Physics · Laws Of Motion · NEET
Tension can only PULL, never push. A string cannot push a body away (a rope goes slack if you try). So tension on any body always acts AWAY from the body, along the string, toward the point where the string is attached or over the pulley. When you draw a free body diagram, the tension arrow on the block points along the string, away from the block.
For a massless (light) string it is. NCERT tells you to 'neglect the mass of the string'. When mass = 0, Newton's second law on any small piece gives T2 - T1 = (mass)(a) = 0, so T1 = T2. That is why one symbol T works for the whole string. If the string had real mass, tension would be larger near the top of a hanging string (it holds more weight below it).
Two reasons for NEET. Massless (light): so tension is the same throughout and the string itself needs no force to accelerate, which keeps the maths clean. Inextensible: it does not stretch, so both connected bodies have the SAME magnitude of acceleration and the SAME speed. These two assumptions are what let you solve pulley and connected-block problems with one value of T and one value of a.
Only when the body is in equilibrium (at rest or moving at constant velocity). Then T = mg. If the body accelerates, T is NOT mg. For a body pulled up with acceleration a, T = m(g + a). For a body going down with acceleration a, T = m(g - a). Always write Newton's second law along the string instead of assuming T = mg.
Both are contact forces, but tension comes from a stretched string and always PULLS along the string. Normal reaction comes from a surface pressing on a body and always PUSHES perpendicular (normal) to the surface. A string pulls; a surface pushes.
One end of a string of length l is connected to a particle of mass m and the other end to a small peg on a smooth horizontal table. If the particle moves in a circle with speed v, the net force on the particle (directed towards the centre) is (T = tension in the string):
Two blocks A and B of masses 3m and m are connected by a massless inextensible string and the whole system hangs from a massless spring. The magnitudes of the accelerations of A and B immediately just after the string is cut are, respectively:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Tension is the pulling force inside a stretched string that it applies on the bodies tied to its ends. It pulls each body inward along the string. Its SI unit is the newton (N).
Because a massless string needs zero force to accelerate. Newton's second law on any piece gives T2 - T1 = mass × a = 0, so the tension has one value everywhere. This only works when we neglect the string's mass, as NCERT does.
Tension is never negative because a string cannot push. It can be zero when the string goes slack (loses its stretch), for example at the top of a vertical circle when the speed drops to the minimum value.
Tension is a contact force. It acts only through the physical string that touches the body. It is not a field force like gravity, which acts at a distance.
For an inextensible (non-stretching) string, both connected bodies share the same acceleration and speed, which is what NEET problems assume. A real stretchy string would allow different motions and store energy, making it far harder to solve.
