Newton's second law about cars

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Discussion Overview

The discussion revolves around the application of Newton's second law to the motion of cars, particularly focusing on the role of friction and reaction forces during acceleration. Participants explore the complexities of forces acting on cars, including static friction, the interaction between tires and the road, and the implications of Newton's third law.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the car accelerates due to an external force, while questioning the existence of such a force when friction opposes motion.
  • Others explain that the force of friction is crucial for the car's acceleration, as it allows for translational motion despite opposing rotational motion.
  • There is a discussion about the analogy between walking and driving, with some participants questioning the direction of reaction forces in both scenarios.
  • Some participants propose that a free body diagram (FBD) could clarify the forces acting on the car and the road.
  • There are conflicting views on whether the reaction force can be balanced by friction, with some arguing that it does not contribute to acceleration while others disagree.
  • Participants discuss the implications of forces acting on the tire and the road, noting that these forces are part of a Newton's third law pair and affect the acceleration of the car differently.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the role of friction and reaction forces in car acceleration. The discussion remains unresolved, with no consensus on the interpretation of forces involved.

Contextual Notes

Some participants highlight the complexity of the forces involved, including the distinction between static and dynamic friction, and the conditions under which these forces act. There are also references to specific scenarios, such as driving on snow or the effects of rocket propulsion, which may influence the discussion.

abdossamad2003
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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
؟

1651486781313.png
 
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abdossamad2003 said:
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.
 
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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.
 
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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?
 
abdossamad2003 said:
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.
 
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abdossamad2003 said:
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.
 
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As PeroK suggests…

1651495467001.jpeg


:)
 
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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.
 
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abdossamad2003 said:
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.
 
  • #10
can you plot a free body diagram (FBD) and forces in car and and explain the below equation?
1651510743228.png
 
  • #11
Car.png
 
  • #12
what is F'R ?
 
  • #13
abdossamad2003 said:
what is F'R ?
It's the reaction force on the wheel from the ground.
 
  • #14
The reaction force can be balanced by friction so there is not force to accelerate car! isn't it?
 
  • #15
abdossamad2003 said:
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.
 
  • #16
Ft is acting against the F'R
 
  • #17
abdossamad2003 said:
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.
 
  • #18
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
 
  • #19
abdossamad2003 said:
do F_R cause e force produce anti-rotational torque
Yes, ##F_R## acts against the rotation of the wheel driven by the engine.
 
  • #20
abdossamad2003 said:
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.
 
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  • #21
abdossamad2003 said:
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.
 

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