Question

Which sentence describes Newton's first law?
A. Forces always come in pairs that have the same magnitude.
B. The action force applied by one object to another is opposite in
direction to the reaction force.
C. It takes a larger net force to accelerate an object at a greater rate.
D. Unbalanced forces cause an object to move.
SM

Answer 1

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Newton's First Law of Motion :

An object stays in rest or in uniform motion unless and until compelled by an external unbalanced force . This is also known as " Law of Inertia " .

Here this statement is best represented by the statement : Unbalanced forces cause an object to move. [ option c ]

More to know :-

Newton's 3 laws of motion :-

Newton's First Law of Motion :-

It states that an object stays in rest or in uniform motion unless and until compelled by an external unbalanced force . This is also known as " Law of Inertia " .

Ex - (i) We tend to fall backwards while standing in the bus , when it starts suddenly.

(ii) We tend to fall forward while standing in the bus , when it stops suddenly.

Newton's Second Law of Motion :-

It states that the rate of change of momentum is directly proportional to the applied force in the direction of the force . That is ∆p $\propto$ Force .

Ex - (i) When we push car & a truck with same force the car will have Greater acceleration due to less mass . (Imaginary case ).

(ii) We feel easy to push a empty cart than a full cart .

Newton's Third Law of Motion :-

It states that every action has equal and opposite reaction .

Ex - (i) We are able to walk on ground due to Newton's third law of motion .

(ii) When we push wall it also applies equal and opposite force .

Answer 2

"Unbalanced forces cause an object to move " is the sentence that describes Newton's first law.

Describe Newton's first law.

   A body remains in the state of rest or uniform motion in a straight line unless and until an external force acts on it.

      The object is to be rest if the velocity and acceleration are zero.  When an object is in motion, velocity is not equal to zero (v ≠ 0) but acceleration (a = 0) is equal to zero. Therefore, the object will continue in motion with constant velocity in the same direction.

Example:

      If we jump from a bus while moving, our body will be like still moving in the same direction of the vehicle. When your feet hit the ground, the grounds act on your feet and stop moving. You will fall because the upper part of your body didn't stop and you will fall in the direction you were moving.

     If the object have the greater mass, then the object will have more inertia. Forces can change the object's motion.

Hence, Option D is the correct answer.

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