Balanced & Unbalanced Forces

Definition of Forces

A force is a push or a pull that acts on an object. Forces can change the motion of an object by speeding it up, slowing it down, or changing its direction. Forces are vector quantities, which means they have both a size (magnitude) and a direction.

For example, when you push a door to open it, you apply a force in a certain direction. The size of the force depends on how hard you push.

Balanced Forces

When two or more forces acting on an object are equal in size but act in opposite directions, they are called balanced forces. Balanced forces have a resultant force of zero, meaning they cancel each other out.

If the forces on an object are balanced:

• The object will remain stationary if it was at rest.
• If the object was moving, it will continue to move at the same speed and in the same direction (constant velocity).

This means there is no change in the motion of the object.

For example, a book resting on a table experiences two balanced forces: the downward force of gravity (weight) and the upward force from the table (normal reaction). These forces are equal and opposite, so the book does not move.

Unbalanced Forces

When the forces acting on an object are not equal and opposite, they are called unbalanced forces. This means the resultant force is not zero.

Unbalanced forces cause a change in the motion of an object. This could be:

• Speeding up
• Slowing down
• Changing direction

In other words, unbalanced forces cause the object to accelerate or decelerate.

Examples in everyday life include:

• A car accelerating when the engine force is greater than the friction and air resistance.
• A ball slowing down as it rolls on the ground due to friction.
• A cyclist turning a corner by applying a force that changes direction.

For instance, if a cyclist pedals harder on a flat road, the forward force from the pedals becomes greater than the resistive forces, causing the cyclist to speed up.

Effects of Forces

Forces can have several effects on objects:

• Change speed or direction: Forces can cause an object to speed up, slow down, or change direction (accelerate).
• Cause deformation: Forces can stretch, compress, bend, or twist objects, changing their shape.
• Start or stop motion: Forces can set an object moving from rest or bring a moving object to a stop.

For example, when you squeeze a sponge, the force you apply causes it to deform. When you release it, the sponge returns to its original shape (elastic deformation).

Forces as Vectors and Resultant Force

Since forces have both magnitude and direction, they are vectors. When multiple forces act on an object, they combine to produce a single force called the resultant force.

The resultant force determines the overall effect on the object's motion:

• If the resultant force is zero, the forces are balanced and there is no change in motion.
• If the resultant force is not zero, the forces are unbalanced and the object accelerates.

For example, if two people push a box in opposite directions with equal force, the resultant force is zero and the box does not move. If one person pushes harder, the resultant force is in that direction and the box moves.

For instance, if a box is pushed with 10 N to the right and 6 N to the left, the resultant force is:

$\text{Resultant force} = 10\,\text{N} - 6\,\text{N} = 4\,\text{N}$ to the right

This unbalanced force will cause the box to accelerate to the right.

For example, if a box is pushed with 8 N to the left and 5 N to the right, the resultant force is $8\,\text{N} - 5\,\text{N} = 3\,\text{N}$ to the left, causing the box to accelerate leftwards.