When hit from behind in a car crash, a passenger can suffer a neck injury called  whiplash. explain in terms of  inertia  how this occurs, and how headrests can prevent the injury

In car crash when it hit from behind the thrust on the car is applied in forward direction

This thrust will produce forward thrust due to which there is sudden increase in the speed of car

Now here as per Newton's first law we will say that

If an object is at rest then it will remain at rest while if object is in motion then it will remain in motion.

So here the person who is sitting in car will experience a sudden change in inertia due to which his head will try to remain at rest while his body moves forward due to sudden thrust, So the person had neck injury.

We can prevent this head injury using head rest because headrest will provide a support which will counterbalance the thrust on the neck and the neck will supported by it.

This will be true as the time of motion will increase that will cause less amount of force on it

c) up is the direction the magnetic force is acting on the charge.