# Heavier object has higher inertia

• sallychan
In summary, inertia refers to an object's tendency to remain unchanged and resist changes in motion. Heavier objects have higher inertia, making them more resistant to change. In the case of a car and its driver, the car's heavier mass results in a higher inertia, but the driver's body is not securely bolted to the car like the car's engine and wheels are. Therefore, the driver's body will maintain its velocity while the car accelerates, causing the driver to be pulled back into their seat. This exemplifies the ambiguous nature of the term "inertia" and why other terms such as mass and linear momentum should be used instead.

#### sallychan

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?

sallychan said:
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.

sallychan said:
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.

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.

sallychan said:
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.

sallychan said:
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.

D H said:
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 see only the mass context.

All macroscopic objects will maintain their velocity unless acted upon by an outside source. In the case of the car driver and the car, the car driver's torso isn't bolted to his seat. It is free to move independently from the car, so he will maintain his velocity as the car accelerates around him. You can understand this concept better if you put a water bottle on your passenger seat and slam on the brakes. The water bottle will go flying forward due to its inertia. This is the exact same phenomenon as the driver falling back in his seat during acceleration.

## What is inertia and how does it relate to the weight of an object?

Inertia is the tendency of an object to resist changes in its state of motion. The weight of an object, which is a measure of its mass and the force of gravity on it, is directly related to its inertia. This means that the heavier an object is, the more difficult it is to change its state of motion.

## How does the mass of an object affect its inertia?

The mass of an object is directly proportional to its inertia. This means that the greater the mass of an object, the greater its inertia will be. This is because more massive objects require more force to change their state of motion, due to their greater amount of resistance to changes in motion.

## Can an object with low mass have high inertia?

Yes, it is possible for an object with low mass to have high inertia. This is because inertia is not solely determined by an object's mass, but also by its velocity. An object with low mass can have high inertia if it is moving at a high velocity, since the resistance to changes in motion will be greater.

## Does the shape of an object affect its inertia?

Yes, the shape of an object can affect its inertia. Objects with irregular shapes or uneven distributions of mass may have different amounts of inertia compared to objects with the same mass and uniform shape. This is because the distribution of mass can affect the object's ability to resist changes in its state of motion.

## Why do heavier objects have higher inertia?

Heavier objects have higher inertia because they have a greater amount of mass. As mentioned before, the greater the mass of an object, the more difficult it is to change its state of motion. This is due to the object's resistance to changes in motion, which is directly related to its mass and the force applied to it.