# A little question

1. Oct 23, 2004

### tyybo

This is my theory. Im just curious what would happen.
Lets say that we have a truck with a long trailer with a ramp dropped down to the ground moving at 70mph. Now we have a car that is directly behind the trailer also traveling at 70 mph. now if the car accelerates just enough to get all 4 tires onto the trailer what would happen?

My theory is that one of these would happen.
1. The car would accelerate at about double the speed of the car and smash into the front of the trailer.
2. the car would slowly advance up the trailer and be able to stop itself?
Tell me what you think would happen

2. Oct 24, 2004

### Cyrus

SMASH! Once all four tires get on the trailer you better get your foot off the gas pedal and fast! Its easy enough to see what happens if the cars sitting on the trailor with the tires not moving, it just gets carried along by the trailor. Look at the nice new cars go down the road when there on the back of a big truck being shipped. So if ur sitting in that car being carried along and decide to floor it, odds are youll go smashing into the cab of the truck. Leaving one not to happy trucker with a dent in the back of his cab, and alot of explaining by you. :-)

3. Oct 24, 2004

### tyybo

hahaha see thats my opinion. but some other people that ive asked say that if you like hit the clutch, you could stop real fast. and i dont htink so i think once you hit, itd just launch across the trailer

4. Oct 24, 2004

Depends if its front wheel, or back wheel drive

5. Oct 24, 2004

### Staff: Mentor

Consider what happens when a plane lands - in a puff of smoke, the tires quickly accelerate to the speed of the plane. If you're not on the gas, the tires of your car will do the same thing (except they will decelerate).

6. Oct 24, 2004

### Cyrus

I dont think that would happen russ. With respect to the airplane, the tire is not rolling. When it comes into contact with the ground, the ground is moving fast. It has to instantaneously speed up to match the forward speed of the airplane. So it will burn out until friction brings it to the same speed.

With respect to the trailor, the tires already moving at a certain speed. The ground is stationary and the tire rolls without burning out. Nothing changes when you drive up onto the trailer. In that case the "ground" of the trailer is just as stationary as the ground of the road. So once you get up onto the trailer, the car will continue going its speed as if it were on the road the whole time. So the result is that it goes 70mph into the trucks cab. I think Ive seen stunt men do this on tv and I dont recall the tires burning out when they go onto a trailer.

Because lets say you dont keep your foot on the gas as you drive up the ramp. The car will just continue to roll forward and loose speed as friction takes over. It would be like coasting, but u run out of road in front of u real fast, because the trailer is short.

Ah, I forgot the fact that the trailor is moving forward. So it would be like you are coasting, but you would coast a much smaller distance. You would coast 70mph and go slower, but the truck always goes 70mph faster. So you would come to rest on the trailer and find yourself rolling out the back if you did not hit the brakes to keep on the trailer.

When the car gets on the trailer, assuming u keep ur foot on the gas, it should now move at a speed of the car + speed of the truck, relative to a person standing on the sidewalk watching this happen.

Last edited: Oct 24, 2004
7. Oct 24, 2004

### T@P

I think that cyrusabdollahi is right. consider it in the frame of the truck. You essentially have two objects that are not moving with respect to each other. Then one of two things can happen: if you have front weel drive your back wheels wil simply roll and you will have to step onto the brake quite hard. Or, your rear wheels would push you on and your front wheels would hit the back of the trailer. (you would have to break harder then)

In a sense russ you are right because the trailer is at an angle and depending on how good your tires are they might not push the car up but simply spin out of control becaus the rest of the car hasnt been pushed up. In that case, there would be many puffs of smoke. The real problem lies in determining with wether your car will make it onto the trailer with or without the smoke, which depends on the wheels. However this changes the problem into a more quantitative one which depends on the friction of the car and road, trailer, weight of the car, etc. It would be interesting to try it out though :)

8. Oct 24, 2004

### Cyrus

The next time i see a trailer truck, im going to floor my honda into the back of it and see what happens. :-)

When the tires burn out it is a result of the tires having to speed up or slow down to move with the same speed as the ground. In the trailer, the "ground" is moving at 70mph. The tires are moving just a little bit above 70mph. So russ is right im sorry, the tires should burn out, but it would be such a small amount i dont think anyone would notice. Whereas an airplane u can really see that puff of smoke. I think the car would just make a slight squeel of the tires, but not any visible smoke. The tires would have to slow down until they are going 70mph too.

Last edited: Oct 24, 2004
9. Oct 24, 2004

### Integral

Staff Emeritus
Think about it.... You and the truck are moving at about 70 mph, your differential speed is small so when your car runs up on to the truck your tires want to be going 70 mph, but relative to your car the truck is moving less then 10mph, your tires must slow down a lot to match that speed. I'll bet that if you are not careful you could destroy a set of tires.

10. Oct 26, 2004

### tyybo

I agree with almost everything you said cyrus sept the movie thing. keep in mind ther is a difference on what you can do with a camra, and reality. he i wish ther was a way i could do this.

11. Oct 27, 2004

### Cyrus

Im wrong tyybo totally wrong, flat out wrong. dont listen to me. I tested out the experiment. Heres what you do, get a nice toy car, it has to be big enough that you can easily see the wheels turning when you roll it. I used a die cast car that you can find at a toy store. There the scale metal cars that are pretty big, like 5 inches or so. You can easily see the tires moving. Put it on your carpet. Then get a "truck" that is moving. I simulated this with a flat newspaper, since it was long incase i overshot i still had plenty of "truck" left, hehe. Try rolling the car on the carpet. The tires spin AND the car moves forward. Now roll the car at some moderate speed from the carpet, so that it rolls onto the newspaper. The instant it gets totally on the newspaper, continually pull the newspaper in the same direction. Now look at the tires. They sit there spinning! The car appears to be stationary on the newspaper when the speeds start to match, but the tires are still moving. A half second or so later ull notice the tires dont spin and it gets carried along. So we can see that the tires are decelerating, they have to be. IF the tires were spinning and gripping the "truck" in our case the newspaper, the truck would go forward AND the car would go forward too. But when you do the experiment, the car sits there! So the tires are spining but the car sure does not go anywhere. So the tires are not having any grip with the newspaper. They are slowing down until friction lets them grip the newspaper and they can propell the car forward once again. If you sat there with your foot on the gas, the tries could never slow down to gain grip, so one of two things could happen. There could be enough static friction between the cars tires and the truck to hold you in place. If not, you would SLIDE not roll right out the back of the truck. You sit there with your foot on the gas, the tires spin but dont grip, and you slide right out.

Now try launching the car faster and faster, you will notice that the momentum carries the car further down the newspaper before it comes to a top (tires still spining though), because of kinetic friction between the tires and the truck. Its like sliding a block on a table, kinetic friction. same deal here, even though the tires are spinning they have no grip, so they just slide right across the truck when the car first rampages onto the ramp.

If you let the tires slow down to get a grip, then the car will move forward while the newspaper also moves forward. You can try this too. Let the car get on the newspaper and watch the wheels slow down as you pull the newspaper. But dont let the wheels stop this time, pull the newspaper much slower, you will see that the tires can grip better now, and the car suddenly launches forward while you also move the newspaper forward at this slower speed.

I dont think I would have understood what russ was saying until i found a way to see it for myself. TRY the experiment it makes it clear as day when you do.

Last edited: Oct 27, 2004