Forces and Motion

Braking Distance of a Car - Mass.

The braking distance of a car increases as the mass increases.

Q1.  A car's brakes apply a force of 1500N.
If the car has a mass of 750 kg, what is its acceleration?
(How quickly does it slow down?)

A1.  Use  F = m x a
 or  a = F ÷ m

a = 1500 ÷ 750
= 2 m/s2.

Strictly speaking we would call the acceleration -2 m/s2
because the car is slowing down.

Q2.  The same car's brakes apply the same force of 1500N.
If the car has double the mass at 1500 kg, what is its acceleration?
(How quickly does it slow down?)

A2.  Use  F = m x a
 or  a = F ÷ m

a = 1500 ÷ 1500
= 1 m/s2.

We have doubled the mass of the car
and applied the same force from the brakes,
the car is now slowing down at only half the rate.

The car with twice the mass will take twice as long to stop
(requiring twice the braking distance - see kinetic energy).

Notice that the acceleration (slowing down)
only depends on the force of the brakes and the mass of the car,
not how fast the car is going.
A faster moving car will take further to stop for the same acceleration
(the same rate of slowing down, see previous page).

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