# Acceleration and Velocity

1. Apr 24, 2006

### jrm2002

We know,
a=F/m
a=acceleration
F=Force
m=mass

Greater the force applied greater would be the acceleration.If force is constant then for a particular mass the acceleration(rate of change of velocity) would remain constant.Right?

Now, let us say I press down the accelerator of my car.I keep the accelerator at a constant level , i.e. after presing the accelerator wioth my foot to a certain distance down, I neither raise my foot above or below.Then, in such a case,

Am I not maintaining a constant force?
Should not the acceleration remain constant?
i.e. should not the velocity vary uniformly(linearly)?
But the needle of the spedometer of the car shows no change in speed in such a case?
But the force is constant as I keep the accelerator at a constant level , i.e. after presing the accelerator wioth my foot to a certain distance down, I neither raise my foot above or below.
Plz. help!!!

2. Apr 24, 2006

### dav2008

Think of this situation: you release the gas pedal completely. You're clearly not applying any force, yet the car experiences acceleration in the opposite direction of motion (deceleration).

Why does the car slow down?

3. Apr 24, 2006

### Staff: Mentor

No, you are not. An engine's torque is not constant with rpm, plus there is an rpm limit - then you have to upshift. See the torque curve:

http://www.auto-ware.com/combust_bytes/p_goal.htm

Last edited: Apr 24, 2006
4. Apr 25, 2006

### jrm2002

To:dav2008

dav2008:

you release the gas pedal completely. You're clearly not applying any force, yet the car experiences acceleration in the opposite direction of motion (deceleration).
Ya-- but I am sorry as I am not familiar with mechanical engineering part of the vehicles.Shall be extremely grateful and obliged if you can explain the answer to my question and also tell me why the car experiences deceleration when the peddle is released fully?

5. Apr 25, 2006

### Kit

i think u miss the contribution of the fuel of the car too.

this is not simply u press the accelerator and the the car would accelerate. there are many other things would be involved.

hope it helps^^

kit

6. Apr 25, 2006

### Hootenanny

Staff Emeritus
The is due to the force of drag or fluid friction. The drag force experienced by a car can be represented as;

$$F_{drag} = \frac{1}{2}\rho CAv^2$$

Where $\rho$ is the density of air, C is the drag co effiecient, A is the cross sectional area of the car and v is the velocity.

This represents an unbalanced force acting on the car, which will cause a negative acceleration. Rolling resistance (of the tyres) and friction of other mechanical parts would produce a retarting force upon the car.

~H

7. Apr 25, 2006

### jasc15

the mechanics of the car just contributes to unnecessary complication of the problem, so lets boil it down...
lets assume that the force you apply to the pedal is directly linearly proportional to the "thrust" of the car (neglecting torque curves, gear changes, fuel and such). now since you are applying a constant force to the pedal, and hence telling the car to apply a constant "thrust" to the ground, why does the car not continue to accelerate??? Well, neglecting air resistance, road friction, drag torque in the drive train, etc., the car would continue to accelerate, much like a rocket in space where there is no resistance to motion. but since we live on earth, and these resistances do occur, the constant thrust of the car is exactly balanced by the external forces mentioned before when the car reaches a constant speed (i.e., a=0). Hope that helps without being redundant in addition to the other helpful answers in this thread

8. Apr 25, 2006

### jrm2002

Acceleration and Velocity(Continued)

That means:

If we pull any body say with a constant force (neglecting effects due to friction,etc.) it would accelerate with a constant acceleration i.e. its velocity keeps changing at every instant(linear variation).Right?

9. Apr 26, 2006

yes, it is