a) A TV lying on a table: TV is interacting with table surface. Applying Newton's third law of motion, action force acts on table due to TV (FTV-Table) and a reaction force acts on TV by the table i.e. (FTable-TV). Both these forces are equal in magnitude but in opposite directions.
⇒ FTable-TV = - FTV-Table
(b) Pushing a wall with your hand: When you pushes a wall, you exert a force (action) on the wall. Similarly, the wall exerts a (reaction) force on your hands in contact with the wall. According to Newton;s third law of motion, both these forces form action-reaction pair and act on different bodies. Both are equal in magnitude but opposite in direction.
(c) Firing a bullet from a gun: When a gun fires a bullet, a force is exerted (action) on a bullet and the gun experiences an equal recoil (reaction) force in opposite direction.
(d) Walking of a person on a ground: When a person pushes the ground (action) backwards by his foot, the ground also exerts an equal force (reaction) on the his foot in forward direction.
Q48: A TV set is lying on a table. The TV set experiences a gravitational force (pull) downwards (FW) by the Earth. It also experiences a force on it due to table in contact i.e. (FTable-TV) in upward direction. Do these forces form an action-reaction pair? Do these forces follow Newton's third Law of motion?
In fact, Newton's first law of motion does apply. Since the TV set is at rest, because these forces are balanced. i.e. FW = - FTable-TV
Q 49(CBSE Exam/NCERT): According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the road side, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and explain why the truck does not move.
Case I: No external force is applied. In this case, the truck weight (gravitational force) acting downwards, is cancelled by the contact force applied on the truck by the ground.
i.e.
FWeight = - Fground
Case II: If a person or group of persons pushes truck, the static friction acting on the truck horizontally, cancels this push-force. Static friction is a self-adjusting force. More push is applied, more static friction will oppose it (upto a limit) and forces are balanced. In order move the truck, the push-force must overcome static friction.
and FPush = - FFriction
Q50 (NCERT): Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.
Answer: Large amount of water coming out from the nozzle of the hose at high velocity will have large momentum. Due to a large momentum, water comes out exerting a large force. According to Newton's third law of motion, the hose will also experience an equal reaction force but in opposite direction. Because of this reaction force, it is difficult for a fireman to hold a hose.
Q51 (CBSE 2010): It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why?
Answer: A frictional force always acts parallel to the surface and is directed to oppose sliding. Banana skin reduces friction (or frictional force) and thus brings body in unbalanced state and we tend to fall.
Q53: State Law of conservation of momentum.
Answer: According to law of conservation of momentum, in the absence of external unbalanced force the total momentum of a system of objects remains unchanged or conserved by collision.
i.e. Total Initial Momentum Before Reaction = Total Initial Momentum After Reaction
This law holds good for any number of objects.
Q54: Law of conservation of momentum is applicable to an isolated system. What do you mean by an isolated system?
Answer: Isolated system refers to absence of external unbalanced force or net force is zero.
Q55(NCERT EXEMPLAR): Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why ?
Answer: The reason is because there exists external unbalanced force i.e. gravitation pull of the earth due to which ball falls down and is accelerating. Therefore it is not an example of conservation of momentum.
Q56: What is collision? What are the types of collision?
Answer: The interaction between two or more bodies causing the exchange of momentum is called
collision. It is categorised as of two types:
- elastic collision (both momentum and kinetic energy are conserved)
- inelastic collision (only momentum is conserved)
Q60: What is impulse?
Answer: The effect of force applied for a short duration is called impulse. It is the product of force (F) and the time duration (t) for which the force is applied.
i.e. Impulse = F × t
Impulse in a vector quantity and its SI unit is N-s.
Since F = ma = m(v-u)/t
⇒ F × t = mv - mu
⇒ Impulse is equal to change in momentum.
Q66(NCERT): The following is the distance-time table of an object in motion:
| Time in seconds | Distance in meters |
|---|---|
| 0 | 0 |
| 1 | 1 |
| 2 | 8 |
| 3 | 27 |
| 4 | 64 |
| 5 | 125 |
| 6 | 216 |
| 7 | 343 |
(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?
(b) What do you infer about the forces acting on the object?
Answer: (a) Since the distance travelled in in equal intervals of time is not equal rather it is increasing. It means the object is moving with a non-uniform velocity. It is accelerating.
By looking at the table we find distance (S) ∝ t3 ...(I)
Let us check if acceleration is constant. We know that for fixed 'a', equation S = ut + ½at2is valid.
In this case, u = 0, (since at t = 0, S = 0), ⇒ S = ½at2 or (S) ∝ t2 ... (II)
But from the table, we get S ∝ t3, we conclude, that 'a' is increasing.
(b) Since the object is in accelerated condition, According to Newton's Second Law, F ∝ a.
We can say, unbalanced force is acting on the object.
Q69: A spring scale reads 20 N as it pulls a 4.0 kg object across a table. What is the magnitude of the force exerted by object on the spring scale?
(a) 40 N
(b) 20 N
(c) 4 N
(d) 5 N
Answer: (b) 20 N (Hint: Newton's 3rd Law Action-reaction force).
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Answer: Newton's laws of motion hold true for common practical applications. However it has certain limitations:
- Second Law (F = ma) assumes mass is constant. When an object travels at the speed of the light, its mass is also affected. Newton's law is not applicable in this case.
- Newton's Laws fail to explain motion of electrons around nucleus.
- These law also fail to explain about black holes and bending of gravity due to star light.
- It also fails to explain why a leaf falls in river stream, its path cannot be determined (chaos theory)
Q72: Two objects having their masses in ratio 3:5 are acted upon by two forces each on one object. The forces are in the ration of 5:3. Find the ratio in their accelerations.
Answer: Since masses are in ratio 3:5.
Let the mass of the objects be 3x and 5x.
Let F1 and F2are the two forces with a1 and a2 accelerations.
∴ F1 = m1a1 = 3xa1
and F2 = m2a2 = 5xa2
Since F1:F2 = 5:3, we have
3xa1:5xa2 :: 5:3
⇒ a1:a2 = 5×5:3×3
⇒ a1:a2 = 25:9
The END
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