# If there is an inclined plane with a pulley fixed to it which further has a string block connection, do we account the tension in the string while making the free-body-diagram of the wedge????Give a reason alsoI found a solution to a problem in which it was accounted(from irodov) and another in which it wasnt(from HC VERMA)

Aman Bansal
592 Points
11 years ago

Dear Ritik,

Science originates by observing nature and making inferences from them followed by devising and doing experiments to verify or refute theories. The three laws of motion discovered by Newton govern the motion of every object in nature all the time but due to the presence of friction and air resistance, they are a little difficult to see.

## Newton’s first law is stated as:

“In the absence of external forces, an object at rest remains at rest and an object in motion continues in motion with a constant velocity (that is, with a constant speed in a straight line)”.

Though this is not what we observe everyday. A ball rolling on the floor eventually stops, faster on a sandy floor as compared to a marble floor. This is due to the force of friction present between the ball and the floor. An opposing force in the direction opposite to that of its velocity slows the ball down and eventually brings it to rest. If the ball were rolling on a frictionless floor(ideal case), it would never stop in the absence of external forces.

### Newton’s second law states:

The acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass.

∑F = ma

This law is a little easier to observe as compared to the first law. You can throw a lighter shot put farther than a heavier one even if you put all your energy (or force) in both the cases. This happens because the lighter one gets more acceleration as compared to the heavier one and it is able to cover more distance before falling down. Though there are a lot of other factors like angle of throwing, air drag, etc. which govern the distance covered by the shot put before landing but assuming those factors to be equivalent in both the throws, this should give you some insight that Newton’s second law holds.

### Newton’s third law states:

If two objects interact, the force F12 exerted by object 1 on object 2 is equal in magnitude to and opposite in direction to the force F21 exerted by object 2 on object 1:

F12 = -F21

This law can be understood by considering the following example. When you hit a football with a barefoot, the foot hurts less if you hit it softly and it hurts more if you hit it with greater strength. Thus, the football exerts as much force on your foot as you hit it with. Now, there can be a confusion regarding this rule when you think of all the bodies that the earth is attracting with its force of gravitation. What about the force they exert back on the earth? This is very true, anything that the earth attracts towards itself also attracts the earth towards themselves with a force equal to the magnitude of g (acceleration due to gravity), but the mass of earth is so large as compared to the magnitude of force applied that it effectively remains at rest whereas the body accelerates towards it and falls on the surface of the earth.

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Thanks

Aman Bansal