NCERT Solutions
Class 9 Science
Force and Laws of Motion

Q.7
An 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:
(a) the net accelerating force;
(b) the acceleration of the train; and
(c) the force of wagon 1 on wagon 2.
(a) Force exerted by the engine, F = 40000N
Frictional force offered by the track, Ff = 5000N
Net accelerating force, Fa = F – Ff = 40000 – 5000 =35000N.
(b) Acceleration of the train = a
The engine exerts a force of 40000N on all the five wagons.
Net accelerating force on the wagons, Fa = 35000N
Mass of the wagons, m = mass of a wagon x number of wagons
Mass of wagon = 2000kg
Number of wagons = 5
Therefore m =2000 x 5 =10000kg.
Mass of the engine m’ = 8000kg
Total mass m + m’ = 18000kg
From Newton’s second law of motion:
Fa = ma
a = ( ) = (
) = 1.944m/s2
(b) Mass of all the wagons except wagon 1 is 4 × 2000 = 8000 kg
Acceleration of the wagons = 3.5 m/s
Thus, force exerted on all the wagons except wagon 1
= 8000 × 3.5 = 28000 N
Therefore, the force exerted by wagon 1 on the remaining four wagons is 28000 N.
Hence, the force exerted by wagon 1 on wagon 2 is 28000 N.