Why does a person on a bus move to the back when the bus accelerates?

In summary, the conversation discusses the concept of inertia through a cartoon about a man on rollerblades on a bus. The man stays in the same position as the bus moves, showing his tendency to keep moving at the same speed. The conversation also touches on the difference between mass and weight and the confusion surrounding the use of "inertia" in different contexts. The topic of car crashes and momentum is briefly mentioned, with the suggestion to wait until learning about Newton's second law and conservation of momentum. The conversation ends with the mention of a discussion about the force a car exerts on a wall during a crash.
  • #1
Peter G.
442
0
I was reading about inertia and there was this cartoon on my textbook.

This guy puts on rollerblades and steps on to a bus. "Magically" as the car accelerates he ends up at the end of the vehicle and when he wants to step out the bus he simply stands up and, "magically" ends up at the entrance as the bus brakes.

So when the bus accelerates it moves forward, the guy ends up on the back of the bus: Is this because the force of the bus acceleration is not great enough to overcome his inertia so as the bus moves forward he, without "moving", ends up at the back of the bus? Or is the bus exerting a force on him making him move?

I don't know, I mean they explained about the car crashes too and I understood that our bodies are moving and if no resultant force great enough to overcome our reluctance of keep moving forward acts on us we will keep on moving in a straight line and how inertia is dependent on weight, not on mass but I can't grasp the inertia concept on this diagram.

Thanks in advance
 
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  • #2
Hi Peter! :smile:
Peter G. said:
So when the bus accelerates it moves forward, the guy ends up on the back of the bus:

Is this because the force of the bus acceleration is not great enough to overcome his inertia so as the bus moves forward he, without "moving", ends up at the back of the bus?

Or is the bus exerting a force on him making him move?

Forget force.

There is no force!

If he starts to stand on the rollers when the bus is moving at 30 mph, then he always keeps moving at 30 mph, whatever the bus does…

i] if the bus keeps moving at 30 mph, then he stays where he is on the bus simply because they're moving at the same speed
ii] if the bus brakes, then he starts overtaking the bus (he's always going at 30 mph)
iii] if the bus accelerates, then the bus starts overtaking him (he's always going at 30 mph) :wink:
… how inertia is dependent on weight, not on mass …

This is very confusing :redface:

even if they mean it the other way round, the difference really only applies on the Moon etc

also, they're confusing two meanings of "inertia" … in your bus cartoon, it means the tendency to keep going, and this has nothing to do with either mass or weight … in a car crash, it means your momentum , which governs the forces on you when your speed is reduced.
 
  • #3


Your explanation is really good. :smile:

Yeah, the mass and weight thing is confusing but they used it to explain that if you kick a rock in the moon it will still hurt :biggrin:

Regarding the car crash I think they also mean the tendency of keep moving because as soon as the car crashes it stops moving but no resultant force acts on you (provided you are not wearing your seat bealt) and you will keep moving through the windshield. I haven't learned about momentum, so I don't understand how it relates, maybe if you can explain o:).

Now, back to the bus: You said that when the bus moves at 30 mph, the man will move at 30 mph and, because of inertia, when the bus changes its motion, he does not, he continues moving the same way as he was before until another force acts upon him.

But he hops in the bus when it is at 0 mph, when the bus accelerates he continues moving at 0 mph? Or he accelerates together with the bus, because if he does, he will be moving at the same speed as the bus and will not end up at the back of the bus, right?

Sorry, I am a bit confused.

Thanks for your patience.
 
  • #4
Hi Peter! :smile:

(just got up :zzz: …)
Peter G. said:
Yeah, the mass and weight thing is confusing but they used it to explain that if you kick a rock in the moon it will still hurt :biggrin:

Yes, but then it is the mass, not the weight! :wink:
Regarding the car crash I think they also mean the tendency of keep moving because as soon as the car crashes it stops moving but no resultant force acts on you (provided you are not wearing your seat bealt) and you will keep moving through the windshield. I haven't learned about momentum, so I don't understand how it relates, maybe if you can explain o:).

uh-uh, leave that until your teacher tells you about Newton's second law and conservation of momentum.
Now, back to the bus: You said that when the bus moves at 30 mph, the man will move at 30 mph and, because of inertia, when the bus changes its motion, he does not, he continues moving the same way as he was before until another force acts upon him.

But he hops in the bus when it is at 0 mph, when the bus accelerates he continues moving at 0 mph? Or he accelerates together with the bus, because if he does, he will be moving at the same speed as the bus and will not end up at the back of the bus, right?

If he's on the rollers when the bus starts, he'll hit the back of the bus.

If he's not (I don't know what the technical term is :redface:), then the friction between his shoes and the bus floor will be a force that will accelerate him, keeping him at the same speed (and acceleration) as the bus. :smile:
 
  • #5


Ok, got it :wink:

Now, because I'm a very curious boy... :shy: I decided to look into momentum (there were only two pages anyway :biggrin:) And I was wondering if you could read two simple problems and say either: Correct or Wrong. In case I get it wrong, I promise I will sit, patiently, waiting for my lesson.

Force a car exerts on a wall during a crash: (Sorry I love cars)

So the car, of mass 200 kg, is traveling down the road at a velocity of 10 m/s. Suddenly, he hits a very rigid wall:

So, I will use the equation derived from Newton's Second Law of Motion: Ft = mv-mu

Ft = (200 x 10) - (200 x 0) (Quite a large Kinetic Energy dissipation :eek:)
Ft = 2000 kg m/s
(For the time of the collision, I will use the figure given in the book for a different problem involving a very rigid stone)
F = 2000 divided by 1/100th of a second
F = The car exerts on the wall is 200 000 N

The second problem involves the theory of conservation of momentum:

This time the car hits a person and I am wondering at what speed will this poor little guy move once struck by this misgoverned automobile:

The car, again, weighs 200 kg and is moving at 10 m/s, momentum of: 2000 kg m/s
The guy had a mass of 10 kg.

So: 2000 kg m/s = 10 kg x Velocity
Therefore: 2000 / 10 = 200 m/s

Thanks once again,
Peter
 
  • #6
Hi Peter! :smile:

Your first example is ok
Peter G. said:
The second problem involves the theory of conservation of momentum:

This time the car hits a person and I am wondering at what speed will this poor little guy move once struck by this misgoverned automobile:

The car, again, weighs 200 kg and is moving at 10 m/s, momentum of: 2000 kg m/s
The guy had a mass of 10 kg.

So: 2000 kg m/s = 10 kg x Velocity
Therefore: 2000 / 10 = 200 m/s

No, momentum is conserved (in any collision), but the momentum of the car is not entirely transferred to the guy.

He and the car will normally have the same speed after the collision (in practice, the car would probably carry on braking, so the guy would fly off) …

so you can use the equation vcar,after = vguy,after, and combine it with the conservation of momentum equation. :wink:
 

What is inertia and how does it relate to a person on a bus?

Inertia is the tendency of an object to resist changes in its motion. This means that a person on a bus will continue to move at a constant velocity unless acted upon by an external force.

Why do people sometimes feel like they are being pushed towards the back of the bus when it starts moving?

When a bus starts accelerating, the people inside it are still at rest due to inertia. As the bus moves forward, the people are still at rest while the bus moves forward. This creates a relative motion, making it feel like they are being pushed towards the back of the bus.

How does a person's mass affect their inertia on a bus?

The greater the mass of a person, the greater their inertia will be. This means that a person with a larger mass will require a greater force to accelerate or decelerate them on a bus.

What happens to a person's inertia when the bus makes a sharp turn?

When a bus makes a sharp turn, the inertia of the person will cause them to continue moving in a straight line. This results in the person feeling like they are being pushed towards the outside of the turn.

Does a person's position on the bus affect their inertia?

No, a person's position on the bus does not affect their inertia. Inertia is a property of an object and is not affected by its position. However, a person's position can affect how they perceive changes in motion on the bus.

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