# Change in forces acting on a car

As a biochemist now teaching physics (middle school), I have what might be two silly questions. I get that for a stationary car to move forward, the forward force (acceleration) needs to be greater than the drag force but QUESTION 1 does the drag force decrease when a car accelerates? I would think it increases as you should experience an increase in air resistance and friction. I also suppose that as long as the acceleration force is greater than the drag force, then the car will move forward. Then QUESTION 2 for that car to slow down, would you see both an increase in the drag force / friction as well as a decrease in acceleration? I suppose if you just take your foot off the accelerator, you will lose just the forward momentum but if you brake, you will add friction to the mix and increase the drag force. This might sound a little simple but I am teaching 11-14 year olds so need to keep it simple :-) Thanks.

• SIgh... what school district allowed someone lacking any eductation in physics to teach the subject? Nov 22 '15 at 14:39
• Wow, this is a rather arrogant attitude. When you teach Science as a general subject, you have to teach physics, chemistry, biology and geology. Can you teach all of these? At least I know my limitations and choose to learn more to be a better teacher. Nov 23 '15 at 10:40
• No, it's not arrogant. It's a flat-out demand from someone who has children and wants properly qualified people teaching them. And, yes, I can teach all of those. I took all those courses in high school (and most of them in college as well). If you don't know physics and knew you'd be teaching it, what were you doing all summer when you could have been reading Halliday and Resnick? Nov 23 '15 at 12:10
• Typically air drag increases as the square of the velocity, not linearly. Nov 23 '15 at 14:59

Answer 1: The Air drag increases as the car accelerates, the Air drag is usually a complicated expression which can be approximated to be $$F_d=bv$$ where $b$ is a parameter that depends on the the structure or the area of the car, and $v$ is the velocity, The friction or rolling friction in particular doesn't depend on speed, It'll remain constant

Answer 2: if on enough acceleration the drag force equals the the car's force you would attain a constant speed, when you take your foot off the accelerator you would be slowed down by the friction and by the drag force, when you apply the brakes as you said the friction force due to braking will also add to this.

• I wonder, wouldn't the effective normal reaction on the car decrease due to aerodynamic lift.Wouldn't there be a less rolling friction, the more velocity it has? Yea there is a drag force due to air. But we should consider the net effective drag on the car, if we want to analyse what actually happends to it ,shouldn't we. Nov 22 '15 at 12:45
• Yes you are right, but that depends on the design of the car. Nov 22 '15 at 12:52

Answer 1:The drag force in the car during the forward acceleration is mainly inertia and friction. The air resistance is taken into consideration only at high speeds. The air resistance will increase due to fluid dynamics but the frictional force will remain constant. Rolling friction is proportional to Weight of the car not the speed and acceleration.

The simple answer is that the drag force will generate whenever an object is accelerated. Justification is Newton's first law of motion.

Answer 2: You have asked whether there is increase in drag force and decrease in acceleration. Your question regarding the acceleration is incorrect.

THERE IS INCREASE IN DECELERATION AND ZERO ACCELERATION. Let us make that point clear. If you take the foot off the accelerator the nature does the work you. If you hit the brakes you add to the natural drag produced due to friction and air resistance to make it fancy.

In both of the cases you loose forward momentum and zero acceleration. You wanted to keep it simple so not mathematical explanation.

• Let me know if you found it useful. I am a rookie review helps. Nov 23 '15 at 14:36
• Review: inertia is not a force. "Drag force," if you mean "air resistance," occurs when an object is moving, not just when accelerated. I fear your answer is lacking in accuracy. Nov 23 '15 at 15:11
• According to the D'Alambert's Law Inertia is considered as a force exerted by body to the cause. Nov 23 '15 at 15:13
• Hi Vinay, yes I agree, that as you take the foot off the accelerator it becomes zero. Sorry, my head was not thinking straight. And thank you for your feedback. One common misconception with 11-13 year olds seems to be that for a car to move forward, it must have increase in force(s) that moves it forward and decrease in force(s) that slow it down. They get the concept of friction pretty easily but don't seem to "see" that it also involved and in fact increases with motion. Nov 25 '15 at 10:08
• I am glad Sasha, that you understood. Don't worry with experience every one gets the concept. I answered it easily because I am a mechanical engineering student. Nov 26 '15 at 5:29