Why does Newton's 3rd Law of Motion not apply to car crashes?

Question

In driving school we learned that if you don't wear a seatbelt during a car crash then your body will be hurled to the site of impact. How does that make any sense when for every action there is an equal and opposite reaction? Wouldn't your body move away from the site of impact?

Think about those little silver beads that hang. When the first one hits the second bead, the second bead doesnt hit the first bead again, it hits the third bead.

I mean if like a car hits you from the side, not if a you hit a car.

Answer 1

If you're driving, hit something, your car stops (decelerates) quickly - you don't. So, your car hits, your car slows/stops, you continue to move until you hit your steering wheel, or your windshield, or if you go through the windshield, you eventually hit the object your car hit.

Or, say you're stopped and someone hits you. Their car slows/stops, your car begins to move away from the impact suddenly, but you don't start to move right away. As your car moves away from the impact, it hits you, and unless you accelerate with your car, you may fly out the window. In actuality, you stay still and the window flies past you.

To illustrate, take a glass of water held still. Move the glass suddenly - the glass moves, but the water wants to stay in the same place while accelerating, so it sloshes up the inside of the glass until it's in motion.

Answer 2

Actually the 3rd Law does apply: the collision generates a vector force away from the site. You are the reactor and are moved by a vector force towards the site.

If someone hits you from behind you are pushed into the seat. From the front you are pulled out of the seat. Since you are not connected to the car - the force is an impulse force so you continue on a trajectory following Newton's first law until you hit the windshield - then Newton's 3rd Law applies again only now you are not the reactor. Sort of like those beads you are talking about.

I would not suggest doing the experiments - I just have been in enough accidents to have verified this truth.

Answer 3

It really has more to do with the first law than the third, which says, to simplify, an object in motion will tend to stay in motion unless an external force acts upon it.
When you are in your car driving you are in motion. The idea here is that when you crash your car comes to a stop, but until something pushes on you, you remain going. That something will be the inside of the car. So you smash into the steering wheel. Now comes Newtons third law: it pushes on you (making you recoil back into your seat and busting up your body) you push on it, smashing it into the dashboard.
Now if there is nothing there, for example if you hit the windshield instead of the steering wheel, you continue going by the first law, although the windshield will have slowed you down a little bit.
Newtons laws have been verified and shown to be correct for every known phenomenon that does not occur at very, very, very fast speeds (approaching the speed of light).

Answer 4

Vector algebra proves the 3rd Law true. I won't get into it here, too lengthy.

Consider a collision from the rear: the target car is accelerated very suddenly. That is the equal reaction. The passengers are not accelerated as quickly (since they are not completely tied into and thus part of the car) and so move "toward" the impact. If seatbelts are worn, only the neck up moves. If a head restraint is worn (as in NASCAR) nothing moves). This is an illustration of the 1st Law: a body at rest tends to stay at rest (a passenger in a car is essentially considered to be "at rest").

Alter the direction of impact and you'll see the same thing happening.

Answer 5

If you view the situation from the perspective of the driver, Newton's 3rd Law of Motion does not hold!

The reason is that your frame of reference (the car) is in accelerated motion. Newton's Laws are not valid in such a system. Instead, you experience an "inertial force" in the direction opposite to the acceleration. This force could indeed launch you through the windshield.

For an outside observer, nothing strange happens. The car is slowed down, and at the same time exerts a force on the obstacle it crashes into (Third Law). The driver, however, continues to move as he did before -- forward and through the windshield, if there was no seatbelt. From the outside perspective there is not force on the driver at all!

Answer 6

Okay, so your car is moving, and you crash into another car. Your car exerts force on the other car, and that car simultaneously exerts force back on your car; that's Newton's 3rd law working perfectly fine. So your car stops moving, but inertia, from Newton's 1st law (an object at constant velocity remains at constant velocity unless acted upon by a force), makes you want to keep moving at the speed your car used to be going. Say your seatbelt is on. it exerts force on you because you're trying to move forward while the seatbelt has stopped moving, with the car. As it's putting a force on you, you also exert a force on it... again, action, reaction.

Same thing if you get hit from the right side... intertia keeps you trying to go in a straight line, while the car is moving to the left.. relative to the car you are moving towards the side of the crash, but that's because the car is actually trying to move without you.

Answer 7

Newton's Law is based on Momentum: Something in motion tends to remain in motion unless acted on by an external force.

An unbelted driver will continue in the direction of travel, at the same speed as the vehicle just prior to the crash. The only thing left to slow the driver down is whatever he can hold onto or the stuff right in front of him.

It is like the last thing that goes through a pilot's mind during a plane crash: the control wheel....

Answer 8

The problem with your logic is you aren't getting hit, the car is. If you were walking down the street and got hit, you'd fly off like one of those silver balls. If you're in your car and it gets hit, the car is going to be redirected by the impact of the collision, but your inertia is going to keep you going until you are acted upon by a force like a seatbelt or a windshield or a tree.

Answer 9

The 3rd law of motion is true. Your problem is that you are assuming that the force is acted on the human which is not true.The force is applied to the car. When you have your seat belt on you become more of a part of the car and you move as the car does. When you do not have your seat belt on you are proving Newtons 1st law of motion. An object in motion stays in motion. Newton's laws of motion go together so you can not ignore one to prove another.

If you were hit by the car you would follow Newton's 3rd law.

Answer 10

That's explained in a different law. Your body isn't moving or is moving at a constant speed when you are hit. Then if a car hits you from the side your car moves in the direction it was hit but your body stays in the same place, so the car moves away from you, you don't actually move towards anything.