Difference in masses
= $m _2 - m _1$ = $20.17 g -20.15 g$
$=0.02 g$ (correct upto two places of decimal)

A surface molecule experiences a net downward force.

Force will be same for all the surfaces.
Hence, the correct option is $(D)$

The friction force is the force exerted by a surface an object moving across it. It acts in a direction to oppose the relative motion between objects.

Frictional force acts between two surfaces to oppose the relative motion between the surfaces. It acts on the body moving across it or trying to move across it.

Friction is a force that is exerted by a surface on an object that is moving on the surface. So, it is a contact force.

It is the force of friction between the surface of water and the boat that brings the boat to rest once we stop rowing.

It is the force of friction between the surface of the ball and the ground that bring the rolling ball to rest.

A weight put at the center of a metal scale placed between two bricks will bend the scale from the middle.

A spring gets compressed by the application of a force. This is an example of force changing the shape of an object.

Effects of force on an object are
(i) to change the state of motion of the object
(ii) to change the shape and size of the object

When a force is applied on a non-rigid body, its shape changes.

Force can also bring change in dimensions and all the above options are its examples.

The contact forces represent the result of physical contact between two objects ,one by which force is exerted and the other on which force is exerted.

It is common experience that many a time application of force does not result in a change in the state of motion.For example,a heavy box may not move at all even if you apply the maximum force that you can exert.

Weight is gravitational force acting on body.

Swimming is account on the newton's third law of motion since as we push the water we feel equal force by water on us and we move front

Body does not move even if forces are applied, that means body is in equilibrium.

Whenever forces act on an object in opposite directions—like the book on the table—the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal, or balanced, the net force is zero, as it is for the book.

Hence, the net force becomes the differences of the two forces.

If two forces act on an object in the same direction, the net force is equal to the sum of the two forces. This always results in a stronger force than either of the individual forces alone.

Whenever forces act on an object in opposite directions—like the book on the table—the net force is equal to the difference between the two forces. In other words, one force is subtracted from the other to calculate the net force. If the opposing forces are equal, or balanced, the net force is zero, as it is for the book.

Hence, force in the same direction by addition and force in opposite direction by subtraction

A car with streamline body suffers less air air friction while moving and car with flat face will suffer higher air friction.As a result car with with streamline body will require less force from the engine to maintain the same speed

An external force applied can only increase or decrease the speed of a moving object or change its direction.

Application of force only changes the physical state of the substance but not the chemical state (properties). As reactivity of substance with water is a chemical property, thus it will not be effected due to application of force.

Newton's first law of motion states that a body can change its state of rest or of uniform motion only when acted upon by some external force

$ 1kg-wt = 1 \times 9.8 N $ 

The action of a force on an objects can result in:

If the force applied on the object is in the direction of its motion, the force aids the motion of the object and hence the speed of the object increases.

A change in either the speed of an object or its direction of motion or both is described as a change in its state of motion.

On pressing a rubber ball placed on a table,the ball compresses and hence, the shape of the ball changes.

Many a times, application of force does not result in a change in the state of motion. For example, no effect of force in the state of motion is observed when you try to push a wall.

The force of friction acts on moving objects and its direction is opposite to the direction of motion.

Force changes the shape of a non-rigid body.

Force can bring change in dimension.A rubber balloon expands when the molecules of air exert force on its walls.

The impact force or impulse is given by $I=F\times t$  

Net force on an object is directly proportional to the acceleration of the particle which is the rate of change of velocity.

We have impulse $\vec{I}=\vec{F}\times t=\vec{p} _{2}-\vec{p} _{1}$ 

The force can net be seen but we can see the effects of the force. The force is a pull or a push. The effect of forces are like as it causes an object to move, it can change the motion of the body, it can change shape or volume of the body. 

If Body is travelling with constant velocity.

When force is applied in the direction of motion it increases velocity,

While splitting or sneezing you will exhale things out with some momentum by applying force on it and in return you will experience a force in the opposite direction according to the $Newton's\quad Third\quad Law\quad of\quad Motion$.So, in this way you will aquire a velocity on a frictionless surface and you will be able to get off it.

The property of a body by virtue of which it resists any change in its state of rest or of motion in a straight line on its own is called its inertia.

A rider on horse falls back when horse starts running, all of a sudden because the rider is in state of rest initially and later inertia of rest keeps the upper part of body at rest while lower part of the body moves forward with horse.

Newton's First law of motion - "Every body continues to be in its state of rest or of uniform motion in a straight line unless compelled by some external force to act otherwise". 

Force cannot be seen as it is a push or pull. But it causes an object to or can also change the motion of an object.

Three force are in equilibrium if 

$U = 8x^2 - 4x + 400$
Differentiate with $dx$ we will get $F$
$F = 16x - 4$
At equilibrium $F = 0$
$0 = 16x - 4$
$x = 1/4 = 0.25 m$
It’s state of equilibrium will be at $x = 0.25 m$

For the application of translational motion the body must be free to move and after applying force, all the particles in that body move with same velocity.

Both in uniform motion and when the body is at rest the net force on the body is zero or the body is said to be in equilibrium.

Answer is A.

Friction prevents objects from moving or slows them down. It also causes wear on surfaces as they rub against each other and generates heat. Thus energy is wasted in overcoming friction. Hence, the force of friction can slow down an object.
Hence, the statement is true.

Force of friction arises because of interlocking of irregularities on the two surfaces in contact. When a heavy object is placed on the floor, the interlocking of irregularities on the surface of box and floor become strong. This is because the two surfaces in contact are pressed harder. Hence, more force is required to overcome the interlocking. Thus to push the heavier box, Seema has to apply a greater force than Eqbal.

Here, body is moving with constant velocity. 

In the case of falling freely under gravity.

Bodies of mass $m _1$ and $m _2$ acted on by force F (say) for time t. Acceleration in mass $m _1$= $\dfrac{F}{m _1}$
Acceleration in mass $m _2$= $\dfrac{F}{m _2}$
Velocity acquired during this time for mass $m _1$
$v _1=\dfrac{F}{m _1} t$
Velocity acquired during this time for mass $m _2$
$v _2=\dfrac{F}{m _2} t$