- #1

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How to solve this problem? I have no idea at all how to start since seem like lack of some others value. We have only the value of v and coefficient of kinetic friction. How to solve this??

- Thread starter frozen7
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- #1

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How to solve this problem? I have no idea at all how to start since seem like lack of some others value. We have only the value of v and coefficient of kinetic friction. How to solve this??

- #2

Pyrrhus

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- #3

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How to find out acceleration as we don`t know the mass of the car?

- #4

Pyrrhus

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- #5

Päällikkö

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Or you can use the conservation of energy principle.

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The only force acting on the car is friction. Newton tells us that the sum of all forces acting on a point of mass equals m*a. So you know the friction equals m*a. That is (on a horizontal plane):

[tex]m\cdot a=f\cdot m \cdot g[/tex]

Divide by m on both sides and you get:

[tex]a=f\cdot g[/tex]

Now you know the acceleration.

Be the time at the moment, where the car starts breaking (the wheels stand still), [tex]t_{i}=0[/tex] and the distance at that point of time [tex]s_{i}=0[/tex].

Notice: we have reduced the problem thru a dynamic consideration (fricition is the only force) to a purely kinematic one. We have now an accelerated motion (no initial distance and no movement with a constant speed). That's it. So we use the kinematic formulas you're surely familiar with:

[tex]s=\frac{1}{2}\cdot a t^2[/tex]

We also know that acceleration a=v/t. Therefore, t=v/a. We can thus compute s:

[tex]s=\frac{1}{2}\cdot a \left(\frac{v}{a}\right)^2=\frac{1}{2}\cdot \frac{v^2}{a}[/tex]

That was it.

- #7

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Some more, why consider the initial speed = 0 when finding the distance?

- #8

Doc Al

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The only horizontal force on the car is that of friction. And the value of the kinetic friction force is [itex]F_f = \mu N[/itex], where N is the normal force between the surfaces (the tires and the road). In this case, the normal force is just the weight of the car (mg). Note that the mass cancels, so you don't need to know what it is.frozen7 said:Thanks..but I am still not really understand why ma = f (mg).

A force is only needed toEven the car is moving with a deceleration, isn`t there any force which cause the car moving forward?

It may be less confusing if you took the initial speed to be 58.0 km/h, the final speed to be 0, and the acceleration to be negative. (Don't forget to change everything to standard units. For example, speed should be in m/s.)Some more, why consider the initial speed = 0 when finding the distance?

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