# Clarification needed about accleration

1. Oct 25, 2009

### logearav

revered members,
if velocity is constant acceleration is zero. now my question is if a vehicle moving with constant velocity, say a train, has constant velocity, so acceleration is zero. now if that train hits a person, why that person gets hurt, inspite of acceleration is zero? my son asked this question and i dont have answer to this? please help

2. Oct 25, 2009

### A.T.

Because the wheels of the train cut the person in 3 parts? You don't need acceleration to hurt someone, just deformation. And the acceleration of the person is not zero if he doesn't get overrun.

Last edited: Oct 25, 2009
3. Oct 25, 2009

### arithmetix

The poor person begins to move at the speed of the train and is accelerated to the velocity of the behemoth in something less than the time the train takes to pass the distance that the person's thickness represents within the train's path. Perhaps the time the body takes to be accelerated to its final velocity for a train moving at about 70km/h is on the order of 1/100 second.
Seconds can be marked rhythmically with a stick, the rate doubled to demonstrate 1/2 seconds, doubled again for 8ths, if you can tap 32 beats a second you're probably doing well.
I wonder if the child has seen the train far away and lacks visual comprehension of its speed.

4. Oct 25, 2009

### Staff: Mentor

In a collision with a person, the train's acceleration is (practically) zero. The person's acceleration is far from zero.