A force is a push or a pull acting on an object. It is measured in newtons (N).
Force can start or stop motion. (Motion)
Force can change the speed or direction of a moving object. (Acceleration and Direction)
Force can also change the shape (stretch, compress, bend, or deform an object.) of an object (like squeezing clay). (Deformation)
Force has both magnitude (how strong it is) and direction. (It is a vector)
Effects of Force
a) Motion - When a force is applied to an object, it can cause the object to move in the direction of the force.
This is explained by Newton’s First Law of Motion, which states: - An object at rest will stay at rest, and an object in motion will stay in
motion at constant velocity unless acted upon by an external force.
So:
A stationary object starts to move when a force is applied.
A moving object can change its speed or direction when acted upon by a force.
b) Acceleration
Force can also cause acceleration, which means a change in velocity over time.
This is explained by Newton’s Second Law of Motion, which states:
Acceleration is directly proportional to the force applied and inversely proportional to the object's mass.
So:
More force = greater acceleration
More mass = less acceleration (for the same amount of force)
c) Deformation - A force can also cause an object to change its shape, which is called deformation.
This is explained by Hooke’s Law, which says: The amount an object deforms is directly proportional to the force applied (within elastic
limits).
So:
The stronger the force, the greater the deformation.
d) Friction
Friction is the force that opposes motion between two surfaces in contact.
When a force is applied to move an object:
Friction can prevent motion or slow it down.
Understanding friction is very important in engineering and design, because it helps ensure that machines and structures function safely
and efficiently.
Types of force
We can classify forces into two main types: Contact Forces and Non-contact Forces
Type Force Name Push / Pull Example
Contact Frictional Force Push Sandpaper rubbing wood
Normal Force Push Book on a table / Cat on the ground
Tension Force Pull Tug-of-war rope / Hanging swing
Muscular force Push/Pull Lifting a school bag, pushing a swing, pedaling a bicycle.
Applied Force Push or Pull Pulling a suitcase / Pushing a cart
Air Resistance Push Air pushing against a flying bird, aeroplane
Spring Force Push or Pull Spring toy stretching (pull) or compressing (push)
Non-contact Gravitational Pull Mango falling from tree
Force
Magnetic Force Pull or Push Magnet attracting or repelling objects
Electrostatic Force Pull or Push Balloon attracting hair / Charged particles
a) Contact Forces - Contact forces happen when two objects physically touch each other.
Require touch to be applied.
Act in the direction in which the force is applied.
Can change the shape, size, or motion of an object.
1. Muscular Force - Force applied using the muscles of our body (humans or animals).
2. Frictional Force - Happens when two surfaces rub against each other, It opposes motion.
3. Normal Force - A force that a surface applies to support the weight of an object resting on it.
4. Tension Force - Force transmitted through a string, rope, or wire when it is pulled from both ends.
5. Applied Force - A direct push or pull by a person or object.
6. Air Resistance - A type of frictional force acting against something moving through the air.
7. Spring Force - Force from a compressed or stretched spring.
If stretched → Attractive force
If compressed → Repulsive force
b) Non-Contact Forces - These forces act without touching the object.
Do not require contact to act.
Act from a distance.
Can cause movement or change in direction without touching.
1. Gravitational Force - A force that pulls everything toward the Earth (or any mass).
2. Magnetic Force - A force of attraction or repulsion between magnets and magnetic materials.
3. Electrostatic (Electrical) Force - The force between charged objects.
There are several types, including: applied (from person or machine), tension (from string or cable), gravity (between masses), normal (perpendicular to
surface), friction (moving across a surface), thrust (from pushing something the other way), drag (resistance from air or other fluid), buoyancy (from
displacing a fluid), and lift (on wings moving through air).
Balanced vs. Unbalanced Forces – Comparison Table
Feature Balanced Force Unbalanced Force
Definition When two or more forces acting on an object are equal in When two or more forces acting on an object are unequal
size and opposite in direction. in size, or not directly opposite.
Net Force Zero Not zero
Effect on No change in motion. Object stays at rest or moves with Causes motion: object may start moving, stop, speed up,
Motion constant speed in the same direction. slow down, or change direction.
Acceleration No acceleration Causes acceleration
Example 1 A book resting on a table (gravity down = normal force up) Pushing a stationary toy car so it starts to move
Example 2 Two people pushing a box with equal force from opposite Two people pushing a box from the same side with different
sides force
