# Motion in a car

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Einstein60
I a car,why do you move back when the car accelerates forward. You move forward when the car accelerates backwards,You move right when the car turns left on a bent road, you move left when the car turns right on a bent road.
What actually makes you move in the opposite direction of the car.

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russ_watters
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I a car,why do you move back when the car accelerates forward. You move forward when the car accelerates backwards,You move right when the car turns left on a bent road, you move left when the car turns right on a bent road.
What actually makes you move in the opposite direction of the car.
Unless you fall out, aren't you compelled to move in the same direction as the car? Isn't that how the car takes you to your destination?

It would be funnny if the car drove north and you moved south, and you ended up in different places. That would be weird!

russ_watters
Einstein60
That's true but I mean when the car accelerates, a certain force pushes you to the seat.
Unless you fall out, aren't you compelled to move in the same direction as the car? Isn't that how the car takes you to your destination?

It would be funnny if the car drove north and you moved south, and you ended up in different places. That would be weird!

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That's true but I mean when the car accelerates, a certain force pushes you to the seat.
Does it really? Or, does the seat simply push you forwards?

Einstein60
It says every body continues to stay at rest or if it is moving continues to move in a straight line unless an external force acts on it to change its direction or state of motion.

Einstein60
Does it really? Or, does the seat simply push you forwards?
I remember one day when I entered a bus. I was standing taking money from my Pocket I didn't know the driver was about to take off. Immediately the driver drove the car, an unknown force pushed me back on the seat.

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I remember one day when I entered a bus. I was standing taking money from my Pocket I didn't know the driver was about to take off. Immediately the driver drove the car, an unknown force pushed me back on the seat.
Isn't that the floor of the bus pulling your feet forwards? Like, if you are standing on a carpet, which someone pulls violently in one direction, pulling your feet in that direction and causing you to fall over?

If your feet were not on the floor of the bus, there would be no force, surely?

Einstein60
Isn't that the floor of the bus pulling your feet forwards? Like, if you are standing on a carpet, which someone pulls violently in one direction, pulling your feet in that direction and causing you to fall over?

If your feet were not on the floor of the bus, there would be no force, surely?
Thank you very much. I now understand.

Lnewqban
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Thank you very much. I now understand.
What you've identified is what is sometimes called a "fictitious force". The example with the carper is a good one. The real force is definitely to the left, say. The carpet gets pulled to the left and your feet get pulled to the left with it. This causes a certain sensation of your upper body having been pushed over to the right.

But, it is only your mind playing tricks. The real force in the car or bus is forward: it's the seat or the floor pushing you in the direction of the vehicle's motion. But, your mind creates a sensation of being pushed backwards relative to your surroundings.

Lnewqban
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You can think of it like laying down a sheet of graph paper horizontally on your seat. When the car accelerates forward, the graph paper moves forward and your butt moves backward relative to the graph paper.

While you are sitting in the car you tend to unconsciously adopt the car (and, by extension, the graph paper) as your standard of rest. It becomes your "frame of reference". You judge your own motion relative to the car.

vanhees71
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Indeed, and if the car is accelerating it's a non-inertial frame and you'll interpret the corresponding inertial forces as forces. It's important to keep in mind that the inertial forces only make sense in a non-inertial frame.

The description wrt. an inertial frame is always simpler. According to this simpler point of view you are accelerated with the car and thus there must be some true force accelerating you, and this force it is you "feel".

jbriggs444
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Although
The description wrt. an inertial frame is always simpler.
Although the description is easier, the complete calculation may or may not be.

For instance, in weather prediction or naval gunfire it is traditional to calculate using the Coriolis force to determine weather for a stationary Chicago under a 10 mile per hour west wind rather than to use a purely inertial analysis to determine weather for a moving Chicago under a 590 mile per hour east wind.

vanhees71
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Of course.

Einstein60
Isn't that the floor of the bus pulling your feet forwards? Like, if you are standing on a carpet, which someone pulls violently in one direction, pulling your feet in that direction and causing you to fall over?

If your feet were not on the floor of the bus, there would be no force, surely?

Isn't that the floor of the bus pulling your feet forwards? Like, if you are standing on a carpet, which someone pulls violently in one direction, pulling your feet in that direction and causing you to fall over?

If your feet were not on the floor of the bus, there would be no force, surely?
But with a bicycle, it is different

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But with a bicycle, it is different
When you push on the pedals that results in the back wheel being forced backwards against the ground. Static friction acts and, by Newton's third law, the ground applies a force to the bike. It's similar to walking in a way.

Einstein60
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I a car,why do you move back when the car accelerates forward.
You don't move backwards. You move forwards, just not as much as the car.

russ_watters and jbriggs444
Einstein60
You don't move backwards. You move forwards, just not as much as the car.
You move forward with the car but your upper body goes back and lie on the seat.
It's like Newton's first law, when the car is at rest, you continue to be at rest with the car. But when the car accelerates forward, you still want to be at rest so your upper body moves a little backwards.

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You move forward with the car but your upper body goes back and lie on the seat.
It's like Newton's first law, when the car is at rest, you continue to be at rest with the car. But when the car accelerates forward, you still want to be at rest so your upper body moves a little backwards.
When one shifts frames of reference like that within a single sentence, one ought to display an awareness of having done so.

PhDeezNutz, russ_watters, PeterDonis and 3 others
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You move forward with the car but your upper body goes back and lie on the seat.
No it doesn't. Watch a video of what happens. The seat moves forward and presses against your back.

You can do the same experiment with a child's wagon and a ball. Place the ball in the wagon. Pull the wagon forward and you may think the ball rolls backwards and hits the back of the wagon. But if you make a movie of it happening you will see that the ball actually moves forward. The wagon moves forward, but faster, so the back of the wagon hits the ball.

vanhees71 and PeroK
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But when the car accelerates forward, you still want to be at rest so your upper body moves a little backwards.
No, your upper body doesn't move backwards - not even a little. It is the now-moving car that pushes on your lower body. Your still-at-rest upper body will just follow when your lower body will pull it (or the seat backrest will push it).
But with a bicycle, it is different
No, it isn't. but the acceleration produced by human force might not create enough force between the bicycle and the biker for the biker to feel it.

But sit on a powerful motorcycle under hard acceleration and without a proper backrest the motorcycle will not push you, it's the handlebars that will pull you (and you better hold on tight).

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Perception is powerful. I was once at a red light and my foot unknowingly let up on the break. Since I was “stopped“ with my foot “on“ the break, it actually felt like the world was moving towards me until I realized what was happening.

sophiecentaur