Introduction
Physics is the science that explains how the physical world around us behaves. It deals with motion, forces, energy, and matter — the very foundations of how things move and interact. Whether it’s a car driving on a road, a ball being thrown, or a satellite orbiting Earth, the principles of physics are at play everywhere.
This article provides a clear overview of fundamental concepts such as distance, displacement, speed, velocity, acceleration, projectile motion, and Newton’s Laws of Motion — essential for understanding how motion works in our everyday world
1. Distance and Displacement
Distance
Distance refers to how much ground an object has covered during its motion. It only measures the total path travelled, regardless of direction.
- Example: If John walks 8 metres east and then 3 metres west, the total distance is 8 + 3 = 11 metres.
- Distance is a scalar quantity, meaning it has only magnitude (no direction). It is always positive.
Displacement
Displacement, however, considers direction. It is the shortest straight-line distance between the starting and ending points.
- Using the same example, John’s displacement is 5 metres east, because he ended up 5 metres away from where he started.
- Displacement is a vector quantity, meaning it has both magnitude and direction.
- It can be positive, negative, or zero depending on direction.
Key difference:
- Distance = total path travelled.
- Displacement = change in position (final position – initial position).
2. Speed
Speed tells us how fast an object is moving, regardless of direction.
If a car moves at 30 metres per second (m/s), it means that every second it covers a distance of 30 metres.
The basic formula for speed is:
Speed (v)=Distance (d)Time (t)\text{Speed (v)} = \frac{\text{Distance (d)}}{\text{Time (t)}}Speed (v)=Time (t)Distance (d)
So, if a car travels 1,000 metres at a speed of 50 m/s, the time taken is:
t=100050=20 secondst = \frac{1000}{50} = 20\text{ seconds}t=501000=20 seconds
Speed is always positive and is measured in m/s, km/h, or mph.
3. Speed vs Velocity
While both describe how fast something is moving, velocity also includes direction.
- Speed is scalar (only magnitude).
- Velocity is vector (magnitude + direction).
For example:
- A train moving at 45 m/s → describes speed.
- A train moving at 45 m/s west → describes velocity.
Velocity can be positive or negative depending on the direction.
If an object reverses direction, its velocity becomes negative, but its speed remains positive.
Relationship:
Speed=∣Velocity∣\text{Speed} = |\text{Velocity}|Speed=∣Velocity∣
4. Average Speed and Average Velocity
Sometimes, an object changes its motion over time. To describe its overall motion, we use average values.
Average Speed
Average Speed=Total DistanceTotal Time\text{Average Speed} = \frac{\text{Total Distance}}{\text{Total Time}}Average Speed=Total TimeTotal Distance
Average Velocity
Average Velocity=Total DisplacementTotal Time\text{Average Velocity} = \frac{\text{Total Displacement}}{\text{Total Time}}Average Velocity=Total TimeTotal Displacement
Example:
An object moves 12 m east and then 20 m west in 4 seconds.
- Total distance = 32 m
- Displacement = 12 − 20 = −8 m
So,
- Average speed = 32 ÷ 4 = 8 m/s
Average velocity = −8 ÷ 4 = −2 m/s (negative because it moved west)
5. Acceleration
Acceleration measures how quickly velocity changes over time.
If a sports car goes from 0 to 60 mph in 5 seconds, while a truck takes 30 seconds, the car has a higher acceleration.
Formula:
a=vf−vita = \frac{v_f – v_i}{t}a=tvf−vi
Where:
- aaa = acceleration
- vfv_fvf = final velocity
- viv_ivi = initial velocity
- ttt = time
Example:
- A truck accelerates from 0 to 60 mph in 30 s → a=2 mph/sa = 2 \text{ mph/s}a=2 mph/s
- A car accelerates from 0 to 60 mph in 5 s → a=12 mph/sa = 12 \text{ mph/s}a=12 mph/s
If acceleration and velocity have the same sign, the object is speeding up.
If they have opposite signs, the object is slowing down.
Formula for Final Velocity
vf=vi+atv_f = v_i + a tvf=vi+at
6. Gravitational Acceleration
On Earth, all objects near the surface experience a constant downward acceleration due to gravity, represented by:
g=−9.8 m/s2g = -9.8 \, \text{m/s}^2g=−9.8m/s2
The negative sign indicates that gravity acts downward (towards the Earth).
- On the Moon, g=−1.6 m/s2g = -1.6 \, \text{m/s}^2g=−1.6m/s2, which is why objects fall slower and people can jump higher.
Examples:
- A ball dropped from rest: velocity becomes more negative (downward) every second.
- A ball thrown upward: velocity decreases until it reaches zero at its highest point, then becomes negative as it falls back down.
7. Projectile Motion
Projectile motion occurs when an object moves under the influence of gravity in both the x (horizontal) and y (vertical) directions.
Example:
A ball kicked off a cliff follows a curved trajectory because gravity pulls it down while it moves forward.
- Horizontal velocity (Vx): remains constant (no horizontal acceleration).
- Vertical velocity (Vy): changes due to gravity.
ax=0,ay=−9.8 m/s2a_x = 0, \quad a_y = -9.8 \, \text{m/s}^2ax=0,ay=−9.8m/s2
At any given moment:
- VxV_xVx = constant
- VyV_yVy = decreases by 9.8 m/s every second (downward)
When the ball rises and falls symmetrically, its speed at the same height on both sides of the trajectory is equal, although the direction of VyV_yVy changes.
8. Newton’s Three Laws of Motion
First Law (Law of Inertia)
An object at rest stays at rest, and an object in motion continues moving in a straight line at constant speed, unless acted on by an external force.
- Example: A hockey puck gliding on ice keeps sliding because friction is minimal.
- If friction were completely absent, it would move forever.
Second Law (Force and Acceleration)
The acceleration of an object depends on its mass and the net force applied:
F=maF = m aF=ma
Where:
- FFF = Force (in Newtons)
- mmm = Mass (in kilograms)
- aaa = Acceleration (in m/s²)
Example:
A 10 kg box is pushed with a force of 80 N:
a=Fm=8010=8 m/s2a = \frac{F}{m} = \frac{80}{10} = 8 \, \text{m/s}^2a=mF=1080=8m/s2
This means its velocity increases by 8 m/s every second.
Third Law (Action and Reaction)
For every action, there is an equal and opposite reaction.
When you push on a wall, the wall pushes back with equal force in the opposite direction.
When a rocket expels gas downward, it propels upward with equal force.
Conclusion
Physics helps us understand motion, energy, and the forces that govern them.
From simple quantities like distance and speed to advanced principles like projectile motion and Newton’s laws, these fundamentals explain nearly everything about how objects move.
Whether calculating how long it takes a car to reach a destination, understanding why a ball curves in midair, or exploring space travel — these core ideas form the basis of all motion and mechanics.
Physics, in its simplest form, is the study of how things move — and why they move the way they do.
