I have mass and velocity, how do I get accel?

[knight4life]

Homework Statement

The driver of a 1750 kg car traveling on a horizontal road at 110 km/h suddenly applies the brakes. Due to a slippery pavement, the friction of the road on the tires of the car, which is what slows down the car, is 25% of the weight of the car.

Homework Equations

(a) What is the acceleration of the car?

(b) How many meters does it travel before
stopping under these conditions?

The Attempt at a Solution

I keep getting -2.7m/s^2 and i cannot figure out the correct way to do it.

 

[Doc Al]

The Attempt at a Solution

I keep getting -2.7m/s^2 and i cannot figure out the correct way to do it.

How did you get that value? (Hint: Use Newton's law.)

 

[knight4life]

How did you get that value? (Hint: Use Newton's law.)

I don't know, I cannot seem to get that value again.

 

[w3390]

For part a, you do not even need the velocity of the truck. If you use Newton's law like Doc Al says, you will have -F(friction)=ma --> F(friction)=-ma, which makes sense because the truck is decelerating. You know the force of friction is (1/4)mg so a should equal (-1/4)g m/s^2. Then to find the distance traveled, use the equation Vf-Vi=2*a*delta x, where Vf is final velocity and Vi is initial velocity. In this case, your final velocity will be zero.

 

[knight4life]

For part a, you do not even need the velocity of the truck. If you use Newton's law like Doc Al says, you will have -F(friction)=ma --> F(friction)=-ma, which makes sense because the truck is decelerating. You know the force of friction is (1/4)mg so a should equal (-1/4)g m/s^2.

Thank you very much. I was trying to figure out using kinematics, and getting very angry. I also was assuming that she wanted the acceleration of the vehicle, not that of the friction force.

 

[w3390]

That is the acceleration of the vehicle. The friction force is used because it is the only force acting on the truck.