Net Force Dispute with Teacher

In summary: In this case, the coefficient of friction for the wet pavement is much greater than for the dry pavement (0.6 vs. 0.24), so the car skids.
  • #1
Ninjamonkey474
2
0
1. A 1000 kg automobile initially moving 72 km/h jams the brakes and skids to a stop in a distance of 24 m on a dry pavement but stops in 81.6 m for a wet pavement. (Neglect reaction delay.)
A. Calculate the applied forces for both weather conditions.
B. What is the coefficient of friction for dry weather?
C. What is the coefficient in wet weather conditions
2. Vfinal2=V02-2ad,
μ=Ffriction/Fnormal,
F=ma
3. I said this was impossible since both friction and applied are pointing in the same direction, you can only calculate the net force and not the applied, without which you cannot find either of the coefficients. She said it was possible but I didn't understand what she meant, can somebody explain to me why I'm wrong or why I'm right?
 
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  • #2
Hi Ninjamonkey474! Welcome to PF! :smile:
Ninjamonkey474 said:
A 1000 kg automobile initially moving 72 km/h jams the brakes and skids to a stop in a distance of 24 m on a dry pavement but stops in 81.6 m for a wet pavement.
…
… both friction and applied are pointing in the same direction …

The friction (from the road) is the applied force. :wink:

(there are no external forces on the car except the weight and the friction from the road)
 
  • #3
So do I also use the combined brake-road force to calculate the coefficients of friction?
 
  • #4
Ninjamonkey474 said:
So do I also use the combined brake-road force to calculate the coefficients of friction?
What combined force? The only force you need to worry about is friction from the road.

Or are you thinking that she meant the force that the driver "applied" to the brake? That's not what she meant. And if she did, you would not be able to calculate that. (Note that any force the driver applies to the brake pedal is internal to the system and has no direct affect on the slowing of the car.)
 
  • #5
When your teacher is talking about the applied forces, what she is talking about is Fnormal and Ffriction. Why don't you start out by first calculating the acceleration rates (or in this example, the deceleration rates) under the two different kinds of road conditions? The next step after that will be to calculate the normal force. The only other force acting on the car besides that is the frictional force exerted by the road on the car, in the direction opposite to the car's motion.

Chet
 

1. What is the Net Force Dispute with Teacher?

The Net Force Dispute with Teacher is a hypothetical situation used to teach students about Newton's laws of motion and how they apply to real-life scenarios. It involves a student and a teacher arguing about the forces acting on an object and how those forces affect its motion.

2. What are the main concepts involved in the Net Force Dispute with Teacher?

The main concepts involved in the Net Force Dispute with Teacher are Newton's first law of motion (inertia), Newton's second law of motion (force and acceleration), and Newton's third law of motion (action and reaction). Other concepts such as net force, balanced and unbalanced forces, and the relationship between force and motion are also important to understand in this scenario.

3. How does the Net Force Dispute with Teacher demonstrate Newton's laws of motion?

By presenting a scenario where a student and a teacher have different ideas about the forces acting on an object, the Net Force Dispute with Teacher allows students to see how Newton's laws of motion come into play. The student represents the role of an object with inertia, while the teacher represents the role of an external force. As they argue and apply the different laws, students can see how forces affect an object's motion and how they interact with each other.

4. What are some common misconceptions about the Net Force Dispute with Teacher?

One common misconception is that the teacher is always right and the student is always wrong. In reality, both the student and the teacher can have valid points and it is important to consider all perspectives. Another misconception is that the dispute is solely about finding the correct answer, when in fact it is more about understanding the underlying concepts and how they apply to the situation.

5. How can the Net Force Dispute with Teacher be used in the classroom?

The Net Force Dispute with Teacher can be used as a hands-on activity or demonstration to help students understand and apply Newton's laws of motion. It can also be used as a discussion or debate topic to encourage critical thinking and problem-solving skills. The scenario can be modified to fit different grade levels and can be used to introduce or review the concepts of forces and motion.

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