Navigation
Learn that acceleration isn't just speeding up; it's any change in velocity.
Have you ever felt pushed back into your seat when a car starts moving, or leaned to the side as it turns a sharp corner? That 'push' you feel is the physical evidence of acceleration in action!
In everyday talk, we use 'acceleration' to mean speeding up. But in physics, acceleration has a much broader meaning: it is the rate at which velocity changes. Remember that velocity is 'speed with a direction' (like km/h North). Because velocity includes direction, acceleration happens whenever an object changes its speed or its path. If you are running in a circle at a perfectly steady speed, you are still accelerating because your direction is constantly changing! We measure acceleration in units like meters per second squared, written as . This tells us how many meters per second your speed changes every single second.
Quick Check
If a race car is traveling at a constant 200 km/h around a circular track, is it accelerating?
Answer
Yes, because its direction is constantly changing, which means its velocity is changing.
Scientists identify three specific ways an object can accelerate. First, speeding up (positive acceleration), like a rocket taking off. Second, slowing down, which is often called deceleration or negative acceleration. Third, changing direction, even if the speed stays exactly the same. Think of a moon orbiting a planet; it stays at a steady speed, but gravity is constantly 'accelerating' it by pulling it into a curve. Any time you change how fast or where you are going, you are accelerating. This is why a steering wheel is just as much an 'accelerator' as the gas pedal!
A sprinter starts from rest ( ) and reaches a speed of in seconds. 1. Identify the change in velocity: . 2. Divide by time: . 3. The acceleration is .
To find acceleration, we use a formula that compares the change in velocity to the time it took for that change to happen. In math terms, we write it as:
Where is the final velocity, is the initial (starting) velocity, and is the time. If the result is a positive number, the object is speeding up. If the result is negative, the object is slowing down. This formula helps engineers design everything from safe elevators to high-speed roller coasters by ensuring the acceleration isn't too 'jerky' for humans to handle.
A cyclist is moving at and sees a red light. They brake and come to a complete stop in seconds. 1. Final velocity () is . 2. Initial velocity () is . 3. Time () is . 4. . 5. The negative sign indicates the cyclist is slowing down (decelerating).
Quick Check
What does a negative acceleration value usually tell you about an object's motion?
Answer
It usually indicates that the object is slowing down (deceleration).
A ball is thrown straight up. At its highest point, its speed is for a split second. Is it accelerating? 1. Even though the speed is , the ball's velocity is about to change from 'upward' to 'downward.' 2. Gravity is pulling on the ball at a constant rate of downward. 3. Because the velocity is in the middle of changing direction, the ball is still accelerating even when its speed is zero!
Which of the following is NOT an example of acceleration?
A skateboarder goes from to in seconds. What is their acceleration?
An object can have a constant speed but still be accelerating.
Review Tomorrow
In 24 hours, try to list the three different ways an object can accelerate without looking at your notes.
Practice Activity
Next time you are in a car or bus, pay attention to when you feel 'pushed' or 'pulled.' Identify if you are speeding up, slowing down, or changing direction.