You are here:

# Physics/Regarding Newton's third law

Question
QUESTION: Hi,

I could not understand Newton's third law.

If I drive a car, where does the equal and opposite reactionary force acts on it. If it is in the car, the car wouldn't move. If it is in my body, I would not move along with the car. Please explain. Also please clarify what causes the creation of this reactionary force?

Regards.

ANSWER: What do you mean, it wouldn't move?  There are two things at work here, first that the car has more mass than you.  If it puts a force F on you and you put a force F on the car, you will accelerate far more than the car.  Second, the car also has a large force on its tires forcing it to accelerate.  There is no such force on you, just the force of the car.  So you accelerate.  You don't "cancel out" the car's push on you just by being there, you merely exert a small force on the car (small compared to the force of the ground on the car's tires).

The ultimate cause of these contact forces is the polarization of molecules at the surface.  The electron clouds of the molecules distort, causing a polarized molecule.  Ultimately the forces are local and electrostatic in nature.

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

>>Second, the car also has a large force on its tires forcing it to accelerate.

If car exerts large force on its tires, where is the reactionary force acting in this case? Does the reactionary forces get dissipated in the ground? please clarify.

Regards

ANSWER: Sure, the car exerts a reactionary force on the Earth.  The Earth's mass is six trillion trillion kilograms, so good luck measuring its acceleration, but it does accelerate.

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

QUESTION: Hi,

If the earth accelerates, will this create another reactionary force on space? I was wondering how Newton figured it out when the atomic models were not understood during his time.

Regards.

Of course not, there's nothing in space.  That's why it's called space, and why the Earth can accelerate (though not much due to its huge mass) without running into anythiing.  Newton didn't understand the nature of the contact force, just that there was one.  His ideas at the time were truly revolutionary, but he was very smart.

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.