Acceleration

The light turns green, the car pulls away and gently presses you into your seat. A few seconds later the speedometer reads 50 km/h. What you feel there is acceleration: the speed is changing. Whether it's a sprint start, a roller coaster or an emergency stop, the same idea is behind all of it.

What is acceleration?

Acceleration tells you how much the speed changes per second. As a formula: a = Δv/t, the change in speed divided by the time. If a car gets 10 m/s faster in 5 seconds, then a = 10 m/s divided by 5 s, which is 2 m/s². The unit m/s² sounds odd, but it just means: every second, another 2 m/s of speed is added. Braking is acceleration too, just with the opposite sign: the speed decreases every second, and the acceleration points against the direction of travel.

Free fall

If you drop a stone, the Earth accelerates it at about 9.8 m/s². After 1 second it moves at about 9.8 m/s, after 2 seconds already 19.6 m/s. The acceleration stays constant while the speed keeps growing and growing, as long as air resistance does not play a big role. That's why jumping from the 3 metre board hurts a lot more than stepping off the pool edge.

From 0 to 100

In car reviews you often read: 0 to 100 km/h in 10 seconds. That's nothing but a statement of acceleration: 10 km/h more every second. A sports car does the same in 4 seconds, so 25 km/h per second. If you convert 100 km/h to about 27.8 m/s, the sports car comes out at roughly 7 m/s², about 70 percent of the free fall acceleration of 9.8 m/s².