# Why does it seem all bodies oppose acceleration?

For example, when you are on a rollercoaster and you go past the tip of the coaster you feel like you are going to fly off your seat... however you are radially accelerating down. Same thing when you take a u-turn in a car. you are radially accelerating to your left (atleast in america) however you feel like you are being pushed to your right.

Why is this?

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ZapperZ
Staff Emeritus
For example, when you are on a rollercoaster and you go past the tip of the coaster you feel like you are going to fly off your seat... however you are radially accelerating down. Same thing when you take a u-turn in a car. you are radially accelerating to your left (atleast in america) however you feel like you are being pushed to your right.

Why is this?
Just to clarify, you are really asking why Newton's First Law is valid, aren't you?

Zz.

Just to clarify, you are really asking why Newton's First Law is valid, aren't you?

Zz.
well what I'm asking is why do we feel like we're going against acceleration? When we are at the top of the rollercoaster the net force is downward.... I would expect that we would feel a downward jerk because we are accelerating downward. Why is it the opposite?

russ_watters
Mentor
I would just say it's because inertia is a property of matter, tied to mass.

ZapperZ
Staff Emeritus
well what I'm asking is why do we feel like we're going against acceleration? When we are at the top of the rollercoaster the net force is downward.... I would expect that we would feel a downward jerk because we are accelerating downward. Why is it the opposite?
I think you got this all wrong.

If you tie a ball to a string, and then spin it round in a circle, what is the direction of the force? And what is the direction of the acceleration?

Now, if you are the ball (i.e. I attach you to a string and then spin you around in a circle), what forces do you feel that are acting on you? In this case, does the fact that you are in a non-inertial reference frame has any bearing to what you THINK are the forces that you experience?

In both cases, try to draw a free-body diagram of the forces. Now use this and see if you can reformulate your question, and whether my re-interpretation of your question makes any sense.

Zz.

I think you got this all wrong.

If you tie a ball to a string, and then spin it round in a circle, what is the direction of the force? And what is the direction of the acceleration?

Now, if you are the ball (i.e. I attach you to a string and then spin you around in a circle), what forces do you feel that are acting on you? In this case, does the fact that you are in a non-inertial reference frame has any bearing to what you THINK are the forces that you experience?

In both cases, try to draw a free-body diagram of the forces. Now use this and see if you can reformulate your question, and whether my re-interpretation of your question makes any sense.

Zz.

It would accelerate to the center of the string (the point of rotation).

I was told that you can not do F=ma in a non inertial frame, so how can I answer the 2nd question if I AM the ball? Wouldn't I be accelerating meaning I can't do F=ma.

jbriggs444
Homework Helper
2019 Award
well what I'm asking is why do we feel like we're going against acceleration?
Because our observed motion (measured against the roller coaster cart or against the interior of your automobile) does not match the felt force from the cart or car. We see ourselves stationary with respect to the cart or car. We physically feel a lowered supporting force (for the cart at the top of its arc) or a leftward push (for a leftward turning car). But we account for the discrepancy between felt force and observed lack of movement by imagining the presence of an additional "inertial" force directed opposite to the true acceleration.

We imagine that it is this inertial force that is lifting us off of the roller coaster seat or pushing us rightward against the passenger side door.

Because our observed motion (measured against the roller coaster cart or against the interior of your automobile) does not match the felt force from the cart or car. We see ourselves stationary with respect to the cart or car. We physically feel a lowered supporting force (for the cart at the top of its arc) or a leftward push (for a leftward turning car). But we account for the discrepancy between felt force and observed lack of movement by imagining the presence of an additional "inertial" force directed opposite to the true acceleration.

We imagine that it is this inertial force that is lifting us off of the roller coaster seat or pushing us rightward against the passenger side door.
Ahh, ok. that makes sense. So what is it about inertial foces that makes it hard to do F=ma? My prof said that those forces (im assuming inertial forces are the same as pseudo forces) makes solving F=ma problems confusing.

jbriggs444
Homework Helper
2019 Award
Ahh, ok. that makes sense. So what is it about inertial foces that makes it hard to do F=ma? My prof said that those forces (im assuming inertial forces are the same as pseudo forces) makes solving F=ma problems confusing.
The key concern is that the "a" in f=ma depends on your choice of coordinate system.

If you are using an accelerating coordinate system or rotating coordinate system then the "a" will have a contribution from your true acceleration and will also have a contribution from the fact that your coordinate system is changing constantly.

As long as you are careful to pick a single coordinate system and stick to it faithfully, then f=ma works fine. If you choose an accelerating coordinate system then you will have inertial pseudo force(s) to account for. In a rotating coordinate system, the inertial pseudo forces are known as the centrifugal and Coriolis forces. Newton's second law works. The thing that breaks down is Newton's third law. Inertial pseudo forces do not have third law partners.

Edit: The confusion creeps in when you do not faithfully stick to your chosen frame.

ZapperZ
Staff Emeritus
I was told that you can not do F=ma in a non inertial frame, so how can I answer the 2nd question if I AM the ball? Wouldn't I be accelerating meaning I can't do F=ma.

toesockshoe said:
well what I'm asking is why do we feel like we're going against acceleration? When we are at the top of the rollercoaster the net force is downward.... I would expect that we would feel a downward jerk because we are accelerating downward. Why is it the opposite?
You are describing what you are feeling in the rollercoaster. This is no different than if you are the ball in my example, is it not?

Zz.

A.T.