Unbalanced Force

When two or more external forces acting on a body produce a net resultant force, i.e, the vector sum of forces is not zero, the forces are unbalanced.

Newton’s Second Law of Motion

Newton’s Second Law of Motion says that the resultant force acting on a body produces a net acceleration and causes the body to accelerate in the direction of the resultant force.

  • If the net resultant force is in the direction of the motion, the body will accelerate (positive acceleration).
  • If the net resultant force is against the direction of motion, the body will decelerate (negative acceleration).

For a body of constant mass, Newton’s Second Law of Motion can be expressed as:

$$F = ma\tag{1}$$

, where

F = force
m = mass
a = acceleration

Since mass (m) is scalar quantity, the direction of acceleration (a) is thus in the same direction as the applied force (F).

To calculate the acceleration of a body, one should use Equation 1, with $F$ being the net resultant force exerted on the body.


All forces observed in nature can be explained in terms of four basic interactions that occur between elementary particles:

  • Gravitational force
  • Electromagnetic force
  • Strong nuclear force
  • Weak nuclear force

Worked Examples

A box of mass 20 kg is pushed with a force of 50 N. What is the acceleration of the box? Neglect the effects of friction.

Show/hide answer

From Newton’s Second Law,

$$\begin{aligned} F &= ma \\ a &= \frac{F}{m} \\ &= \frac{50}{20} \\ &= 2.5 \text{ m s}^{-2} \end{aligned}$$

A car of mass 1000 kg accelerates from rest to $20 \text{ m s}^{-1}$ in 5 s. Calculate the driving force of the engine. (Neglect the effects of friction)

Show/hide answer

Let’s calculate the acceleration first.

$$\begin{aligned} a &= \frac{v-u}{t} \\ &= \frac{20-0}{5} \\ &= 4 \text{ m s}^{-2} \end{aligned}$$

From Newton’s Second Law,

$$\begin{aligned} F &= ma \\ &= 1000 \times 4 \\ &= 4000 \text{ N} \end{aligned}$$

Additional notes on the four fundamental forces stated above:

(Not Important. Just for fun. 🙂 )

  • The four forces are thought to be split off from one super-force.
  • The Universe is thought to be created from a big bang. (The beginning of time and space)
  • The Universe is very hot at the beginning and the particles in the Universe have a lot of energy.
  • As time went by, the Universe cools.
  • With the drop in the particles’ energy, the super-force split into the familiar four forces as observed today.

Back To Forces & Turning Effect Of Forces (O Level)

Back To O Level Topic List

Sharing is caring:
Mini Physics

Administrator of Mini Physics. If you spot any errors or want to suggest improvements, please contact us.

4 thoughts on “Unbalanced Force”

Leave a Comment