Easy physics question

[sahtt]

You are accelerating on a motorcycle. While still accelerating, you pull the clutch in stopping any power from the engine at 60mph.

Intuitively I'd expect velocity to still increase [go beyond 60mph] before wind resistance and friction eventually decrease the velocity of the motorcycle. However, with no additional force I can't think of what is causing the acceleration since if velocity is still increasing then acceleration is positive.

I want to say it's positive-still accelerating, but at a decreasing rate. Once velocity starts to decrease, say it goes from 69mph->70mph->69mph, acceleration becomes negative. I guess what I'm missing is without the force of the engine, what is responsible for the increase in velocity [assuming I am thinking about this the right way]? I want to say momentum and know-

F t = m v and force = mass x (velocity / time) = (mass x velocity) / time = momentum / time but I still can't figure it out and it's pissing me off.

Ok, so have at it, should be easy.

 

[DRIFTER]

My guess would be reduced parasitic drag from the engine / trans/ gears?

 

[sahtt]

I think I managed to leave out the actual question.

Upon pulling on the clutch lever and disengaging the engine from the drivetrain, does the motorcycle's velocity still increase given it was accelerating up to that point?

 

[Daedalus]

with no additional force I can't think of what is causing the acceleration since if velocity is still increasing then acceleration is positive.

I believe this is the case--you immediately begin to slow down the moment you grab the clutch.

 

[sahtt]

I believe this is the case--you immediately begin to slow down the moment you grab the clutch.

If you are going at a constant velocity, then you definitely begin to instantly slow down. But I have to believe that if you are accelerating up to that point, you still carry some momentum and your velocity increases a little.

For instance, if I am on my sport bike and am going WOT up to 50mph in first gear, then instantly let off, there is no way my bike is not going to go above 50mph. I actually just tested it this way-on the 4 lane feeder road going over a bridge I went WOT in 1st gear until redline, due to the powerband it is still accelerating at that point sharply. Redline was something like 70mph at 15.5k RPMS. However, I eventually reached 82mph, then it steadily started to slow down. The bike literally will not go 82mph in 1st gear and the digi speedo is not that far off, 82 in 1st is a couple thousand RPMS past redline.

 

[Sig]

Assuming flat ground, all things equal, blah blah.... if it had not done so already prior to pulling the clutch, the rate of acceleration will immediately begin to decrease. However, that does not mean that you will immediately stop accelerating.

 

[sahtt]

Assuming flat ground, all things equal, blah blah.... if it had not done so already prior to pulling the clutch, the rate of acceleration will immediately begin to decrease. However, that does not mean that you will immediately stop accelerating.

Right, that's what I inititially thought as well. But in order for that to occur, acceleration would still have to be positive [but decreasing towards zero then negative] without any force acting on the bike, which is impossible.

So to the original proposition, would your velocity [mph] still increase at all after you pulled the clutch in? If acceleration is still positive, it does increase, otherwise it does not.

 

[cojones]

I think I managed to leave out the actual question.

Upon pulling on the clutch lever and disengaging the engine from the drivetrain, does the motorcycle's velocity still increase given it was accelerating up to that point?

I think it would accelerate a little bit... assuming conservation of momentum (= mass x velocity), pulling the clutch in releases the drag of the transmission/etc; the friction reduction effectively acts like a weight reduction, and the speed will increase. At some point rolling friction kicks in and you'll eventually coast to a stop, hopefully before you hit the wall.

Its kinda like when you go off in the grass... the friction coeff of tire on grass is much lower than grass on blacktop, thus the vehicle accelerates...

 

[Daedalus]

Acceleration cannot be stored.

F = m * a

What are the sum of forces on the vehicle? F must be positive for a to be positive. If the engine isn't hooked up to the rear wheel, what provides the force that overcomes the others?

 

[ALSWEL]

I am not a physics major but I have to disagree with this. When a pitcher throws a pitch it immedately begins losing speed as soon as it leaves his hand. Whe you hit a golf ball it begins to lose speed as soon as it leaves the club head. I don't supose in a vacuum it would make a difference, the ball would just keep moving at the same rate. What accelerates a 'ball' once it leaves the the driving force? Nothing, it can't accelerate anymore.

Waiting to be corrected.

 

[Vega$ NSX]

I am not a physics major but I have to disagree with this. When a pitcher throws a pitch it immedately begins losing speed as soon as it leaves his hand. Whe you hit a golf ball it begins to lose speed as soon as it leaves the club head. I don't supose in a vacuum it would make a difference, the ball would just keep moving at the same rate. What accelerates a 'ball' once it leaves the the driving force? Nothing, it can't accelerate anymore.

Waiting to be corrected.

He is correct.

Once you disengage the clutch, you disengage any forces associated with acceleration. You should theoretically immediately start slowing down. As to why you may have sped up in your experiment, I'm not sure. Perhaps you had a tailwind or a slight decline to the road. However, in a controlled experiment, you should slow down as soon as you depress the clutch. To confirm this, wear an accelerometer and repeat the experiment. It should immediately go negative as soon as you push the clutch in. Imagine a drag strip, where people accelerate as fast as possible. As soon as you clutch to change gears, the car decelerates and your head lurches forward. That is a clear indication that forces are trying to push your car backwards.

Momentum (which has the equation of p=mv) really has nothing to do with acceleration. It is best used in collisions and particle interaction, but does nothing to influence forces or speed. It explains why a huge semi can cause the same kind of damage going very slowly as a motorcycle would make going very fast.

 

[C-speed]

^ Correct.

Once you remove the force accelerating an object, the object will remain at a constant speed until (unless), another force acts upon it, in this case, friction and drag.


Just like firing a gun, the bullets maximum velocity after it is fired is when it exits the muzzle. It will never be any faster than that point in its trajectory (muzzle velocity) - since the accelerant (exploding gun powder) no longer accelerates the bullet once it leaves the muzzle.

 

[cojones]

Once you remove the force accelerating an object, the object will remain at a constant speed until (unless), another force acts upon it, in this case, friction and drag.

My answer re: increase in speed (i.e., effective reduction in mass = increase in velocity) is partly dependent on what gear are you in, wrt to road speed... when you start to coast.

If you are off the gas (feet off all pedals) at 60 mph, in high gear, chances are you are already entering some amount of engine braking, depends on the gear/diff ratios, etc. Engaging the clutch disconnects the tranny load, and releases the engine braking effect.

Eventually rolling friction and drag resistance/downforce will slow you down.

 

[stuntman]

^OP says the clutch is disengaged, not lifting off the throttle.

As soon as you stop the acceleration force (in gear, under load) you immediately start slowing down.

I've datalogged professional racecar drivers lose 2-3mph during each of their shifts in production-based cars. Using the clutch (same concept as you), in few tenths of a second it takes to upshift, they already have lost speed. The really good drivers, overly abusive, some wreckless drivers can shift fast enough to have the next gear engaged with the clutch out before loosing speed.

To answer your question - you will lose speed the second you pull in the clutch.

 

[sahtt]

I have come to the same conclusions mathematically. Once you draw the graphs it's clear. Without a force, it cannot continue to accelerate. If that was the case, in outer space or a 'zero' friction area if you accelerated to any degree you would infinitely accelerate without any extra force which is again impossible.

The hard part is you have to rely on your speedometer [and a digital one at that] as the more reliable RPMS are not useful once you pull in the clutch [obviously].

 

[C-speed]

There is always the exception to the rule...

Was the bike on a treadmill? :biggrin:

 

[BD]

So, why would the bike be any different from the car? If you accelerate in a car, the car slows down the moment you let go the gas peddle, unless you're going down hill.

 

[usafdarkhorse]

I am not a physics major but I have to disagree with this. When a pitcher throws a pitch it immedately begins losing speed as soon as it leaves his hand. Whe you hit a golf ball it begins to lose speed as soon as it leaves the club head. I don't supose in a vacuum it would make a difference, the ball would just keep moving at the same rate. What accelerates a 'ball' once it leaves the the driving force? Nothing, it can't accelerate anymore.

Waiting to be corrected.

He's right.

You can accelerate in a negative direction ( decelerate ) and still be moving in a positive direction.

In a pendulum, the weight swings left and right and has a dip in the middle.
When the pendulum swings from right to left, it's accelerating until it hits the dip in the middle, where it starts decelerating, but it still moves to the left until it stops at the extent of the travel. Then acceleration is completely in the right direction and the cycle repeats to the right.

 

[slurpee]

If its a digital speedometer, there is an algorithm involved. As digital speedometer cannot simply indicate the speed 10 times per second, they must do some kind of sampling or sampling and averaging. Perhaps yours also includes change in velocity over time (acceleration) in its algorithm, effectively 'predicting' your speed.

 

[Miner]

There is always the exception to the rule...

Was the bike on a treadmill? :biggrin:

I thought about saying it, but thought better.:wink:

 

[PeterM]

the only thing affecting acceleration on any object is its net force and of course its mass. being that mass is constant, then the only thing affecting its acceleration must be the net force. but upon pulling the clutch in, what force do you have accelerating your bike in the forward(positive) direction?

what are you feeling can be due to the fact that engine and gearbox drag which acts similar to brakes(engine braking) and so when the you pull in the clutch to disengage the wheels from the gear box, you are going from negative acceleration to constant velocity which can produce a false perception of positive acceleration, when in fact it's not.

 

[JonBoy]

If you are accelerating (ie, non-constant velocity) and pull in the clutch, the motorcycle could still increase in velocity. It happens all the time in my S2000 - 2nd gear ends at 57 mph but I can still hit 60 mph between the 2-3 shift, since I've given the car an IMPULSE. The acceleration decreases when I put in the clutch but the velocity continues to increase. The drag on the car is not sufficient to keep the car from still accelerating but rather, it just decreases the acceleration.

However, if you are not accelerating (ie, constant velocity) and pull in the clutch, the car's velocity will immediately start to decrease.

The kicker is that at higher speeds, the drag forces are typically high enough to reduce the effect of the impulse, in which case the motorcycle (or car) may immediately start to decrease in speed as soon as you put in the clutch. I saw a test where a Champ Car (Bourdais' car) had 1.2g of deceleration as soon as they let off the gas at 200+ mph! So, obviously, velocity plays a large factor in drag, which will also affect whether or not the vehicle continues to increase in velocity.

Remember, velocity can still increase even if acceleration is decreasing.

Short answer:

Pull in the clutch and acceleration will definitely be reduced but velocity may continue to increase for a short period of time. Higher velocity will significantly impact how quickly the vehicle stops accelerating positively and may, in fact, cause acceleration to (more or less) immediately become negative.

 

[C-speed]

If you are accelerating (ie, non-constant velocity) and pull in the clutch, the motorcycle could still increase in velocity. It happens all the time in my S2000 - 2nd gear ends at 57 mph but I can still hit 60 mph between the 2-3 shift, since I've given the car an IMPULSE. The acceleration decreases when I put in the clutch but the velocity continues to increase. The drag on the car is not sufficient to keep the car from still accelerating but rather, it just decreases the acceleration.

However, if you are not accelerating (ie, constant velocity) and pull in the clutch, the car's velocity will immediately start to decrease.

The kicker is that at higher speeds, the drag forces are typically high enough to reduce the effect of the impulse, in which case the motorcycle (or car) may immediately start to decrease in speed as soon as you put in the clutch. I saw a test where a Champ Car (Bourdais' car) had 1.2g of deceleration as soon as they let off the gas at 200+ mph! So, obviously, velocity plays a large factor in drag, which will also affect whether or not the vehicle continues to increase in velocity.

Remember, velocity can still increase even if acceleration is decreasing.

Short answer:

Pull in the clutch and acceleration will definitely be reduced but velocity may continue to increase for a short period of time. Higher velocity will significantly impact how quickly the vehicle stops accelerating positively and may, in fact, cause acceleration to (more or less) immediately become negative.

This is absolutely and physically impossible. Just like the previous examples given above, if a pitcher throws a ball, the fastest the ball will EVER be in flight (velocity) is when the ball leaves the pitchers' hand. From that point on, it decelerates. Why? Because the pitcher is accelerating the ball, once he lets go, there is no more acceleration.

Same goes with the example I gave with the bullet. The fastest the bullet will ever be is the moment it leaves the muzzle. Again for the same reason.

This is true for a vehicle.

sahtt said it just right like 6 posts above. If your analogy were true, then in space if you threw a ball (for example), the ball would continue to accelerate - infinately - even after you let go of the ball, since there is no force slowing it down (in this case, drag). That is impossible! If you threw to ball and accelerated it to say 45mph. The ball would simply continue in a straight line at 45mph - its velocity will never exceed 45mph (unless/without another external force acting upon it)

 

[Vega$ NSX]

Impulse, like momentum, is typically used in describing collisions. Impulse is the change in momentum, or more specifically the rate of change in momentum (force over time). It would describe how quickly the change in momentum occured (i.e. let off the clutch fast or slowly). It isn't a force that would keep pushing the motorcycle further.

 

[Kitaka]

Linear Physics: V=Vo + at
where V= speed, Vo= speed at time zero
a= aceleration. t= time

Assuming no friction loses, you would continue at the speed that's occurring at the time (t= 0)you depress the clutch (there is no acel or decel) You would continue at this speed until friction, wind resistance, gravity, or another mass interferes with your travel.