Force of Friction: Calculating Car's Acceleration

AI Thread Summary
To calculate the average force of friction on a 1200 kg car that coasts to a stop from 30 m/s in 40 seconds, first determine the car's acceleration by dividing the change in speed by the time taken. The acceleration is negative, indicating deceleration, which can be calculated as the speed change over 40 seconds. Once the acceleration is found, apply Newton's second law (F = ma) to find the force of friction acting on the car. This force is responsible for the car's deceleration until it comes to a stop. Understanding these calculations is crucial for solving the problem effectively.
elissadi
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I'm stuck! Please help! I have been looking @ this problem for 2 hours! :rolleyes:
A 1200 kg car has a speed of 30 m/s - it runs out of gas and coasts, stopping in 40 secs. What is average force of friction on the car?
Do you find the car's acceleration first? There is no force, so you can't use a=F/m
 
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You should be able to find the acceleration. Acceleration is the change of speed divided by the time taken. You know the time, how about the speed?
 
You get the acceleration by using the speed divided by 40 seconds.
 
Ok...

Ok, thanks...I found the acceleration, now how does that fit into the rest of the situation?
 
The friction force causes the acceleration of that mass (car). Use F = ma.
 
find the deceleration, then apply the eqn F=ma
 

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