A bus slams on its breaks and goes from 30 km/hr to 15km/hr in 4 seconds. What is its acceleration?

Acceleration = (change in speed) / (time for the change).

Change in speed = (15 - 30) = - 15 km/hr

Time for the change = 4 sec

Acceleration = (-15 km/hr) / (4 sec) = - 3.75 km/hr per second.

Is that a lot? Not much?

Let's convert it to a unit that we can think а bout:

(-15 km/hr) x (1,000 m/km) x (1 hr / 3,600 sec) =

(-15 x 1,000) / (3,600) = - (4 and 1/6) m/sec.

So the acceleration of the bus is - (4 and 1/6) m/sec².

The negative sign means that it slowed down.

(4 and 1/6) m/sec² is about 42% of the acceleration of gravity ...

the acceleration the bus would have if it drove off of a cliff.

When the car or the bus you're riding in slows down at that rate,

you feel 42% of your weight pulling you forward against your

seat belt. That's quite a drastic acceleration!