A freight train rolls through a rail yard at walking speed, about 1.5 m/s. A jogger beside the track moves at the same speed. The jogger can stop in one step. The train needs a long stretch of braking.
The speeds match. Why is the train so much harder to stop?
Momentum is mass times velocity: p = mv. Stopping difficulty follows the momentum, not the speed alone.
The train's speed is small, but its mass is enormous: about 200000 kg, even for a short train.
A huge mass times a small velocity is still a huge product.
The jogger's mass is about 70 kg. At the same 1.5 m/s, the jogger's momentum is a tiny fraction of the train's.
So speed alone cannot measure stopping difficulty. The mass counts too, and momentum counts both.
A fully loaded dump truck creeps down a driveway. Which quantity measures how hard the moving truck is to stop?
The loaded dump truck creeps along at walking speed, yet it is very hard to stop. Why?
Mass alone fails the same test. A parked train has a huge mass and zero velocity, so its momentum is zero: there is nothing to stop.
Dmitri says: 'A car parked on a level street has zero momentum.' Is Dmitri correct?
That is why momentum, the product of the two, is the measure of how hard a mover is to stop.
The 200000 kg train rolls at 1.5 m/s. State its momentum.
Write down the values:
m = 200000 kg, v = 1.5 m/s, p = ?
Write down the equation:
p = mv
Substitute in the values, and calculate:
p = 200000 × 1.5 = 300000
The train's momentum is 300000 kg·m/s.
The 70 kg jogger moves at the same 1.5 m/s. State the jogger's momentum.
Write down the values:
m = 70 kg, v = 1.5 m/s, p = ?
Write down the equation:
p = mv
Substitute in the values, and calculate:
p = 70 × 1.5 = 105
The jogger's momentum is 105 kg·m/s. The same speed, a far smaller momentum.
An empty pickup truck and a fully loaded moving truck travel at the same speed. The loaded truck is harder to stop. Why?
Which of the following has the largest momentum?
A dragonfly flies across a parking lot. A parked bus sits nearby. Which has more momentum?