# Heavier object has higher inertia

## Main Question or Discussion Point

I learn that inertia means the tendency an object will remain from changes. Heavier object has higher inertia, which means higher tendency to resist change.

So, here is an example from my teacher.
A car accelerates, the driver is pulled backward and hit the rear of his seat. This is due to inertia of the driver-- he tends to remain in rest rather than accelerating.

My problem is, the car is definitely heavier than the driver, which means the car should have higher inertia than the driver. Why the car moves, but the driver tends to remain in rest?

## Answers and Replies

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A.T.
the car is definitely heavier than the driver, which means the car should have higher inertia than the driver
That's why the ground applies more force to the car, than the car to the driver.

Why the car moves, but the driver tends to remain in rest?
Both accelerate, when a force is applied. The car requires more force for the same acceleration.

SteamKing
Staff Emeritus
Homework Helper
Newton's First Law of Motion: An object at rest tends to remain at rest unless acted upon by an external force.

When the car starts to accelerate, the driver's body wants to remain stationary, if but for a moment. When the driver's body is pushed back into the seat, the seat is also pushing back on the driver's body. This is where Newton's Third Law applies: For every action, there is an equal and opposite reaction. After this, Newton's Second Law applies (F = ma) until the car and driver have accelerated and reached a constant speed.

NascentOxygen
Staff Emeritus
My problem is, the car is definitely heavier than the driver, which means the car should have higher inertia than the driver. Why the car moves, but the driver tends to remain in rest?
The car is securely bolted to its engine and wheels, so whatever the powerful engine dictates, the rest of the car follows along. (I described the engine as "powerful"; it needs to be powerful to overcome the enormous inertia of the heavy car body and engine.) The man's head is only loosely positioned on his body, and in most sitting positions the driver's head is not supported against the head rest. So in effect, his head is just flopping about in mid-air. The head rest is really a safety feature, so that if the car is rear-ended the driver's head is not flung back so far (due to inertia) as to cause neck damage.

D H
Staff Emeritus
I learn that inertia means the tendency an object will remain from changes. Heavier object has higher inertia, which means higher tendency to resist change.

So, here is an example from my teacher.
A car accelerates, the driver is pulled backward and hit the rear of his seat. This is due to inertia of the driver-- he tends to remain in rest rather than accelerating.
This exemplifies why I don't like the term "inertia". It's an ambiguous term that sometimes means mass, other times linear momentum. In this one example it is used in both contexts.

I'm not alone in my dislike. Once one progresses beyond the introductory calculus-based physics class you just won't see that word used with one exception, "moment of inertia" or "(moment of) inertia tensor".

There are two perfectly good and unambiguous phrases that mean mass and linear momentum, respectively. They are of course mass and linear momentum.

A.T.