# Car in motion

Im new here and I really dont know the standard sorry if this is too easy for you,
There is a car moving at constant speed and suddenly the driver applies the brakes. So everyone in the car moves forward. But as the car holts the people go forward. Why? With reasons.
I have a decent solution but I dont think its right.

I thought it this way........... imagine the brakes are a force opposing the motion of the car so due to the 1st law the people will go forward. But after that to achieve equilibrium the car goes backwards. Do you think the answer is right?

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brewnog
Gold Member
People here won't just do your homework for you.

I'm not exactly sure what you're saying, but I think you're close..

Find a connection between the first law and the passengers. What are they doing when the car freezes?

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they are moving forward when the car freezes...... how can that help? But I have noticed that the whole car moves backward when the car freezes. That means it is not related to the passengers. I think that when the brakes stop the car the car exert a force on the brakes so they displace and so to come back to their original position the exert force on the car which makes it move backward.

Newton's First Law. The passengers in the car travels with the velocity of the car, but when the car is stopped (by the braking force), the passengers keep going at the velocity of the car because there are no forces stopping it.

HallsofIvy
Homework Helper
Do you mean that the car moves backward relative to the people? Certainly putting the brakes on applies a force to the car, not directly to the people. The car will slow down, the people will not- until something in the car (seat belts, the dashboard, the windshield) applies a force to the people to slow them also. During that time, the people will move forward, hopefully not far, in the car and we might say that the car will appear to be moving backward to those people. Of course, relative to the ground, the car is still moving forward, only not as fast.

Danger
Gold Member
HallsofIvy said:
Do you mean that the car moves backward relative to the people?
I'm wondering if he might mean the very slight rebound effect of the deformed tires and suspension system regaining their original shape after a panic stop...

brewnog
Gold Member
Danger said:
I'm wondering if he might mean the very slight rebound effect of the deformed tires and suspension system regaining their original shape after a panic stop...
I think he might also be getting at the way that (say) a passenger's head will move forwards under braking, and then after the car has stopped, the passenger's head will move back. This is, of course, just down to the passenger returning to its normal seated position accompanying the restoration of balance of forces on the passenger.

Also, (and here I'm just making sure that the OP does understand), a car's brakes do not provide a 'backwards' force on the car, they provide a force in opposition to that of movement.

hey,
danger understood me, thats what I mean, and the relative thing is not what I mean because the car does go backward as seen from an outside inertial frame slightly.
And Brewnog the passengers head do move back but because of the force of the brakes not the normal seating position.
And something else, brakes stop the tyres from moving so when the tyres stop suddenly they exert a force on the brakes (3rd law) so the brakes move from their initial position and try to come back(elasticity) which in turn exerts a force which I think is the possible answer

What are you talking about? I think futb0l gave you the right answer. Also, the backward motion might be the frame of the car realigning itself with the tires.

what??? fytbol is talking about the jerk forward not backward. Which is not my question.

Gokul43201
Staff Emeritus
Gold Member
Just look at this whole thing in the original moving frame (of the car before brakes were applied). In this frame the action of the brakes is equivalent to a backward force on the seat, causing separation between the person's head/torso and the seatback. When the brakes stop acting (ie : car in in rest in the earth frame), this backward force is removed, and the person's head/torso return to the upright position only due to (i) the action of seatbelts, and (ii) the fact that this is the equilibrium/comfort position of the person....unless I'm missing something.

PS : Stupidkid, you have a typo in the OP. You say "forwards" when you mean "backwards" for the second jerk.

hey gokul43201,
i dont think your answer is right because even if you dont have a seatbelt this happens
and the WHOLE car moves backward. did you understand?
PS: whats a "typo in the OP.

brewnog
Gold Member
stupidkid said:
hey gokul43201,
i dont think your answer is right because even if you dont have a seatbelt this happens
and the WHOLE car moves backward. did you understand?
PS: whats a "typo in the OP.

Ok, I think I know what you're getting at now.

The person will move back because that is how the human body naturally rests in such a seat, under equilibrium conditions. If I pulled you up out of a chair, and then let go, you'd return to the chair. The passenger is simply returning to its equilibrium condition, much like pushing a pendulum and it returning to its lowest position.

I don't know about you, but when I apply the brakes from travelling forwards in my car, I don't end up going backwards. There may be a slight rebound effect, as Danger (?) mentioned, caused by the elasticity in the tyres, and any slack in the suspension linkages.

brewnog
Gold Member
Ok, I've got it.

stupidkid said:
And Brewnog the passengers head do move back but because of the force of the brakes not the normal seating position.
If you crash into a wall, the passengers' heads will move forwards at first (you understand this, Newton's First Law). They'll then move backwards again. It IS because of the natural resting position of the human in its seated position. The force pulling the heads (and torsos!) back is done by the human body (and slightly by inertia-reel seatbelts). There is no other external force pushing the people back into their seats, assuming the car eventually comes to rest.

you all are saying stuff about natural positions and all blahblahblah but I think the answer lies somewhere in the displacement of the brakes because when this happens the force backward is quite alot when braking suddenly. That force cannot be due to any natural head position. It is found that the WHOLE CAR MOVES BACKWARD. so how can that be due to naturalblahblhal blah.

brewnog
Gold Member
stupidkid said:
you all are saying stuff about natural positions and all blahblahblah but I think the answer lies somewhere in the displacement of the brakes because when this happens the force backward is quite alot when braking suddenly. That force cannot be due to any natural head position. It is found that the WHOLE CAR MOVES BACKWARD. so how can that be due to naturalblahblhal blah.
The whole car does NOT move backward considerably. Any (small, perhaps a couple of millimeters) backwards movement once the car has come to a halt is due, as you have been told several times by several members, to the elastic nature of the tyres, and possibly some slack in the suspension mechanisms.

If you don't believe us, perhaps you'd care to explain how a braking system would cause motion of the car in the backwards direction.

EXPLANATION
Lets say that the brakes apply a force on the tyres to stop them. NOW due to NEWTONS 3rd LAW there will be a force on the brakes. So by a small quantity the brakes will move, now considering that they dont brake the elastic limit they have to come back to their original position. And hence they exert a force on the car which makes the WHOLE car MOVE BACKWARD.
If anyone has a different answer to this pls post.

Doc Al
Mentor
Forget about the brakes applying a force to the wheels. While true, the external force that stops the car is the force of friction that the road exerts on the tires. That's the force that stops the car. (Step on the brakes all you want. But if there's no friction--say you're on a patch of ice--the car won't stop.)

The answer to your original question is much simpler that you seem to want to believe. As others have explained, Newton's 1st law says that an object will keep moving in a straight line at constant speed unless a force acts on that object. The friction slows the car, but the contents of the car keep going until they are slowed: by seat belts, seat friction, or hitting the dashboard.

brewnog
Gold Member
Doc Al said:
Forget about the brakes applying a force to the wheels. While true, the external force that stops the car is the force of friction that the road exerts on the tires. That's the force that stops the car. (Step on the brakes all you want. But if there's no friction--say you're on a patch of ice--the car won't stop.)

The answer to your original question is much simpler that you seem to want to believe. As others have explained, Newton's 1st law says that an object will keep moving in a straight line at constant speed unless a force acts on that object. The friction slows the car, but the contents of the car keep going until they are slowed: by seat belts, seat friction, or hitting the dashboard.

This isn't what he's asking. He's being unclear, and not listening to the valid answers he has been given.

stupidkid said:
EXPLANATION
Lets say that the brakes apply a force on the tyres to stop them. NOW due to NEWTONS 3rd LAW there will be a force on the brakes. So by a small quantity the brakes will move, now considering that they dont brake the elastic limit they have to come back to their original position. And hence they exert a force on the car which makes the WHOLE car MOVE BACKWARD.
If anyone has a different answer to this pls post.

Yes, as myself, Danger (and perhaps some others) have told you, this is due to slight deformation in the braking and suspension systems, and tyres.

The backwards movement of the car once it stops is NEGLIGIBLE (perhaps a millimeter or so, depending on the deceleration under braking). This is NOT what causes the passengers to return to their seats following an emergency stop.

okay so the natural head position causes it then?

brewnog
Gold Member
stupidkid said:
okay so the natural head position causes it then?
Causes what?

You're talking about two separate things.

The reason the CAR moves backwards EVER SO SLIGHTLY is due to slight deformation of the braking and suspension systems, and tyres.

The reason the PASSENGERS move backwards is due to their natural seating position.

OlderDan
Homework Helper
My \$.02

I've been sort of following this discussion, and think perhaps a fresh look is in order. I'll make two observations.

1) This problem is not about the car moving backwards because of some elastic extension of tires and brake pads. It is about the occupants moving forward during the braking phase and moving backward when the car stops. (I assume Gokul43201 correctly observed that the OP was mistyped, and the motion in question is the backward motion of the occupants relative to the car once it stops decelerating.)

2) Talk about "natural seating position" and references to equilibrium positions are, in my opinion, not helpful and not particularly relevant. The occupants of the car are not oscillators subject to some position dependent restoring force.

What has been correctly observed by some is that the occupants have to be stopped by some force (seatbelts, windshield, etc., have been mentioned) other than the direct force stopping the car, but the actual force that stops the people (in most cases) has barely been mentioned, though it was touched on here

brewnog said:
The force pulling the heads (and torsos!) back is done by the human body.
Before there were seatbelts people still managed not to crash into their windshields. They did this by applying a force to hold their bodies in a rigid configuration with pressure from the seat and floor applying a torque that keeps them from rotating and a frictional force by seat and floor, or pressure on feet against the firewall that decelerates their center of mass. Human muscles account for the torque that stops the rotation of the head and torso about the hips, and of the head about the neck.

The reason the occupants move forward in a panic stop is because they are out of phase with the forces stopping the car. They don't get their force working until there has been a forward displacement or rotation that they manage to counter eventually to maintain a stable configuration relative to the car. And the reason they move backward is because they don't stop applying this force in phase with the forces acting on the car. They keep applying the torque to their upper bodies and the push against the seat and floor because they do not anticipate the end of the car's deceleration. There is no "equilibrium position" that they fall back into. The occupants do what they have to do to decelerate, and they keep on doing it for a while after the need is gone. They simply over-react and produce a force/torque that pushes them backward relative to the car and straightens the torso once the car stops, or greatly reduces its deceleration

Every good driver in the world learns to control deceleration to give self and occupants time to adjust their body forces to maintain their seating position without jerking around. Pay attention to yourself the next time you drive and you will notice that you gradually release pressure on the brakes as you stop to gradually lower the rate of deceleration instead of having it abruptly end. There is a reason why the rate of change of acceleration is called "jerk."

brewnog