# My minivan defies what I know of physics. **URL:** \ **Category:** Factual Questions\ **Created:** [October 13, 2005, 3:14am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015 "2005-10-13T03:14:26Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![CastleAnthrax](https://avatars.discourse-cdn.com/v4/letter/c/ecd19e/32.png) [@CastleAnthrax](https://boards.straightdope.com/u/CastleAnthrax)\ **Post date:** [October 13, 2005, 3:14am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015/1 "2005-10-13T03:14:26Z")
I’ve noticed that my minivan does not accelerate when I go downhill without pressing the gas pedal. It actually slows down. This makes no sense to me. If I put a ball on a hill, it accelerates as it rolls, right? The van is running, which means that the engine is generating enough power to overcome inertia on a flat plane, so any mechanical resistance should be countered. The only thing I can think of is wind resistance, but it doesn’t seem like that would be a real factor at regular highway speeds. So what am I missing? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [October 13, 2005, 3:19am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015/2 "2005-10-13T03:19:18Z")
Nearly all cars do this. It’s called “engine braking”. Even, though the car “wants” to roll downhill under the force of gravity, the coupling between the drive train and the engine is strong enough so that the compression of the engine acts to slow the car down. It’s also why downshifting on a long hill is a good way to keep the brakes from overheating, by making the engine do some of the work of keeping the vehicle speed manageable. ---
**Author:** ![chaoticbear](https://avatars.discourse-cdn.com/v4/letter/c/f07891/32.png) [@chaoticbear](https://boards.straightdope.com/u/chaoticbear)\ **Post date:** [October 13, 2005, 3:21am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015/3 "2005-10-13T03:21:02Z")
Your engine isn’t producing enough power at idle RPMs to accelerate your car. Try putting it in neutral. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [October 13, 2005, 3:24am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015/4 "2005-10-13T03:24:39Z")
> [@HennaDancer](#): > > I’ve noticed that my minivan does not accelerate when I go downhill without pressing the gas pedal. It actually slows down. This makes no sense to me. > If I put a ball on a hill, it accelerates as it rolls, right? The van is running, which means that the engine is generating enough power to overcome inertia on a flat plane, so any mechanical resistance should be countered. The only thing I can think of is wind resistance, but it doesn’t seem like that would be a real factor at regular highway speeds. > So what am I missing? When you don’t press the fuel pedal the car is partly driving the engine as if it were a pump. The engine is at idle which would mean maybe 600 rpm and the minivan is going at a speed that would require maybe 2000-2500 rpm so the car is driving the engine. This takes energy out of the car so it either slows down or accelerates little. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [October 13, 2005, 5:51am UTC](https://boards.straightdope.com/t/my-minivan-defies-what-i-know-of-physics/326015/5 "2005-10-13T05:51:14Z")
> [@](#): > > The only thing I can think of is wind resistance, but it doesn’t seem like that would be a real factor at regular highway speeds. Almost 90% of the energy expended by your engine at highway speeds is simply to push the air out of the way.