Please could someone explain the following situation. A truck weighs 4 tonnes and travels at 18m/s. A car on the same road, weighs 1.4 tonnes and is travelling at 35 m/s. The car has more kinetic energy but less momentum. My book says that momentum is how hard it is to stop something which is where my confusion comes in. To stop the car would require a force over a certain distance to bring it to rest and the work done would be equal to the KE. Since the car has more kinetic energy the force will need to be applied over a longer distance (or a greater force over the same distance) so surely the car is harder to stop? But it has less momentum so according to that definition it shouldn't?