TN 10th Science Guide

1. Law of Motion

 I. Choose the correct answer

1) Inertia of a body depends on

a) weight of the object

b) acceleration due to gravity of the planet

c) mass of the object

d) Both a & b

2) Impulse is equals to

a) rate of change of momentum

b) rate of force and time

c) change of momentum

d) rate of change of mass

3) Newton’s III law is applicable

a) for a body is at rest

b) for a body in motion

c) both a & b

d) only for bodies with equal masses

4) Plotting a graph for momentum on the Y-axis and time on X-axis. slope of momentum time graph gives

a) Impulsive force     b) Acceleration

c) Force                     d) Rate of force

5) In which of the following sport the turning of effect of force used

a) swimming              b) tennis

c) cycling                   d) hockey

6) The unit of ‘g’ is ms^-2. It can be also expressed as

a) cms^-1                    b) Nkg^-1

c) Nm²kg^-1                d) cm²s^-2

7) One-kilogram force equals to

a) 9.8 dyne                 b) 9.8 × 10⁴ N

c) 98 × 10⁴dyne       d) 980 dyne

8) The mass of a body is measured on planet Earth as M kg. When it is taken to a planet of radius half that of the Earth then its value will be____kg

a) 4 M         b) 2M       c) M/4         d) M

9) If the Earth shrinks to 50% of its real radius its mass remaining the same, the weight of a body on the Earth will

      a) decrease by 50%     b) increase by 50%

      c) decrease by 25%   d) increase by 300%

10) To project the rockets which of the following principle(s) is /(are) required?

            a) Newton’s third law of motion

            b) Newton’s law of gravitation

            c) law of conservation of linear momentum

            d) both a and c

 

II. Fill in the blanks

1. To produce a displacement ___________is required (force)

2. Passengers lean forward when sudden brake is applied in a moving vehicle. This can be explained by ___________(the inertia of motion)

3. By convention, the clockwise moments are taken as negative and the anticlockwise moments are taken as positive.

4. ___________ is used to change the speed of car.(Acceleration)

5. A man of mass 100 kg has a weight of ___________ at the surface of the Earth(980N)

 

III. State whether the following statements are true or false. Correct the statement if it is false

1. The linear momentum of a system of particles is always conserved.

        Answer:  False.

Correct Statement: The linear momentum of a system of particles is always conserved if no external force acts.

2. Apparent weight of a person is always equal to his actual weight

        Answer:  True.

3. Weight of a body is greater at the equator and less at the polar region.

        Answer:  False.

Correct Statement: Weight of a body is lesser at the equator and greater at the polar region.

4. Turning a nut with a spanner having a short handle is so easy than one with a long handle.

        Answer:    False.
Correct Statement: Turning a nut with a spanner having a short handle is so harder than one with a long handle.

5. There is no gravity in the orbiting space station around the Earth. So the astronauts feel weightlessness.

        Answer:    False.

Correct Statement: There is a gravity in the orbiting space station around the earth. Since space station and astronauts have equal acceleration. Both the astronauts and space station are in the state of weightlessness.

IV. Match the following

    Column I                       Column II

a. Newton’s I law - Stable equilibrium of a body

b. Newton’s II law- Law of force

c. Newton’s III law- Flying nature of bird

d. Law of conservation of Linear momentum - propulsion of a rocket

 

V. Assertion & Reasoning Mark the correct choice as

(a) If both the assertion and the reason are true and the reason is the correct   explanation of assertion.

(b) If both the assertion and the reason are true, but the reason is not the correct explanation of the assertion.

 (c) Assertion is true, but the reason is false.

 (d) Assertion is false, but the reason is true.

1. Assertion: The sum of the clockwise moments is equal to the sum of the anticlockwise moments.

Reason: The principle of conservation of momentum is valid if the external force on the system is zero.

        Answer:
(b) If both the assertion and the reason are true, but the reason is not the correct explanation of the assertion.

 

2. Assertion: The value of ‘g’ decreases as height and depth increases from the surface of the Earth.

Reason: ‘g’ depends on the mass of the object and the Earth.

        Answer:

(c) The assertion is true, but the reason is false.

 

VI. Answer briefly.

1. Define inertia. Give its classification.
Answer:
        Inertia: The inherent property of a body to resist any change in its state of rest or the state of uniform motion, unless it is influenced upon by an external unbalanced force, is known as ‘inertia’.
Types of Inertia

• Inertia of rest
• Inertia of motion
• Inertia of direction

2. Classify the types of force based on their application.
Answer:

1.     Like parallel forces

2.     Unlike parallel forces

3. If a 5 N and a 15 N forces are acting opposite to one another. Find the resultant force and the direction of action of the resultant force.
Solution:
The two forces are unlike parallel forces
Let P = 5N, Q = 15N
Resultant force (R) = P – Q = 5 + (-15) = -10N
                            R = -10N.
The resultant force acting along the direction of “Q”.

4.Differentiate mass and weight.

Answer:

Mass	Weight
The quantity of matter contained in the body	The gravitation force exerted on it due to the Earth’s gravity alone.
Scalar quantity	Vector quantity
Unit: Kg	Unit: N
Constant at all the places	Variable with respect to gravity.

5. Define moment of a couple.

Answer:
Rotating effect of a couple is known as moment of a couple.
Moment of a couple = Force × perpendicular distance between the line of action of forces
M = F × S

6.State the principle of moments.
Answer:
When a number of like or unlike parallel forces act on a rigid body and the body is in equilibrium, then the algebraic sum of the moments in the clockwise direction is equal to the algebraic sum of the moments in the anticlockwise direction.

7.State Newton’s second law.
Answer:
“The force acting on a body is directly proportional to the rate of change of linear momentum of the body and the change in momentum takes place in the direction of the force”.

8.Why a spanner with a long handle is preferred to tighten screws in heavy vehicles?
Answer:
This is because turning effect to tighten the screws depends upon the perpendicular distance of the applied force from the axis of rotation is power arm. Larger the power armless is the force required to turn the screws. So spanner is provided with a long handle.

9. While catching a cricket ball the fielder lowers his hands backwards. Why?
Answer:
(i) When the fielder lowers his hands backwards, he increases the value of time of collision and so retardation is decreased.
(ii) Hence retarding force becomes lesser than before and the palm of the fielder is not hurt very much.

10. How does an astronaut float in a space shuttle?
Answer:
An astronaut float in a space shuttle because both are in the state of weightlessness. Both are experiencing equal acceleration towards earth as free fall bodies. Astronauts are not floating but falling freely.

VII. Solve the given problems

1. Two bodies have a mass ratio of 3:4 The force applied on the bigger mass produces an acceleration of 12 ms-2.What could be the acceleration of the other body, if the same force acts on it.

Answer:

                      Let mass of the body A = 3 m
            Mass of the body B = 4 m
                    Force applied = F
For body A

                         F = mass ×acceleration

                         F = 3m × 12 ms–1

                          F = 36N.

For body B

                           

2. A ball of mass 1 kg moving with a speed of 10 ms-1 rebounds after a perfect elastic collision with the floor. Calculate the change in linear momentum of the ball.

Answer:

3. A mechanic unscrew a nut by applying a force of 140 N with a spanner of length 40 cm. What should be the length of the spanner if a force of 40 N is applied to unscrew the same nut? 

Answer:

Equating the torque in both the cases.

4. The ratio of masses of two planets is 2:3  and the ratio of their radii is 4:7 Find the ratio of their accelerations due to gravity.

Answer:

VIII. Answer in detail.

1.  What are the types of inertia? Give an example for each type.

Answer :

There are 3 types of Inertia. They are;

👉 Inertia at rest : The resistance of a body to change its state of rest is called inertia of rest.

Example : When you vigorously shake the branches of a tree, some of the leaves and 

fruits are detached and they fall down (Inertia of rest).

👉Inertia of motion : The resistance of a body to change its state of motion is called inertia of 

motion.

Example : An athlete runs some distance before jumping because this will help him jump

longer and higher.

👉Inertia of direction : The resistance of a body to change its direction of motion is called inertia of 

direction.

Example : When a bus turn towards right, the passangers are thrown towards left.

2. State Newton’s laws of motion?

Answer:

Newton's First Law : This law states that everybody continues to be in its state of rest (or) the state of uniform motion along a straight line unless it is acted upon by some external force.

Newton's Second Law : According to this law, the force acting on a body is directly proportional to the rate  of change of linear momentum of the body and the change in momentum takes 

place in the direction of the force.

Newton's Third Law : Newton's third law states that for every action, there is an equal and opposite  reaction. They always act on two different bodies.

3. Deduce the equation of a force using Newton’s second law of motion.

Answer:

This law helps us to measure the amount of force. So it is also called as "law of force". Let 'm' be the mass of a moving body, moving along a straight line with an initial speed 'u' after a time interval of  't' the velocity of the body changes to 'v' due to the impact of an unbalanced external force 'F'.

No external force is required to maintain the motion of a body moving with uniform velocity. When the  net force acting on a body is not equal to zero, then definitely the velocity of the body will change.

4. State and prove the law of conservation of linear momentum. There is no change in the linear momentum of a system of bodies as long as no net external force acts on them.

Answer:

Let us prove the law of conservation of linear momentum with the following illustration.

Proof : Let two bodies A and B having masses m₁ and m₂ move with initial velocity u₁ and u₂ in a straight line. Let the velocity of the first body be higher than that of the second body. i.e., u₁ > u₂. During an interval of time 't' second, they tend to have a colliusion. After the impact, both them move along the same straight line with a velocity V₁  and V₂ respectively.

Force on body B due to A

The above equation confirms in the absence of an external force, the algebraic sum of momentum after collision is numerically equal to the algebraic sum of the momentum before collision.

Hence the law of conservation of linear momentum is proved.

5. Describe rocket propulsion.

Answer:

👉 Propulsion of rockets is based on the law of conservation of linear momentum as well as Newton's III law of motion. 

👉Rockets are filled with a fuel in the propellant tank.

👉 When the rocket is fired, this fuel is burnt and a hot gas is ejected with a high speed from the nozzle of the rocket, producing a huge momentum. 

👉 To balance this momentum, an equal and opposite reaction force is produced in the combustion chamber,  which makes the rocket project forward.

👉 While in motion, the mass of the rocket gradually decreases, until the fuel is completely burnt out. 

👉 Since, there is no net external force acting on it, the linear momentum of the system is conserved. 

👉 The mass of the rocket decreases with altitude, which results in the gradual increase in velocity of the  rocket. 

👉 At one stage, it reaches a velocity, which is sufficient to just escape from the gravitational pull of the Earth. This velocity is called escape velocity.

6. State the universal law of gravitation and derive its mathematical expression.

Answer:

This law states that every particle of matter in this universe attracts every other particle with a force. This  force is directly proportional to the product of their masses and inversely proportional to the square of the distance between the centers of these masses. The direction of the force acts along the line joining the masses. 

        Force between the masses is always attractive and it does not depend on the medium where they are  placed.

7. Give the applications of universal law gravitation.

Answer:

👉 Dimensions of the heavenly bodies can be measured using the gravitation law. Mass of the Earth, radius of the Earth, acceleration due to gravity, etc. can be calculated with a higher accuracy.

👉 It helps in discovering new stars and planets.

👉 One of the irregularities in the motion of stars is called 'Wobble' lead to the disturbance in the motion of  a planet nearby. In this condition the mass of the star can be calculated using the law of gravitation.

👉 Helps to explain germination of roots is due to the property of geotropism which is the property of a root responding to the gravity.

👉 Helps to predict the path of the astronomical bodies.

IX. HOT Questions

1. Two blocks of masses 8 kg and 2 kg  respectively lie on a smooth horizontal surface in contact with one other. They are pushed by a horizontally applied force of 15 N. Calculate  the force exerted on the 2 kg mass. 

  

2. A heavy truck and bike are moving with  the same kinetic energy. If the mass of the truck is four times that of the bike, then calculate the ratio of their momenta. (Ratio of momenta = 2:1)

3. “Wearing helmet and fastening the seat belt is highly recommended for safe journey” Justify your answer using Newton’s laws of  motion.

Answer:

1. Wearing a helmet is strongly recommended for safe journey, because when a person fall from bike he exerts a force equal to product of mass of the person and acceleration of the bike (Newton's II law).  According to Newton's III law, in turn the ground offers and equal and opposite force on the persion,  which will porduce large damage. In order to immense damages the person must wear helmet.

2. Fastening of seat belt will not allow a person to move from seat why the vehicle comes to rest  suddenly by applying brake or by having some accidents.

    This is due to inertia of motion. (Newton's I law). When the speeding vehicle stops suddenly the lower part  in contact with the seat stops while the upper part of the body tends to maintain its uniform motion. Hence the person will turn forward and obtain injuries. In-order to avoid this, fastening of seat belt is important.