Category: 04 Forces

Rotational Equilibrium

In this video, you can see a rigid body, formed by joining two balls, being thrown across the screen. Tracing the trajectory of either ball shows a complicated path. On the other hand, the trajectory of the CM is the familiar parabolic arc of a projectile motion. This shows that an object’s motion can be treated as a translational motion of the CM juxtaposed with a rotational motion about its CM.

3.4.1 Frictional Force

Friction does not oppose motion. Friction opposes relative motion. In fact, when walking, we use friction both to initiate and stop motion.

One useful thing to know about friction is this: the friction between two surfaces increases if the two surfaces are pressed more strongly into each other.

Explanation at

When friction is multiplied thousands of times…

Explanation at Laws of Friction

  • The frictional force between two stationary surfaces is called the static friction,  fs.
  • The frictional force between two sliding surfaces is called the kinetic friction, fk.
  • It can be shown empirically that between two particular surfaces
    • static friction fs ranges from zero to a maximum value.

0 < fs < μsN

    • kinetic friction fk is constant and independent of speed.

fk = μkN 

    • maximum static friction is larger than kinetic friction.

This video shows clearly how the static friction increases until reaching its maximum value, then drops to the constant kinetic friction.

Unlike drag force, friction is unaffected by the sliding speed. It is also unaffected by the area of the surfaces in contact. All that matters is the material of the two surfaces and how hard they are pressed against each other.

Static Equilibrium

Here is the “proof” for the summation of moments is constant regardless of pivot point when net force is zero. This is not required by H2 syllabus. So watch it only if you’re interested.

Below are a few worked examples