Acceleration: Examples

Have you noticed something else? ... About acceleration, that is. It's not only when the bus driver presses the accelerator or the brake pedal that the bus accelerates - and you need to hold on. You must have noticed that when a bus turns - changes direction - you lean - with all the other passengers - and you're pushed to one side, even if the bus keeps a steady pace. Because the bus is actually accelerating when it's turning, too.

The bus can accelerate without changing its speed! Check this out! This bus is driving straight ahead at a speed of 12 meters per second. And we're going to be picky now. The speed is 12 meters per second.

But the velocity not only tells how fast it's going, but also the direction in which the bus is moving. If only one of these changes, the velocity changes, and you experience acceleration. The bus keeps a constant speed, but then makes a sharp turn to the left. If you're standing up in the bus you'll have to hold on, to keep from falling. And that is a sure sign the bus is accelerating.

The harder you have to hold on, the stronger is the acceleration. Acceleration has, just like velocity, a direction. The acceleration of the bus is in the opposite direction to where you are about to fall. When the bus is accelerating forwards, you are pushed backwards. When the bus brakes - and has a negative acceleration - then you move forwards.

If you are pushed to the right inside the bus, that's because the bus is accelerating to the left. We illustrate velocity like this, with an arrow. But this isn't any old arrow. It's a vector. The bus is moving in the direction of the vector.

The longer the vector, the faster the bus is moving. That was the velocity vector. But there's another vector here, and this vector represents the acceleration of the bus: that is, the rate of change in velocity. And right now, the acceleration is pointing sideways. That means the direction of the bus is changing.

As long as the bus is changing direction, it is accelerating. Now the bus has finished turning, and it's no longer accelerating. The acceleration vector has disappeared. The velocity vector has changed direction. And now the bus is braking, and slows down.

The acceleration vector points backwards, and the velocity vector gets shorter. At a constant speed, in the same direction, the bus isn't accelerating at all. All of this about acceleration applies to more than buses, of course. The moon travels around the Earth in an orbit, and moves at a constant speed. or about one thousand metres per second.

One revolution in just over 28 days, But the moon does not move at a constant velocity. The moon is accelerating all the time -- but sideways! In every instant, the moon is on a path straight ahead! And if it wasn't affected by any force at all, it would fly straight off into space and never come back! But gravity keeps the moon constantly accelerating sideways - towards the Earth.

The velocity forwards is constantly affected by the acceleration sideways; and together they make the moon travel around the Earth. Acceleration: that's the rate of change in velocity. And because velocity has direction, a change in direction is also a change in acceleration. To turn is also to accelerate.