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Contact forces and non-contact forces - class-VIII

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Consider two conducting plates $A$ and $B$, between which the potential difference is $5 V$, plate $A$ being at a higher potential. A proton and an electron are released at plates $A$ and $B$ respectively. The two particles then move towards the opposite plates - the proton to plate $B$ and the electron to plate $A$. Which one will have a larger velocity when they reach their respective destination plates?

  1. Both will have the same velocity

  2. The electron will have the larger velocity

  3. The proton will have the larger velocity

  4. None will be able to reach the destination point


Correct Option: B
Explanation:

Between two conducting plates, there is the uniform electric field and therefore the same charge is applicable on both proton and neutron. Since Proton mass is more than that of the electron, it will have less acceleration and hence less speed achieved by it. Therefore electron will reach plate with larger velocity.

$F _g$ and $F _e$ represent gravitational and electrostatic force respectively between electrons situated at a distance $0.1\,m. F _g/F _e$ is of the order

  1. $10^{-41}$

  2. $10^{-45}$

  3. $10^{40}$

  4. $10^{-42}$


Correct Option: D
Explanation:
${ F } _{ y }$ and ${ F } _{ e }$ represent gravitational and electrostatic force respectively between electrons situated at a distance $=0.1m$
let, us take the ratio
$\dfrac { { F } _{ e } }{ { F } _{ g } } =\dfrac { \dfrac { 1 }{ 4\pi { \epsilon  } _{ 0 } } \dfrac { { q }^{ 2 } }{ { d }^{ 2 } }  }{ G\dfrac { { m }^{ 2 } }{ { d }^{ 2 } }  } $
$\dfrac { { F } _{ e } }{ { F } _{ g } } =\dfrac { 1 }{ 4\pi { \epsilon  } _{ 0 } } \times \dfrac { 1 }{ G } \times \dfrac { { q }^{ 2 } }{ { m }^{ 2 } } $
$\dfrac { { F } _{ e } }{ { F } _{ g } } =9\times { 10 }^{ 9 }\times \dfrac { 1 }{ 6.67\times { 10 }^{ -11 } } \times \dfrac { { \left( 1.6\times { 10 }^{ -19 } \right)  }^{ 2 } }{ { \left( 9.1\times { 10 }^{ -31 } \right)  }^{ 2 } } $
$\Rightarrow \dfrac { { F } _{ e } }{ { F } _{ g } } =4.17\times { 10 }^{ 42 }$
or,  $\dfrac { { F } _{ e } }{ { F } _{ g } } $ is of the order of ${ 10 }^{ -42 }$.

Four point charge of $+10^{-7}C, -10^{-7}C, -2\times 10^{-7}C$ and $+2\times 10^{-7}C$ are placed respectively at the corners $A, B, C, D$ of a $0.05\ m$ square. Find the magnitude of the resultant force on the charge at $D$.

  1. $0.2\ dyne$

  2. $0.2\ newton$

  3. $2\ dyne$

  4. $0.02\ newton$


Correct Option: A

An $\alpha$-particle is the nucleus of a helium atom. It has a mass $m=6.64 \times 10^{-27}$kg and a charge $q = + 2e =  3.2 \times 10^{-19}$ C. Compare the force of the electric repulsion between two a-particles with the force of gravitational attraction between them.

  1. $\displaystyle \dfrac{F _e}{F _g} = 3.1 \times 10^{32}$

  2. $\displaystyle \dfrac{F _e}{F _g} = 3.1 \times 10^{35}$

  3. $\displaystyle \dfrac{F _e}{F _g} = 3.1 \times 10^{38}$

  4. $\displaystyle \dfrac{F _e}{F _g} = 1 \times 10^{32}$


Correct Option: B
Explanation:

Electrostatic repulsion = $F _e=\dfrac{1}{4 \pi \epsilon } \dfrac{q^2}{r^2 }=\dfrac{1}{4 \pi \epsilon } \dfrac{4e^2}{r^2 } $

gravitational attraction = $ F _g = G\dfrac{m^2}{r^2} $

$\dfrac{F _e}{F _g} = \dfrac{1}{4 \pi \epsilon } \dfrac{4 e^2}{G m^2} = 3.1 \times 10^{35} $

Electrostatic force and gravitational force differ in which respect?

  1. Conservative force

  2. Central force

  3. Principle of superposition

  4. Dependence on the intervening medium


Correct Option: D
Explanation:

In electrostatic force, the force of medium depend on charges while the force of medium due to gravity does not depend on masses. There is no concept of induced mass.

The ratio of electric force between two electrons to the gravitational force between them is of the order

  1. 10$^{42}$

  2. 10$^{39}$

  3. 10$^{36}$

  4. 1


Correct Option: A
Explanation:

The electric force between two electrons is $F _e=\dfrac{ke^2}{r^2}$

and gravitational force between two electrons is $F _g=\dfrac{Gm _e^2}{r^2}$
Thus, $\dfrac{F _e}{F _g}=\dfrac{ke^2}{Gm _e^2}=\dfrac{(9\times 10^9)(1.6\times 10^{-19})^2}{(6.67\times 10^{-11})(9.1\times 10^{-31})^2}=4.17\times 10^{42} \sim 10^{42}$

During winter, clothes tend to stick to our body as an action of ______ force.

  1. electrostatic 

  2. muscular

  3. magnetic

  4. contact


Correct Option: A
Explanation:

Due to electrostatic force clothes tends to stick to our body. 

When an object is rubbed with some materials, it acquires charges and becomes ________ object.

  1. uncharged

  2. charged

  3. neutral

  4. heavy


Correct Option: B
Explanation:

When object is rubbed with some materials it acquires charges. This object is called as charged object and develops electrostatic force.

Gravitational force is a/an ______ force.

  1. repulsive

  2. attractive

  3. contact

  4. neutral


Correct Option: B
Explanation:

Gravitational force is an attractive force as it tends to move all objects towards itself.

Under action of which force does water flow from an open tap towards ground?

  1. Electrostatic force

  2. Magnetic force

  3. Gravitational force

  4. None of the above


Correct Option: C
Explanation:

Objects tend to move downwards to the earth because of gravity of the earth. This force, exerted on the object is known as force of gravity. 

Does moon have its own gravity?

  1. No

  2. Yes

  3. Not known yet

  4. Only during full moon


Correct Option: B
Explanation:

Gravity is not a property of the earth alone. Every object in the universe exerts a force on every other object. This is called gravitational force. Naturally, moon also has its own gravity.

Which of the following is the weakest force?

  1. Nuclear force

  2. Gravitational force

  3. Electromagnetic force

  4. None of these


Correct Option: B
Explanation:

Answer is B.

Gravity is a very, very weak force. In a hydrogen molecule it pulls the two protons together with a force about 36 powers of ten weaker than the electric force between them.
Therefore, on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the sun by gravity. 
Hence, among the given options, gravitational force is the weakest.

There are circumstances in which two objects exert forces on one another even though they are not touching. Such forces are called (Mark the most appropriate answer)

  1. non-contact forces

  2. contact forces

  3. electric forces

  4. attractive forces


Correct Option: A
Explanation:

A non-contact force is a force applied to an object by another body that is not in direct contact with it. The most familiar example of a non-contact force is weight.

It is easier to drive a screw rather than a nail of the same size into a wall because:

  1. The nail moves a larger distance for the same force applied

  2. The screw moves a larger distance for the same force applied

  3. The direction of force applied in the screw is perpendicular to its movement

  4. The screw acts as a wedge


Correct Option: C
Explanation:

In screws the direction of force applied is perpendicular to the wall and due to its cuts present their is a force applied tangential to the wall which helps in easy movement of the screw.

Which of the following is a non contact force?

  1. muscular

  2. electrostatic attraction

  3. elastic spring force

  4. friction force


Correct Option: B
Explanation:

Coulomb electrostatic force of attraction acts between the two bodies which are separated by a finite distance, hence it is non contact force since the two bodies need not to be in contact with each other whereas the rest of the forces are contact forces.

Let E,G and N represents the magnitude of electromagnetic, gravitational and nuclear forces between protons and electrons at a given separation (1 fermi meter). Then

  1. N < E < G

  2. E > N > G

  3. G > N > E

  4. N > E > G


Correct Option: D
Explanation:

$r=1fermimeter$ which is same for all.
$F _e=\frac{k _eq _1q _2}{r^2}  ;F _g=\frac{Gm _1m _2}{r^2}$ ; 
$F _N=\frac{\alpha}{r^2}$
where$k _e=8.988\times10^9 N^2m^2c^{2} $;
$q _1=-1.6\times10^{-19}c$; 
$q _2=1.6\times10^{19}$;
$G=6.67\times10^{-11}m^3kg^{-1}s^2$;
$m _1=9.1\times10^{-31}kg$; 
$m _2=1.67\times10^{-27}kg$;
$\alpha=2.3103\times10^{-28}$
$F _e=2.307\times10^{-28}$  ; ${F _g}=1.868 \times10^{-64}$ ;
$F _N=2.3103 \times10^{-28}$
hence,$F _N>F _e>F _g$


Gravitational force acts

  1. only between the sun and the planets moving around it.

  2. only between the earth and the bodies on it.

  3. between all bodies in the universe.

  4. only between the Sun and the earth.


Correct Option: C
Explanation:

Answer is C.

The force of gravity is the force with which the earth, moon, or other massively large object attracts another object towards itself. Gravitational force exists between all the bodies having mass in the universe.

The force of gravity is a ______ force

  1. non contact

  2. contact force

  3. body force

  4. surface force


Correct Option: A
Explanation:

Gravitational force acts at a distance. Therefore it is a non-contact force.

When a force is exerted on an object without any contact, the force is known as ____

  1. contact force

  2. non-contact force

  3. muscular force

  4. none of the above


Correct Option: B
Explanation:

A force applied on an object without any contact it is non-contact force, Examples are magnetic force, electrostatic force etc.

Which of the following is NOT a non-contact force?

  1. Magnetic force

  2. Electrostatic force

  3. Gravitational force

  4. Muscular force


Correct Option: D
Explanation:

Magnetic force and electrostatic forces are non-contact force as the force applied on an object without any contact with it, similarly, with gravitational force this is force applied without being aware of it and all time.

Muscular force can be applied only while in contact with the object. So, it is a contact force, not a non-contact force.

Unlike poles of two magnets ______ each other.

  1. attract

  2. repel

  3. neutralize

  4. exert no force on


Correct Option: A
Explanation:

Unlike poles of the two magnets attracts each other. the force applied by it is non-contact forces and magnetic force.

Like poles of two magnets _____ each other.

  1. attract

  2. repel

  3. exert no force on

  4. neutralize


Correct Option: B
Explanation:

Like poles of two magnets repel each other, The force between them is non contact force and magnetic force

When a ball is hit by a bat, forces acting on the ball are

  1. muscular force

  2. gravitational force

  3. both A and B

  4. neither A nor B


Correct Option: C
Explanation:

Being hit by a bat, muscular force acts on the  ball. And, all the bodies on the earth are acted upon by  the gravitational force. So, the ball is acted upon by both muscular and gravitational force.

Gravity is shown by

  1. earth only

  2. earth and moon

  3. earth and some selected planets

  4. all objects in the universe


Correct Option: D
Explanation:

Gravity is not a property of the earth alone. Every object in the universe exerts a force on every other object. This force is known as the gravitational force.

Attraction or repulsion between two objects can also be seen us another form of ____

  1. pull or push

  2. pull only

  3. push only

  4. none of the above.


Correct Option: A
Explanation:

The attraction or repulsion can also be referred as pull or push force between two objects.

The force exerted by a charged body on another charged or uncharged body is known as ______ force.

  1. magnetic

  2. electrostatic

  3. gravitational

  4. friction


Correct Option: B
Explanation:

Electrostatic force is a exerted by a charged body on another changed or uncharged body.

Gravitational force is a _____ force. 

  1. contact

  2. non-contact

  3. magnetic

  4. electrostatic


Correct Option: B
Explanation:

Gravitational force is a non-contact force. For action of gravitational force, contact with an object is not required. The force of gravity acts on all object at all the time even while not in contact with the earth.

Attraction and repulsion between charged objects are examples of _______ force.

  1. non-contact

  2. contact

  3. either contact or non-contact

  4. neither contact nor non-contact


Correct Option: A
Explanation:

The attraction and repulsion between charged objects happen due to the action of electrostatic forces. Electrostatic force act between bodies not in contact. So, attraction and repulsion between charged objects are examples of non-contact force.

When you ride a cycle, force(s) acting on the cycle is/are

  1. muscular force

  2. force of friction

  3. gravitational force

  4. all of the above


Correct Option: D
Explanation:

To ride a cycle, you are to apply muscular force on the pedals of the cycle. The tyres of  the moving cycle are acted upon by the force of friction from the road. And, all objects on the earth are acted upon by the gravitational force.

Objects fall towards the earth because the earth pulls those. This pull is called _________ force.

  1. contact

  2. muscular

  3. gravitational

  4. electrostatic


Correct Option: C
Explanation:

Objects tend to fall downward towards the earth as the earth pulls the objects due to its gravity. The pull is known as gravitational force.

Electrostatic force is also a ____ force.

  1. contact

  2. muscular

  3. non-contact

  4. gravitational


Correct Option: C
Explanation:

In electrostatic force, force is exerted by an object on another object not in contact.

Which of the following is not a contact force?

  1. Tension force

  2. Normal force

  3. Air resistance

  4. None


Correct Option: D
Explanation:

Tension force, normal force and air resistance force act on the objects which are in physical contact (touch) with each other and thus all these forces are often called as contact forces.

Hence option D is correct.

Magnetic force is a non contact force because for magnetic force to apply

  1. the bodies have to be in contact.

  2. the bodies need not be in contact.

  3. the bodies may or may not be in contact.

  4. none of these.


Correct Option: B
Explanation:

Magnetic force, electric force and gravitational forces are non contact forces because for these forces to apply, the bodies need not be in contact (physically touch) with each other.

Which of the following is a non-contact force?

  1. Gravitational force

  2. Magnetic forces

  3. Electric force

  4. All


Correct Option: D
Explanation:

It is not necessary for the two objects to be in contact (touch) with each other to exert gravitation force, magnetic force and electric force. These three forces can be exerted even if the two objects are separated by a finite distance. Thus these forces are non-contact forces.

Fruits detached from a tree fall down due to force of gravity. We know that a force arises due to interaction between two objects. Name the objects interacting in this case.

  1. Earth and tree

  2. Earth and fruit

  3. Earth and branch

  4. None of the above


Correct Option: B
Explanation:

Earth and Fruits.
Here Force of gravity acts between the fruits and earth.
And due to this the fruit detached from the tree falls to the ground due to Force of gravity.

State whether true or false:

A boy throws a ball down from the second floor of a building. In this case, electrostatic force is acting on the ball.

  1. True

  2. False


Correct Option: B
Explanation:

When boy throws a ball down only gravitational force will act, as electrostatic force act between two charge particle , so there is only gravitational force that is acting on ball. 

Which one of the following forces is a contact force?

  1. Force of gravity

  2. Force of friction

  3. Magnetic force

  4. Electrostatic force


Correct Option: B
Explanation:

Friction is a surface phenomenon. When two surfaces have irregularities that come in contact do they get interlocked  preventing them against sliding against each other. So, frictional force is a contact force dependent on surface.
Here, option (B) is correct. 
Gravitational force, electrostatic force and magnetic force does not depend on the surface.

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