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

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Homework Help Overview

The problem involves a car with a mass of 1750 kg that is decelerating due to friction on a slippery road. The initial velocity is given as 110 km/h, and the friction force is specified as 25% of the car's weight. The questions posed are about calculating the acceleration of the car and the distance it travels before stopping.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the use of Newton's law to relate friction and acceleration. There are attempts to clarify the relationship between the forces acting on the car and the resulting acceleration. Some participants express confusion about the calculations and the relevance of the initial velocity.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some guidance has been offered regarding the application of Newton's law and the role of friction in determining acceleration. There is acknowledgment of the confusion surrounding the calculations and the assumptions made about the forces involved.

Contextual Notes

Participants note that the initial velocity may not be necessary for calculating acceleration, and there is mention of the frustration experienced when using kinematic equations. The distinction between the acceleration of the vehicle and the friction force is also highlighted.

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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.
 
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knight4life said:

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

I don't know, I cannot seem to get that value again.
 
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.
 
w3390 said:
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.
 
That is the acceleration of the vehicle. The friction force is used because it is the only force acting on the truck.
 

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