Speed and acceleration are two words people use interchangeably in everyday conversation. A sports commentator says a sprinter has great acceleration. A driver says their car has a top speed of 150mph. In everyday language this makes sense. In Physics it does not. Speed and acceleration are fundamentally different quantities, and confusing them in an exam will cost you marks that should have been straightforward to get. This guide makes the difference between speed and acceleration impossible to forget.
Speed is how fast an object is moving. It is the distance travelled per unit of time and is measured in metres per second (m/s) or kilometres per hour (km/h). Acceleration is the rate at which speed or velocity changes over time. It is measured in metres per second squared (m/s²). Speed tells you how fast something is moving right now. Acceleration tells you how quickly that speed is changing.
Difference Between Speed and Acceleration: Comparison Table
| Feature | Speed | Acceleration |
|---|---|---|
| Definition | Distance travelled per unit of time | Rate of change of velocity over time |
| What it measures | How fast an object is moving | How quickly speed or velocity is changing |
| Formula | Speed = Distance / Time | Acceleration = Change in velocity / Time |
| Units | m/s or km/h | m/s² |
| Can be zero? | Yes, when stationary | Yes, when moving at constant speed |
| Can be negative? | No, speed is always positive | Yes, negative acceleration means decelerating |
| Scalar or vector? | Scalar (magnitude only) | Vector (magnitude and direction) |
What is Speed?
Speed is a measure of how fast an object is moving. It tells you the distance an object covers in a given amount of time. Speed does not tell you anything about the direction of movement, only the rate of movement. This is why speed is described as a scalar quantity — it has magnitude but no direction.
The formula for speed is straightforward:
Speed = Distance / Time
Or written as a triangle: S = D/T
If a car travels 120 kilometres in 2 hours, its average speed is 60 km/h. If a runner covers 100 metres in 10 seconds, their average speed is 10 m/s.
Key things to remember about speed:
- Speed is always a positive value. You cannot have negative speed
- Average speed is the total distance divided by total time. It does not tell you how fast the object was moving at any particular moment
- Instantaneous speed is the speed at a specific moment in time, like the reading on a car speedometer
- The standard unit for speed in Physics is metres per second (m/s)
- An object moving at constant speed has zero acceleration
What is Acceleration?
Acceleration is the rate at which an object’s velocity changes over time. This is the crucial point that most students miss. Acceleration is not about how fast something is moving. It is about how quickly the speed (or direction of movement) is changing. An object can be moving very fast but have zero acceleration if its speed is not changing.
The formula for acceleration is:
Acceleration = (Final velocity – Initial velocity) / Time
Or: a = (v – u) / t
Where v is final velocity, u is initial velocity, and t is time.
If a car accelerates from 0 m/s to 20 m/s in 4 seconds, its acceleration is (20 – 0) / 4 = 5 m/s².
Key things to remember about acceleration:
- Acceleration can be positive (speeding up) or negative (slowing down). Negative acceleration is also called deceleration
- An object moving at constant speed has zero acceleration
- Acceleration is a vector quantity — direction matters
- Changing direction without changing speed still involves acceleration (because velocity, which includes direction, has changed)
- The unit for acceleration is metres per second squared (m/s²)
Speed vs Velocity: An Important Distinction
To fully understand acceleration, it helps to know the difference between speed and velocity. Speed is scalar — it has magnitude only. Velocity is vector — it has both magnitude and direction.
A car travelling at 60 km/h has a speed of 60 km/h regardless of direction. If it is travelling north at 60 km/h, its velocity is 60 km/h north. If it turns around and travels south at 60 km/h, its speed is still 60 km/h but its velocity has changed.
This matters for acceleration because acceleration is technically the rate of change of velocity, not just speed. This means an object can accelerate even if its speed stays constant, as long as its direction changes. A car going around a roundabout at constant speed is still accelerating because its direction is constantly changing.
Example 1 – A sprinter:
At the start of a 100m race, a sprinter accelerates from 0 m/s to around 10 m/s in the first few seconds. Their acceleration is high. Once they reach top speed and maintain it, their acceleration drops to zero even though they are moving very fast. At the finish line, they decelerate (negative acceleration) as they slow down. Speed and acceleration are both changing throughout the race but at different rates and different times.
Example 2 – A car on a motorway:
A car joining a motorway accelerates from 30 mph to 70 mph in the slip road. Once it reaches 70 mph and maintains that speed, acceleration is zero. The car is moving fast but not accelerating. If it brakes, it decelerates. The speedometer shows speed. The feeling of being pushed back into your seat or forward against your seatbelt is the sensation of acceleration.
Example 3 – A falling object:
When you drop a ball, gravity accelerates it downward at approximately 9.8 m/s² (on Earth). The ball starts with zero speed and gains speed continuously as it falls. After 1 second it is moving at about 9.8 m/s. After 2 seconds it is moving at about 19.6 m/s. The acceleration is constant (9.8 m/s²) but the speed keeps increasing. Acceleration and speed are changing at completely different rates.
Example 4 – A satellite in orbit:
A satellite orbiting the Earth at constant speed is still accelerating. This confuses many students. Because the satellite is constantly changing direction as it follows its curved orbit, its velocity is constantly changing even though its speed remains the same. The gravitational pull of the Earth provides the centripetal acceleration that keeps it in orbit. This is a perfect example of the difference between speed (unchanged) and acceleration (constant, directed towards Earth).
Example 5 – A rollercoaster:
On a rollercoaster, speed and acceleration both change constantly and dramatically. Climbing the first hill, the cars move slowly (low speed) but are decelerating slightly due to gravity working against them. At the top, speed is at its minimum. Descending, speed increases rapidly (positive acceleration). At the bottom of a dip, speed is maximum. Going around a loop, direction changes constantly so acceleration is present even where speed is relatively constant. Rollercoasters are essentially acceleration machines.
Example 6 – Braking distances:
When a driver sees a hazard and brakes, the car decelerates (negative acceleration). The braking distance depends on the initial speed and the rate of deceleration. A car travelling at 30 mph requires far less braking distance than one at 60 mph even if both cars have the same braking force (same deceleration). This is because speed affects the kinetic energy of the car, which must be dissipated to stop. Understanding the difference between speed (how fast you are going) and acceleration (how quickly you are slowing down) is directly relevant to road safety.
The speedometer and the feeling:
Speed = what the speedometer shows. It is the number on the dial right now. How fast are you moving at this moment?
Acceleration = what you feel. When a car accelerates, you feel pushed back into your seat. When it brakes, you feel pulled forward. When it turns, you feel pushed sideways. If you feel nothing, acceleration is zero and speed is constant.
If you can read it on a speedometer, it is speed. If you can feel it in your body as a push or pull, it is acceleration. Zero acceleration means no feeling of force, even if the speedometer shows a high number.
Quick Quiz: Speed or Acceleration?
1. A car is travelling at a constant 60 mph on a straight motorway. Its acceleration is:
2. A ball accelerates from 0 m/s to 15 m/s in 3 seconds. What is its acceleration?
3. A runner covers 400 metres in 50 seconds. What is their average speed?
4. A car slows from 20 m/s to 0 m/s in 5 seconds. Its acceleration is:
5. What are the units of acceleration?
6. A satellite travels around the Earth at constant speed. Is it accelerating?
Difference Between Speed and Acceleration in Exams
The difference between speed and acceleration is tested in every GCSE Physics and Combined Science paper. Questions typically ask you to calculate speed using distance and time, calculate acceleration using change in velocity and time, interpret distance-time and velocity-time graphs, describe the motion of an object from a graph, and explain what is happening physically when acceleration is zero, positive, or negative. Always include units in your answers and show your working clearly.
Common Mistakes to Avoid
Thinking high speed means high acceleration:
A Formula 1 car travelling at 200 mph on a straight has zero acceleration if its speed is not changing. A bicycle starting from rest has positive acceleration even though its speed is very low. Acceleration is about change, not about the current speed. This is the most important distinction to keep in mind.
Forgetting that deceleration is negative acceleration:
When an object slows down, it is experiencing negative acceleration, not “no acceleration.” Students sometimes say an object has no acceleration when it is braking. It does have acceleration — it is just negative. Always use the formula and include the sign in your answer.
Confusing speed with velocity:
Speed has no direction. Velocity has both magnitude and direction. Acceleration is technically the rate of change of velocity, which means changing direction counts as acceleration even if speed stays constant. This distinction matters at GCSE and becomes even more important at A-Level.
Misreading distance-time and velocity-time graphs:
On a distance-time graph, the gradient (slope) represents speed. On a velocity-time graph, the gradient represents acceleration and the area under the graph represents distance. Students frequently mix these up. Always identify what type of graph you are looking at before reading it.
Frequently Asked Questions
Can an object accelerate without changing speed?
Yes. Acceleration is technically the rate of change of velocity, and velocity includes direction as well as speed. If an object changes direction while maintaining the same speed, its velocity has changed and therefore it is accelerating. A car going around a roundabout at constant speed, or a satellite in circular orbit, are both accelerating even though their speed is not changing. This is called centripetal acceleration.
What is the acceleration due to gravity?
On Earth, the acceleration due to gravity is approximately 9.8 m/s², often rounded to 10 m/s² in GCSE calculations. This means that any object in freefall (ignoring air resistance) increases its speed by 9.8 metres per second for every second it falls. After 1 second it is travelling at 9.8 m/s. After 2 seconds it is at 19.6 m/s. The acceleration is constant but the speed keeps increasing as long as the object is falling.
What does a flat line on a velocity-time graph mean?
A flat horizontal line on a velocity-time graph means the object is travelling at constant velocity with zero acceleration. The line shows that velocity is not changing over time. A steep upward slope means high positive acceleration. A steep downward slope means high negative acceleration (deceleration). A line at zero velocity means the object is stationary. Learning to read velocity-time graphs fluently is one of the most valuable skills in GCSE Physics.
What is terminal velocity?
Terminal velocity is the maximum speed a falling object reaches when the drag force (air resistance) acting upward equals the gravitational force acting downward. At terminal velocity, the net force on the object is zero and therefore acceleration is zero. The object continues to fall but at a constant speed. A skydiver reaches terminal velocity of around 55 m/s in a standard position. Opening a parachute increases drag dramatically, reducing terminal velocity to a safe landing speed of around 5 to 6 m/s.
For more Physics help on motion and forces, visit Khan Academy: One-Dimensional Motion.
Speed and acceleration connect closely to other Science topics on this site. Reading about the difference between speed and velocity will give you the complete picture of motion quantities that GCSE Physics requires.
The difference between speed and acceleration is the difference between where you are in your journey and how quickly the journey is changing. Speed is the snapshot. Acceleration is the story of how that snapshot is evolving. Once that distinction is clear in your mind, the difference between speed and acceleration will never cause confusion again, and a whole range of Physics questions that once seemed tricky will start to feel straightforward.
Next time you are in a car, try thinking about the difference between speed and acceleration in real time. The number on the speedometer is your speed. The sensation of being pushed back into your seat or pulled forward is acceleration. When the number stays constant and you feel nothing, acceleration is zero. When the number changes rapidly and you feel a strong push, acceleration is high. Experiencing the difference between speed and acceleration physically like this makes the concepts stick far better than any amount of reading. The difference between speed and acceleration is not just something to understand for an exam. It is something you experience every time you travel.
