Forces in inertial and non-inertial frame of reference

In summary, a man standing on a bus that is moving at a constant speed will change his relative position when the bus starts braking with constant negative acceleration. If we ignore the force of inertia and analyze the situation in an inertial frame, the only force causing the man's motion is the force of friction between him and the bus. If the floor of the bus is made very slippery, the man will still move relative to the bus. This is because the force of friction is not the only force acting on the man, as there are no more forces in the x direction. In the analysis presented, it is important to note that the man does not start moving due to the force of friction, but rather keeps moving because he was already in
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ChessEnthusiast
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Imagine such situation:
A bus is moving at constant speed, a man is standing on the bus and is not holding to anything - he is simply standing.
Now, the bus starts breaking with constant negative acceleration and so the man will change his relative position due to the force of inertia.
Now, let's say we don't believe in the existence of the force of inertia and want to solve this problem in an inertial frame of reference.

The only force of contact between the man and the bus is the force of friction.
The man will move forward due to braking.
Therefore, is the force of friction the force causing his motion relative to the floor on the bus?
 
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  • #2
ChessEnthusiast said:
Therefore, is the force of friction the force causing his motion relative to the floor on the bus?
What do you think would happen if we made the floor very slippery* so the friction force was 0? Would he still move relative to the bus or not?

*perhaps he should not remain standing for this portion of the experiment, it might be difficult to pass the ethics committee review!
 
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Right, the force of friction was not the right guess.
However, there are no more forces in the x direction.

What about this analysis:
There is no force acelerating him towards the windshield, there is simply a force opposing his motion - the force of friction. The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
 
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ChessEnthusiast said:
Right, the force of friction was not the right guess.
However, there are no more forces in the x direction.

What about this analysis:
There is no force acelerating him towards the windshield, there is simply a force opposing his motion - the force of friction. The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
Yes. Don't talk about "relative to the bus" if you decided to use the inertial frame.
 
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ChessEnthusiast said:
The fact that this man starts moving is wrong - the man simply KEEPS moving because he was in motion and no negative acceleration was applied directly to him.
Yes, exactly. In the inertial frame the bus is accelerating, not the man. The change in relative position is due to the bus’ acceleration
 
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Possibly useful:
Frames of Reference (1960) .. go to 13m27s
 

FAQ: Forces in inertial and non-inertial frame of reference

What is the difference between inertial and non-inertial frame of reference?

Inertial frame of reference is a frame of reference in which Newton's first law of motion holds true, meaning an object at rest will remain at rest and an object in motion will continue in a straight line at a constant speed. Non-inertial frame of reference is a frame of reference in which Newton's first law does not hold true, usually due to the presence of external forces.

How do forces act in an inertial frame of reference?

In an inertial frame of reference, forces act in a linear and predictable manner. This means that forces are directly proportional to the mass and acceleration of an object, and they act in the direction opposite to the acceleration of the object.

What is the role of fictitious forces in a non-inertial frame of reference?

Fictitious forces, also known as pseudo forces, are forces that appear to act on an object in a non-inertial frame of reference, but are actually due to the acceleration of the frame itself. These forces are necessary to explain the motion of objects in non-inertial frames, as they account for the deviation from Newton's first law of motion.

What are some examples of inertial and non-inertial frames of reference?

Inertial frames of reference include a stationary room, a car moving at a constant speed on a straight road, and a spaceship traveling through deep space at a constant velocity. Non-inertial frames of reference include a car turning on a curved road, an elevator accelerating upwards or downwards, and a rotating amusement park ride.

How do inertial and non-inertial frames of reference affect the measurement of forces?

In an inertial frame of reference, forces can be measured accurately using instruments such as spring scales. However, in a non-inertial frame of reference, the presence of fictitious forces can make the measurement of forces more complex, and specialized instruments may be needed to accurately measure forces.

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