Newton's second law about cars

  • #1
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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  • #2
PeroK
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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
؟

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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  • #3
Delta2
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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.
 
  • #4
abdossamad2003
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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?
 
  • #5
PeroK
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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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  • #6
jack action
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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.
 
  • #7
ergospherical
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As PeroK suggests…

1651495467001.jpeg


:)
 
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  • #8
Baluncore
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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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  • #9
bob012345
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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
؟

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
abdossamad2003
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can you plot a free body diagram (FBD) and forces in car and and explain the below equation?
1651510743228.png
 
  • #12
abdossamad2003
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what is F'R ?
 
  • #14
abdossamad2003
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The reaction force can be balanced by friction so there is not force to accelerate car! isn't it?
 
  • #15
PeroK
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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
abdossamad2003
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Ft is acting against the F'R
 
  • #17
PeroK
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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
abdossamad2003
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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
PeroK
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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
bob012345
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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
Mister T
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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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