Mendeh

I've been trying to find the answer to this, but haven't been able to track one down. It seems pretty simple, though, and I wonder whether one of you nice people could explain it to me:

Why do we get pressed back in our seats when we accelerate (in, say, a car)? What is it that is doing the 'pressing back', as it were? How come we can tell when we're accelerating, but we can't tell if we're moving at a constant speed?

Fastfalcon

I think that it's to do with Newton's second law of motion (force = mass x acceleration) and Inierta(sp?).

enigma555

Mendeh,

The simple answer is inertia. If I'm reading your question right, however, you aren't asking the simple question... you're asking the "why?" which is related to the order of the universe.

I'll answer the simple question, and hope it gets you rolling.

Mass resists change in motion. That means that regardless of if a mass is moving at a speed or is sitting at rest, it requires an applied force to change it's speed.

If you are in a car moving at constant speed, all the mass of both you and the car are moving at that speed. When the driver hits the brakes, a force is applied to the spinning wheel, attempting to slow them down, which causes a friction force to show up in the contact point between the wheels and the ground, which slows the car.

Your body still wants to go forward, however. You are slowed down by the seatbelt pulling back on you. The seatbelt is slowed by the frame, which is slowed by the suspension which is slowed by the wheels which is slowed by the ground.

I think it's beautiful, really... but I'm a big physics dork, so I'm biased.

Skydancer

Okay... where to begin? First thing, we have to shoot all the misconceptions. Lots of 'em here.

1) You don't feel acceleration. Ever. What you feel are forces, which may _cause_ acceleration. You also cannot feel the force of gravity directly, unless (theoretically) you are so close to a point mass that the acceleration of gravity is significantly different across your body. This is not an issue in the real world.

2) If you are moving at constant speed, or standing still (which is a constant speed of zero), the forces acting on you, if any, are balanced. Since you cannot feel gravity, the only force you detect with your direct senses is the one that balances gravity - the surface you are resting on pushes upward with a strength equal to your weight so that you don't fall through it.

3) If you are _changing_ your velocity, you are accelerating, by definition. Newton first realized that this requires an unbalanced force acting on you - one that isn't cancelled out. If you are seated in an accelerating car, it is the back of the seat which pushes on you that makes you accelerate - and this is a force you can feel, so your body reacts to it. Note that you still feel the seat pushing up on you as well, since you are not falling through it. The combination usually makes you feel slightly heavier and the apparent direction of down shifts toward the back. It is especially noticeable in a jet take-off - it usually feels to the passengers like the jet is moving up a significant slope, but this is simply the combination of the different force directions.

4) If you are in a falling elevator or airplane or in orbit in a spacecraft, you are accelerating, but in these cases you are accelerating due to unopposed gravitational force. Since you cannot feel gravity, this results in the absence of a sensorily detectable force on you, and the resulting condition (the absence of the expected pressure of some underlying surface) is inaccurately called 'weightlessness', a term popularized by clueless journalists during the Apollo Project. Your weight is still there, however - the space shuttle in orbit has about 95% of its ground weight - and the unbalanced weight causes you to accelerate downward (yes, an orbital vehicle is technically falling. It is also moving sideways so fast that by the time it reaches the point where the ground should be, the earth's curvature has moved it away. An orbit is basically falling _and missing the ground_.)

Edited to remove comment about the forum - forgot which forum I was in. Doh!

[ May 31, 2002: Message edited by: Skydancer ]</p>

Vinnie

An object at rest remains at rest unless acted upon by an outside force. Basically, as the car moves faster you would not move faster unless it was pushing you. Its like when you slam on the breaks. The car stops but you don't automatically stop. You stop when acted upon by an outside force (e.g. a seatbelt or possibly the windshield).

An object in motion remains in motiuon unless acted upon by an outside force. When you are traveling with a constant velocity. It all has to do with inertia and newton's laws I think as fastfalcon pointed out.

Actually, turning corners is acceleration as well because its a change in direction. That is why if you turn right reel fast in a car you may slide or move to the left. Your body wants to stay where it is until its acted upon by another force (the friction of the seat, a seat belt, the door). If you are in physics now you will probably talk about a washing machine and why the clothes stick to the side and how changing directions is acceleration even if you are going the same speed.

Take a baloon with you the next car ride. See what happens to it as you speed up, slow down and turn corners. Check out Newton's laws too.

""""How come we can tell when we're accelerating, but we can't tell if we're moving at a constant speed? """"

Technically I think that question is wrong and only applies to linear acceleration. As I said above, turning is considered acceleration even when one turns at a constant speed.

Vinnie

Godless Dave

However none of us are really at rest because we're standing (sitting in my case) on the surface of the earth, which is turning. Am I right that we can't feel that acceleration because the force of gravity holding us to the ground is so much stronger?

JoshuaT

Mendeh,

Skywalker provided an excellent description of the forces involved. However, let me offer a simple thought experiment to make these ideas a bit more concrete (pun intended) to you.

Imagine swinging your fist at a concrete wall. When it hits, it stops very quickly, ie it deaccelerates. The wall does not move (much) so it presses against your fist. Can you imagine this? Are you confused about why your hand would hurt if you did this? I suspect not!

The car seat pressing into your back is exactly the same phenomena. The car is punching you in the back. However, you are more movable than a concrete wall, the car's accelaration is much more gentle than punching a wall, and the force is spread out in a much larger area, so you just feel a gentle pressure rather than eye-popping agony.

Hope that makes it vividly clear.

Skydancer

HEY! That's Skydancer, not Skywalker. I am not a Star Wars freak. Hmmph. The very idea....

Kosh

We aren't accelerating. We're already moving at
that speed, constantly, day in, day out (relatively
that is).

Vinnie

"""We aren't accelerating. """

Isn't a change in direction acceleration? Couldn't the earth be said to be continually accelerating as it travels round and round the sun?