# Inertia of person on a bus

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.

tiny-tim
Homework Helper
Hi Peter!
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)
… how inertia is dependent on weight, not on mass …

This is very confusing

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.

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

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 .

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.

tiny-tim
Homework Helper
Hi Peter!

(just got up :zzz: …)
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

Yes, but then it is the mass, not the weight!
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 .

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 ), 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.

Ok, got it

Now, because I'm a very curious boy... :shy: I decided to look into momentum (there were only two pages anyway ) 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 )
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

tiny-tim
Homework Helper
Hi Peter!

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.