# Newton's second law about cars

• B
In summary, the car that accelerates must follow the equation for external force, but there is no external force present. The force of friction between the car and the road acts in the opposite direction of motion. The reaction force, ##F_R##, is the force of the tire on the road, and it can be balanced by friction, but this does not eliminate the force needed to accelerate the car. In fact, the reaction force and the force of the tire on the road are part of a Newton-third-law pair of forces that act on different bodies and cause both the car and the road to accelerate relative to each other. Additionally, ##F_R## also acts against the rotational motion of the wheel, producing an anti-rotational torque
The car that accelerates must enter according to the equation below the external force, while such an external force does not seem to exist.
The force of friction between the car and the road is in the opposite direction of motion
؟

The car that accelerates must enter according to the equation below the external force, while such an external force does not seem to exist.
The force of friction between the car and the road is in the opposite direction of motion
؟

View attachment 300901
The car tyres push on the road the way our feet do when we are walking. By Newton's third law the road pushes the car in the direction of motion.

bob012345 and Chestermiller
Well the truth is just a bit more complex , static friction opposes the rotational motion of the wheels, but is the one that accelerates and gives translational motion to the wheels and the car. The fact that proves that is that if there is no friction (e.g. road with snow ) the wheels rotate freely but the car doesn't get translational motion at all.

So yes the friction is a double personality , regarding car physics. Opposes rotational motion but accelerates translational motion.

Lnewqban
PeroK said:
The car tyres push on the road the way our feet do when we are walking. By Newton's third law the road pushes the car in the direction of motion.
When a person walks, it is clear that an external force enters the sole of the foot from the ground and agrees to move in that direction, but in the case of a car, where is the direction of the reaction force and where does it enter?

When a person walks, it is clear that an external force enters the sole of the foot from the ground and agrees to move in that direction, but in the case of a car, where is the direction of the reaction force and where does it enter?
The bottom of the tyre acts like a foot.

Last edited:
SammyS, hutchphd and russ_watters
The force of friction between the car and the road is in the opposite direction of motion
That is the force from the tire acting on the road.

The third law says that for every action there is an equal and opposite reaction. In this case, this reaction is that the road is thus pushing on the tire.

Do a free body diagram (FBD) for the tire and another one for the road. The force you are talking about will be on the road FDB and the one I'm talking about will be on the tire FDB. Since the road's acceleration is zero by definition ("ground"), then the car must be the one accelerating. It is the same thing with the sole of the foot.

bob012345
As PeroK suggests…

:)

cjl, RDM70, jack action and 10 others
jack action said:
Since the road's acceleration is zero by definition ("ground"), then the car must be the one accelerating. It is the same thing with the sole of the foot.
When the car accelerates forwards, the Earth must turn imperceptibly backwards.

jack action and bob012345
The car that accelerates must enter according to the equation below the external force, while such an external force does not seem to exist.
The force of friction between the car and the road is in the opposite direction of motion
؟

View attachment 300901
This would also be just as true in the case of a rocket powered car but in that case the friction between the road and tire is not the driving force but only prevents slipping. The Earth still moves, just not as much. Momentum of the car is conserved with the Earth and the rocket exhaust.

can you plot a free body diagram (FBD) and forces in car and and explain the below equation?

what is F'R ?

what is F'R ?
It's the reaction force on the wheel from the ground.

The reaction force can be balanced by friction so there is not force to accelerate car! isn't it?

The reaction force can be balanced by friction so there is not force to accelerate car! isn't it?
No. There's no friction acting against the motion.

Ft is acting against the F'R

Ft is acting against the F'R
##F_t## is the force of the tyre on the road.

Note that in a Newton-third-law pair of forces each force, although equal and opposite, acts on a different body. Hence, both bodies accelerate relative to each other.

If you looked at the centre of mass of the car-Earth system, then ##F_t## and ##F_R## would be internal forces that cancel. But, treating the car and Earth as separate bodies, ##F_R## accelerates the car to the left in the diagram.

F_t act on road so you should remove this force because they don't act on car.
do F_R cause e force produce anti-rotational torque

do F_R cause e force produce anti-rotational torque
Yes, ##F_R## acts against the rotation of the wheel driven by the engine.

The reaction force can be balanced by friction so there is not force to accelerate car! isn't it?
I think the reaction force shown is the friction. Friction acts between the tire and the road so there is an equal and opposite force on each. Here is a short video explanation.

Last edited:
Delta2
The force of friction between the car and the road is in the opposite direction of motion
If the car is coasting, yes. But not for the drive wheels when the car is speeding up. The tires form a contact patch with the pavement. When the car is speeding up the force of friction on the contact patch of the drive wheels is in the direction of motion.

## 1. What is Newton's second law about cars?

Newton's second law states that the force applied to a car is directly proportional to its mass and acceleration. This means that the heavier the car and the faster it accelerates, the more force is needed to move it.

## 2. How is Newton's second law applied to cars?

In the context of cars, Newton's second law is used to calculate the necessary force to move a car at a certain acceleration. This can be used to determine the power and torque needed for a car's engine, as well as the braking force needed to slow down or stop the car.

## 3. What is the relationship between acceleration and force in Newton's second law?

According to Newton's second law, the acceleration of a car is directly proportional to the force applied to it. This means that the greater the force applied, the greater the acceleration of the car will be.

## 4. How does mass affect the force needed to move a car?

According to Newton's second law, the force needed to move a car is directly proportional to its mass. This means that the heavier the car, the more force is needed to accelerate it at a certain rate.

## 5. How does Newton's second law apply to braking in cars?

When a car is braking, the force applied to it is in the opposite direction of its motion. According to Newton's second law, this force is directly proportional to the car's mass and the rate at which it decelerates. This means that a heavier car will require more force to brake at a certain rate than a lighter car.

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