Forces and Laws of Motion Test - 21

(a) Block has a tendency to fall towards the ground. Therefore, block exerts a downward force on the string which is equal to its weight i.e. $$15 N$$.
(b) Since, system is balanced and motionless, all the forces will be balanced. Due to weight of the block, an equal and opposite tension force is acting on the string. Therefore, string exerts a force of $$15 N$$ in the upward direction.

One body applies action force on the other body and the second body applies the same amount of reaction force on the first body in the opposite direction.

$$\textbf{Hint}$$ Apply newton's second law of motion.

STEP (1):  SI Unit

 Force acting on the body may affect the shape of the body or it will accelerate or decelerate the body, or it will give motion to the static body or it will stop the motion of the dynamic body.

                                     $$F = ma$$

   Where   $$'F'$$ = force applied,  $$'m'$$ = mass  and   $$'a'$$ = acceleration 

                           Acceleration   $$a = \dfrac vt$$

    Where   $$v$$  = velocity     (in m/s)

    Here   $$'m'$$  is mass  and  $$'s'$$   is  seconds

    Time   $$'t'$$  is in seconds

Therefore  acceleration unit is = $$\dfrac{\dfrac ms}s = \dfrac m{s^2}$$

Hence unit of force  =  $$kg \dfrac m{s^2} =  Newton$$   (in S.I. unit)  i.e, option $$D$$ is a unit of force

Newton can converted into Pound  

that is  1 Newton = 0.22481  pound  ( in FPS system )  i.e, option $$A $$ is unit of force

  $$\textbf{Step2:CGS Unit}$$

   $$F = \dfrac{g.cm}{s^2} = Dyne$$  (in C.G.S. unit)  i.e, option $$ B $$ is a unit of force

Here  $$'g'$$  is grams  , $$'cm'$$  is centimeters  and  $$'s'$$ is  seconds

Conclusion:  Joule is the correct answer because it is the unit of energy.

To lift an object upwards it has to be pulled upwards, vertically.

Answer is A.

According to Newton's third law, the stone exerts an equal and opposite force on the earth.
Since the earth is very massive and the effect of a small force on its motion is negligible. Force is the mutual interaction between two bodies. Forces between two bodies are always equal and opposite which is given by Newton's third law of motion.

A body moves only when a force of certain magnitude in certain direction is applied to it.
The moving body can be stopped when an opposite force of equal magnitude is applied to it.
The mass of a moving tennis ball is less than that of a cricket ball moving at the same velocity. Thus the tennis ball has less momentum than the cricket ball, because of which it is easier to stop a tennis ball than a cricket ball moving with the same velocity. Also it offers more friction. So, it is easier to grip tennis ball.

From newton's IInd law of motion the net force on object is equal to mass times it's acceleration, mass is a scalar quantity, hence the direction of force and acceleration will be same, correct answer is D.

The mass can be described as inertia of object which resist its changes in state of motion and rest. Hence correct answer is B.