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# Physics/Braking early vs. braking late

Question
QUESTION: I was wondering about the effect on gas used of braking early or late when approaching a stop sign.

(This is not a homework question. I have been out of school forover 40 years.)

ANSWER: Well, while you still have to remove the kinetic energy of the car, then resupply it from the engine (hence the benefit of a hybrid vehicle), braking early means that you're taking your foot off the gas earlier.  You'll be a little slower, but you'll use a tiny bit less gas and you'll save a bit on your brake pads.

---------- FOLLOW-UP ----------

QUESTION: Thanks very much for your prompt reply.

I'm somewhat confused by your mentioning that "braking early means that you're taking your foot off the gas earlier." Very often in fact my foot is already off the gas, so that I'm not doing that earlier or later. Can you clarify?

ANSWER: Generally, people who are driving take a certain set time between gas and brakes.  Statistically, you might drift and coast a certain amount, but the *average* driver is (if they were consciously thinking about braking slower and earlier) going to take their foot off the gas earlier.  Maybe not you, personally.

---------- FOLLOW-UP ----------

QUESTION: Thanks again, and I'm sorry to prolong this dialog, but I probably didn't make my concern clear. I have no interest in what average drivers do. I know my own usual practice in street driving as I approach a stop sign is first to stop accelerating and then to brake. My question really relates to how soon I might apply the brakes in that circumstance. My foot is already off the gas pedal. What I'm trying to find out is the effect on gas consumption in that situation of braking early or later.

From a physics standpoint, there's really no difference if you're taking your foot off the gas at the same time in either situation.  You still remove the kinetic energy of the car with the brakes, and that goes back in when you accelerate again.  When you brake is unimportant.

Physics

Volunteer

#### Dr. Stephen O. Nelson

##### Expertise

I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.

##### Experience

I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Education/Credentials
Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.