Revision Note 4: Newton’s Second Law

Newton’s Second Law:

F_net = ma

Here,

·        F_net = the net force (sum of all forces) on the body

·        m = the mass of the body

·        a = the acceleration of the body

When F_net = 0, a = 0 Newton’s second law becomes equivalent to his first law.

Problem Solving Tips:

Tip 4.1:

The following diagram relates the techniques in Revision Notes 2 (Static equilibrium), Revision Note 3 (Kinematics) and Revision Note 4 (Newton’s second law):

Tip 4.2:

When multiple bodies are pulled in a chain (see picture), then Newton’s second law is satisfied by (i) both bodies together, (ii) Body A, (iii) Body B.

In the example below, F = 2N, T = 1N. Where did the extra 1N force go? It goes to feed the acceleration of Body B. The ‘rough’ statement of this:  Acceleration eats Force.

Tip 4.3:

Impulse is defined as F.∆t, and that is equal to change in momentum:

F.∆t = ∆p

Tip 4.4:

These vectors have the same directions: F, ∆p, ∆v, and that direction is unrelated to the directions of v, p. Note that ∆v and v may not have the same direction. In Physics, v is not so important, ∆v is.