QUESTIONS FOR SHORT ANSWER
1. The apparent weight of an object increases in an elevator while accelerating upward. A moongfaliwala sells his moongfali using a beam balance in an elevator. Will he gain more if the elevator is accelerating up?
Answer: A beam balance compares the weight of an object put at one side with respect to the standard weight put at the other side. So while the elevator is going up, weights on both sides of the balance will gain equally. The moongfaliwala thus would not gain more.
2. A boy puts a heavy box of mass M on his head and jumps down from the top of a multi-storied building to the ground. How much is the force exerted by the box on his head during his free fall? Does the force greatly increase during the period he balances himself after striking the ground?
Answer: During his free fall the box does not exert any force on the head of the boy. It can be explained by taking the frame of reference in the boy. Since the boy is accelerating downward with "g" acceleration, this frame will be a non-inertial frame. To apply Newton's law in a non-inertial frame we add a pseudo force(-ma) to the system along with the other forces acting on it, where a is the acceleration of the frame. In this case a=g. Consider the box as a system. The forces on it are gravity force weight of the box Mg downward, and the pseudo force upward (-Mg). Adding these forces we get zero. So no force is applied by the box on the boy.
Now consider the period when he balances himself after striking the ground. The feet of the boy has come to rest and he tries to bring the box (that has gained velocity) to rest by pushing it up. So he applies a great force upward to decelerate it. And according to Newton's third law of motion, the box also applies equal and opposite force on the boy.
3. A person drops a coin. Describe the path of the coin as seen by the person if he is in (a) a car moving at a constant velocity and (b) in a free-falling elevator.
Answer: (a) A car moving with a constant velocity is an inertial frame of reference. So the person will see the dropped coin falling to the floor in a straight line.
(b) A free falling elevator is a non-inertial frame of reference because it is accelerating towards the ground with g. So along with the force on the coin, its weight 'mg', we shall add a pseudo force '-mg' to it, where m is the mass of the coin. It results in zero force or no force on the coin. So the person in the free-falling elevator will see no movement in the coin. It will remain stable to the point where he dropped it.
4. Is it possible for a particle to describe a curved path if no force acts on it? Does your answer depend on the frame of reference chosen to view the particle?
Answer: The answer actually depends upon the frame of reference chosen. If it is an inertial frame then with no force there will not be a curved path for the particle. But if the frame of reference chosen is non-inertial, a pseudo force -ma will have to be applied to the particle where m is mass of the particle and a is the acceleration of the non-inertial frame. It will give the particle a pseudo acceleration '-a' as viewed from this frame. If the particle was moving with a uniform velocity in a line with some angle to the direction of 'a' with no force in the inertial frame, the path of the particle will appear curved if viewed from the non-inertial frame with acceleration a.
5. You are riding a car. The driver suddenly applies the brakes and you are pushed forward. Who pushed you forward.
Answer: When riding a car our body also moves with the velocity of the car and it does not want to change its state of uniform velocity due to inertia, as Newton's first law says. When the driver suddenly applies brakes the lower part of our body in contact with the car's seat come to rest with the car (due to friction) while the upper part moves forward in inertia. So due to the inertia of our body, our body is pushed forward.
6. It is sometimes heard that the inertial frame of reference is only an ideal concept and no such inertial frame actually exists.
Answer: In an inertial frame, acceleration a of a particle is zero if and only if Net force F on the particle is zero. For example, a book kept on a table has a=0 and sum of all forces on the book is zero, so we call earth an inertial frame but if accurately measured the sum of all forces on the book would not be zero. It will be very close to zero, so for all practical purposes we take the earth as an inertial frame but it is an ideal concept and no such inertial frame exist.
7. An object is placed far away from all the objects that can exert force on it. A frame of reference is constructed by taking the origin and axes fixed in this object. Will the frame be necessarily inertial?
Answer: Since the origin and the axes are fixed in the object, the frame of reference will always move with it. The acceleration of the object with reference to this frame will always be zero. Now the sum of forces on the object will also be zero because no other object can exert force on it. So in this frame of reference always a=0 for F=0. So it is an inertial frame.
8. Figure (5-Q1) shows a light spring balance connected to two blocks of mass 20 kg each. The graduations in the balance measure the tension in the spring. (a) what is the reading of the balance? (b) will the reading change if the balance is heavy, say 2.0 kg? (c) what will happen if the spring is light but the blocks are unequal masses?
The figure for Q. No. - 8
Answer: (a) The reading in the balance is 20 kg.
(b) Yes, if the balance is heavy the reading will change because the tension in the string will change.
(c) When the blocks are of unequal masses, there will be a net force on the system and it will start to move if the pulleys are frictionless. The tension in the string will change and so will be the reading of the balance.
9. The acceleration of a particle is zero as measured from an inertial frame of reference. Can we conclude that no force acts on the particle?
Answer: It can not be concluded that no force acts on the particle. We can only say that net force or the resultant of all forces acting on the particle is zero.
10. Suppose that you are running fast in a field when you suddenly find a snake in front of you. You stop quickly. Which force is responsible for your deceleration?
Answer: It is the force of friction in backward direction between our feet and the ground that decelerates.
11. If you jump barefooted on a hard surface, your legs get injured. But they are not injured if you jump on a soft surface like sand or pillow. Explain.
Answer: On a hard surface our downward velocity quickly comes to zero, means the rate of change of velocity that is acceleration (in the upward direction) is high. It is due to the stronger force exerted by the hard surface on our legs so they get injured. On the other hand, if we jump on the soft surfaces like sand or pillow it takes more time to achieve zero velocity as our legs plunge to a distance. So the rate of change of velocity ie deceleration is lower because the soft surface exert lesser force on our legs and they are not injured.
12. According to Newton's third law, each team pulls the opposite team with equal force in a tug of war. Why then one team wins and other team loses?
Answer: Newton's third law connects the forces exerted by two bodies on one another but these two forces act on two different bodies and they never appear together on one body when it is considered as a system. So these forces will not cancel each other. In a tug of war when we consider a team as a system, the forces acting on it will be pull in the rope, the weight of the team, Normal force by the floor and force of friction between floor and feet. The net of these forces for each team may not be equal that is why one team wins and the other team loses.
13. A spy jumps from an airplane with his parachute. The spy accelerates downward for some time when the parachute opens. The acceleration is suddenly checked and the spy slowly falls on the ground. Explain the action of a parachute in checking the acceleration.
Answer: When the spy jumps the forces acting on him is - 1. His weight in the downward direction and 2. resistance of the air in the upward direction. The magnitude of resistance of air depends upon the area of the object available perpendicular to the direction of motion and velocity of the object. Before opening the parachute, this area is due to his body which is very small, so the air resistance is also very small. So the net downward force is hardly reduced and the spy accelerates downwards. But as the parachute opens, air begins to fill in it and its area gradually increases. In this period air resistance starts to increase and net downward force begins to reduce but it is still not zero that is why for some time he accelerates. But as the parachute opens fully, due to the wide area of the parachute and high velocity, the air resistance in upward direction gets greater than the weight of the spy and not only the acceleration is checked the high velocity achieved begins to decrease (deceleration) due to net upward force. But as the velocity reduces so does the air resistance. And both the forces adjust to become equal and opposite at some slower velocity at which he falls on the ground.
14. Consider a book lying on a table. The weight of the book and the normal force by the table on the book are equal in magnitude and opposite in direction. Is it an example of Newton's third law?
Answer: Yes.
15. Two blocks of unequal masses are tied by a spring. The blocks are pulled stretching the spring slightly and the system is released on a friction-less horizontal platform. Are the forces due to the spring on the two blocks equal and opposite? If yes, is it an example of Newton's third law?
Answer: The forces due to the spring on the two blocks are equal and opposite. Yes, it is an example of Newton's third law.
16. When a train starts, the head of a standing passenger seems to be pushed backward. Analyze the situation from the ground frame. Does it really go backward? Coming back to to the train frame, how do you explain the backward movement of the head on the basis of Newton's law?
Answer: From the ground frame the head of the passenger in the train will look static while his lower part of the body will be moving forward with the train. Because according to the first law of motion, the head does not want to change the state of rest due to inertia.
From the train's frame, the lower part of the passenger moves forward with the frame because it is in contact with the seat and it will look at rest. Since the accelerating frame of the train is a non-inertial frame, so to explain it by Newton's laws of motion a pseudo force opposite to the direction of movement equal to 'ma' will have to be applied on the passenger. The upper part of the passenger not with contact with the seat moves backward due to this pseudo force.
17. A plumb bob is hung from the ceiling of a train compartment. If the train moves with an acceleration 'a' along a straight horizontal track, the string supporting the bob makes an angle tan-1(a/g) with the normal to the ceiling. Suppose the train moves on an inclined straight track with uniform velocity. If the angle of incline is tan-1(a/g), the string still makes the same angle with the normal to the ceiling. Can a person sitting inside the compartment tell by looking at the plumb line whether the train is accelerated on a horizontal straight track or it is going on an incline? If yes, how? If no, suggest a method to do so.
Answer: Since in both of the cases the angle that string makes with normal to the ceiling is same, a person sitting inside the compartment can not tell by looking at the plumb line whether the train is accelerated on a horizontal track or it is going on an incline.
To determine this he should know the weight of the bob and measure the tension in the string with a spring balance. If the tension and the weight of the bob are the same then the train is going with uniform velocity on an incline. If the tension in the string is more than the weight of the bob, the train is moving on a horizontal track with an acceleration.
It can be explained as in the figure below:--
Comparison of both situations
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| Comparison of both situations |
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Links to the Chapters
Links to the Chapters
CHAPTER- 19 - Optical Instruments
CHAPTER- 18 - Geometrical Optics
CHAPTER- 17 - Light Waves
CHAPTER- 16 - Sound Waves
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES -Q-21 TO Q-30
EXERCISES -Q-31 TO Q-40
EXERCISES -Q-41 TO Q-50
EXERCISES -Q-51 TO Q-60
EXERCISES -Q-61 TO Q-70
EXERCISES - Q-71 TO Q-80
EXERCISES - Q-81 TO Q-89
CHAPTER- 15 - Wave Motion and Waves on a String
OBJECTIVE-II
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES - Q-21 TO Q-30
EXERCISES - Q-31 TO Q-40
EXERCISES - Q-41 TO Q-50
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 19 - Optical Instruments
CHAPTER- 16 - Sound Waves
OBJECTIVE-I
OBJECTIVE-II
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES -Q-21 TO Q-30
EXERCISES -Q-31 TO Q-40
EXERCISES -Q-41 TO Q-50
EXERCISES -Q-51 TO Q-60
EXERCISES -Q-61 TO Q-70
EXERCISES - Q-71 TO Q-80
EXERCISES - Q-81 TO Q-89
CHAPTER- 15 - Wave Motion and Waves on a String
EXERCISES - Q-1 TO Q-10
EXERCISES - Q-11 TO Q-20
EXERCISES - Q-21 TO Q-30
EXERCISES - Q-31 TO Q-40
EXERCISES - Q-41 TO Q-50
CHAPTER- 14 - Fluid Mechanics
CHAPTER- 13 - Fluid Mechanics
CHAPTER- 12 - Simple Harmonic Motion
CHAPTER- 11 - Gravitation
CHAPTER- 10 - Rotational Mechanics
CHAPTER- 9 - Center of Mass, Linear Momentum, Collision
CHAPTER- 8 - Work and Energy
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Question for Short Answers
Click here for → OBJECTIVE-I
Click here for → OBJECTIVE-II
Click here for → Exercises (1-10)
Click here for → Exercises (11-20)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
Click here for → Exercises (21-30)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
Click here for → Exercises (31-42)
Click here for → Exercise(43-54)
CHAPTER- 7 - Circular Motion
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CHAPTER- 6 - Friction
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Click here for → EXERCISES (1-10)
Click here for → EXERCISES (11-20)
Click here for → EXERCISES (21-30)
Click here for → OBJECTIVE-I
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CHAPTER- 6 - Friction
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Click here for → Questions for Short Answer
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Click here for → Friction - OBJECTIVE-II
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For more practice on problems on friction solve these- "New Questions on Friction".
Click here for → OBJECTIVE-I
Click here for → Friction - OBJECTIVE-II
Click here for → EXERCISES (1-10)
Click here for → Exercises (11-20)
Click here for → EXERCISES (21-31)
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CHAPTER- 5 - Newton's Laws of Motion
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for → QUESTIONS FOR SHORT ANSWER
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→ Newton's laws of motion - Objective - I
Click here for → Newton's Laws of Motion - Objective -II
Click here for → Newton's Laws of Motion-Exercises(Q. No. 1 to 12)
Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)
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CHAPTER- 4 - The Forces
The Forces-
"Questions for short Answers"
Click here for "The Forces" - OBJECTIVE-I
Click here for "The Forces" - OBJECTIVE-II
Click here for "The Forces" - Exercises
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CHAPTER- 3 - Kinematics - Rest and Motion
Click here for "Questions for short Answers"
Click here for "OBJECTIVE-I"
Click here for EXERCISES (Question number 1 to 10)
Click here for EXERCISES (Question number 11 to 20)
Click here for EXERCISES (Question number 21 to 30)
Click here for EXERCISES (Question number 31 to 40)
Click here for EXERCISES (Question number 41 to 52)
CHAPTER- 2 - "Vector related Problems"
CHAPTER- 2 - "Vector related Problems"
