What happened to Force in this equation?

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The discussion centers on calculating the deceleration of a 2000 kg car that skids to a stop due to locked brakes, with a kinetic friction coefficient of 0.8. The normal force is calculated as 19,600 N, leading to a friction force of 16,000 N. Participants clarify that in this scenario, the only force acting on the car is the kinetic friction, which directly causes deceleration. The confusion arises from the absence of an external force, as the friction force alone is sufficient to describe the car's motion. The key takeaway is that the friction force equals the mass times acceleration, confirming that the car's deceleration is solely due to kinetic friction.
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Homework Statement



a 2000 kg car, after its brakes are locked up, skids and comes to a rest. The coefficient of kinetic friction between rubber and pavement is 0.8. Find deceleration of the car.




Homework Equations



M= 2000 kg
W= mg
V= 0




The Attempt at a Solution



N= mg = 2000 x 9.8 = 19600 N

Fk = N * Mk = 16000

F = ma

-Fk = 2000 a ...Where did F go? In most cases I thought F-Fk = ma. Here the F is missing. Is it because the car is skidding? I'm not sure and I was wondering if you guys could help me out.
 
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laxboi33 said:

The Attempt at a Solution



N= mg = 2000 x 9.8 = 19600 N

Fk = N * Mk = 16000

F = ma

-Fk = 2000 a ...Where did F go? In most cases I thought F-Fk = ma. Here the F is missing. Is it because the car is skidding? I'm not sure and I was wondering if you guys could help me out.

F-Fk=ma gives the resultant force.

Fk causes the car to decelerate so if we use ma=Fk, 'a' gives the deceleration of the car.
 
laxboi33 said:

Homework Statement



a 2000 kg car, after its brakes are locked up, skids and comes to a rest. The coefficient of kinetic friction between rubber and pavement is 0.8. Find deceleration of the car.




Homework Equations



M= 2000 kg
W= mg
V= 0




The Attempt at a Solution



N= mg = 2000 x 9.8 = 19600 N

Fk = N * Mk = 16000

F = ma

-Fk = 2000 a ...Where did F go? In most cases I thought F-Fk = ma. Here the F is missing. Is it because the car is skidding? I'm not sure and I was wondering if you guys could help me out.

Newton's Second Law:

\Sigma \vec F = m\vec a

The sum of all forces is equal to the mass of the object times its acceleration.
For there to be a sum of forces, there doesn't have to be more than one force!

In this case, the only force acting on the car in the horizontal direction is the force of the kinetic friction, so:
\vec f_k = m\vec a

The friction force in this case isn't a reaction force to an external force applied to the object, but it is present because there is relative motion between the car and the surface it is on.
 

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