The “elephant effect” and its relation to Newton’s laws

In a collision, the mass of the occupant does not change (obviously they always weigh the same), so to say that “in the event of a collision our body weighs 20 times more” is a simplification to help us understand the forces involved in an impact. If we could put a scale under the occupant during the whole collision that would only measure their weight, we would see how they always weigh the same. The explanation for all this lies in Newton’s laws of motion.

Newton’s first law, also known as the law of inertia, states that “every body will remain in its state of rest or uniform, rectilinear motion unless it is forced to change it by the forces acting upon it.”

This law describes the concept of inertia, which is the tendency of an object to remain in its present state (either at rest or in uniform rectilinear motion) unless an external force acts upon it. Put simply, a stationary object tends to remain stationary, and an object in motion tends to remain in motion at the same speed and in the same direction, unless an external force changes this.

Let’s imagine we have a collision at 50 km/h. The vehicle will begin to decelerate sharply, while our body will continue to move forward at 50 km/h, and according to Newton it will do so until something forces it to change its state. That “something” is the seat belt, or the child restraint system in the case of children. This is the point at which we apply Newton’s third law of motion.

Newton’s third law, also known as the law of action and reaction, states the following: “If body A exerts a force on a body B, then body B exerts an equal force on body A, but in the opposite direction.”

To stop us moving forward, the seat belt or the child seat, must exert a force on us that is the same intensity that we are exerting, but what is that force?

Once again Newton helps us to solve this issue, through his second law, which establishes the relationship between force and acceleration, where the force is the product of the mass of the body (of the adult or the child) times the acceleration (or deceleration) to which they are subjected.

Now we have it. As our body is abruptly held in place by the seat belt, it experiences a change in velocity (a deceleration). The product of this deceleration and the mass of the occupant results in the force exerted on the seat belt, which in turn is exerted on us. The force will not be constant, but will vary according to the intensity of the deceleration in the collision.

This force derived from the seat belt restraining us is what is called the “elephant effect”. The deceleration in the event of a car crash is considered to be between 20 and 40 times the force of gravity, and since we are not talking about force, but acceleration, that is the factor we have to multiply the mass by.

What about the objects inside the vehicle?

Any objects will continue to move at the same speed they were moving, in our example 50 km/h, until they find something or someone to hold them back.

At that moment the resultant force will be the product of the object’s mass (generally small) times by the deceleration it experiences, which will actually be much higher than that experienced by the vehicle.

In summary, to say that on impact we “become” elephants is a simplification of Newton’s laws to help us understand the effect of a collision on our body. In addition, this principle is not equally applicable to all the occupants or objects in a vehicle.