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Forces and Motion - Balanced and Unbalanced Forces

Grade 6IGCSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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A force is a push or a pull acting upon an object, measured in Newtons (NN) using a force meter.

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The Resultant Force (FnetF_{net}) is the single overall force that represents the combined effect of all individual forces acting on an object.

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Balanced Forces occur when the resultant force is 0 N0\text{ N}. In this state, an object at rest stays at rest, and an object in motion continues to move at a constant speed in a straight line.

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Unbalanced Forces occur when the resultant force is not 0 N0\text{ N}. This causes the object to speed up (accelerate), slow down (decelerate), or change its direction.

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Forces act in specific directions; when calculating the resultant force, forces in the same direction are added, while forces in opposite directions are subtracted.

📐Formulae

Fnet=F1+F2 (for forces acting in the same direction)F_{net} = F_{1} + F_{2} \text{ (for forces acting in the same direction)}

Fnet=Flarger−Fsmaller (for forces acting in opposite directions)F_{net} = F_{larger} - F_{smaller} \text{ (for forces acting in opposite directions)}

F=m×a (Force = mass × acceleration)F = m \times a \text{ (Force = mass } \times \text{ acceleration)}

💡Examples

Problem 1:

A box is being pushed to the right with a force of 20 N20\text{ N} and pulled to the left with a force of 5 N5\text{ N}. What is the resultant force and its direction?

Solution:

Fnet=20 N−5 N=15 NF_{net} = 20\text{ N} - 5\text{ N} = 15\text{ N}

Explanation:

Because the forces are acting in opposite directions, we subtract the smaller force from the larger one. The resultant force is 15 N15\text{ N} to the right. Since the resultant force is not zero, the forces are unbalanced.

Problem 2:

An apple sits still on a desk. The Earth pulls it down with a weight of 2 N2\text{ N}. What is the magnitude of the upward force (Normal Force) exerted by the desk on the apple?

Solution:

Fupward=2 NF_{upward} = 2\text{ N}

Explanation:

Since the apple is stationary (at rest), the forces must be balanced. For the resultant force to be 0 N0\text{ N}, the upward force must exactly equal the downward force of 2 N2\text{ N}.

Problem 3:

A cyclist is pedaling with a forward force of 50 N50\text{ N}. If the air resistance and friction combined equal 50 N50\text{ N}, what happens to the cyclist's speed?

Solution:

The speed remains constant (no change).

Explanation:

The forces are balanced because 50 N−50 N=0 N50\text{ N} - 50\text{ N} = 0\text{ N}. According to the laws of motion, if the forces on a moving object are balanced, it will continue to move at a constant speed in the same direction.

Balanced and Unbalanced Forces - Revision Notes & Key Formulas | IGCSE Grade 6 Science