Physics · Laws Of Motion · NEET
No, and this is the most common mistake. Static friction is self-adjusting: it exactly matches the applied force so the body stays still. If you push a box with 5 N and it does not move, static friction is exactly 5 N, not the maximum. It only reaches its maximum value (called limiting friction = mu_s N) at the instant the body is about to start sliding. So static friction can be anything from 0 up to mu_s N.
Once a body is already sliding, the tiny surface bumps (called asperities) do not get enough time to interlock and settle into each other. When the body is at rest, these bumps lock together more firmly, so you need a bigger force to break free. That is why the maximum static friction (mu_s N) is a little larger than kinetic friction (mu_k N), meaning mu_s > mu_k. This is why a box suddenly jerks forward once it starts moving.
For NEET, the standard model says NO. Friction depends only on the normal reaction N and the nature of the two surfaces (through mu), not on the apparent area of contact. A brick lying flat and a brick standing on its small end feel the same friction if N and the surfaces are the same. Do not get tricked into using area in a friction formula.
Friction acts along the surface (parallel to it), opposite to the direction of relative motion or the tendency of relative motion. It is not always backward. If a box is on the floor of a car that accelerates forward, static friction on the box points forward, because it is the force pushing the box along with the car. Always ask: which way would the body slide if there were no friction, then friction opposes that.
Limiting friction is the MAXIMUM static friction, its value is mu_s N, and it occurs at the exact moment just before sliding begins. Kinetic (or sliding) friction acts once the body is actually moving and equals mu_k N, which is slightly smaller. So limiting friction is the last value of static friction; kinetic friction takes over the instant motion starts.
Which one of the following statements is incorrect?
Calculate the maximum acceleration of a moving car so that a body lying on the floor of the car remains stationary. The coefficient of static friction between the body and the floor is 0.15 (g = 10 m/s^2).
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Friction is a contact force that acts along the surface between two bodies and opposes their relative sliding. It is why a moving object on the ground slowly stops and why you can walk without slipping.
Static friction (acts when the body is not moving, self-adjusting up to mu_s N), kinetic or sliding friction (acts while the body slides, equal to mu_k N), and rolling friction (acts when a body rolls, and is the smallest of the three).
Maximum static friction (mu_s N) is greater than kinetic friction (mu_k N). So mu_s > mu_k. This is why it is harder to start moving a heavy box than to keep it moving.
Friction depends on the normal reaction N, not directly on mass. On a horizontal surface N = mg, so friction rises with mass. But on an incline or with extra vertical push, N changes, so always use f = mu N, not f = mu mg blindly.
The coefficient of friction mu = f/N is a ratio of two forces, so it is a pure number: it is dimensionless and has no units. It also depends only on the nature of the two surfaces in contact.
