# Help with a car under braking and friction force

• eazyduzit
In summary, the conversation discusses the concept of forces acting on a car in motion, including driving force, friction, normal reaction, weight, and braking force. The question at hand is about the direction of the frictional force during deceleration and the correct representation of forces in a force diagram. It is clarified that weight is not acting against the vehicle, but rather against the ground. The conversation concludes that the direction of acceleration is opposite to the direction of motion during deceleration and that the inertia must also be considered in the force diagram. The driving force is deemed irrelevant to finding the answer to the question.

#### eazyduzit

Got a question for you mechanics enthusiasts out there. Here https://i.imgur.com/Px0Hakg.jpg from what I gather is a basic force diagram depicting a car accelerating along a straight road, Dr represents the driving force of the engine, Fr the friction acting on the tyres against the direction of motion, Nr the normal reaction of the car to the road and W the weight of the car. I have to find the coefficient of friction of the tyres to the road and I'm guessing if I find the value for friction I can then use the formula to solve the coefficient value.My question is in what direction would the frictional force be acting on the car if it were decelerating under a braking force, my common sense would tell me it would be opposite to the braking force but I can't be sure.

I'm talking about something that would look like this https://i.imgur.com/DU42tTp.jpg with Br representing the braking force.

Here https://i.imgur.com/nOoNHWf.jpg is a link to the original question text for anyone wondering.

We need to consider all forces as those acting on the vehicle, not the ground. If we neglect aerodynamic forces we have just the upthrust from the ground and friction.
When the car is decelerating, friction applies a force on the vehicle which acts against the motion.
Braking Force is something that is acting, not against the vehicle but against the road, and is equal and opposite to Friction.
Weight is something that is acting, not against the vehicle, but against the road, and is equal and opposite to upthrust.

Please disregard my words regarding weight, which is incorrect. The vehicle experiences the attraction of gravity in equilibrium with the upthrust from the ground. Apologies.

You are right, the direction of acceleration ##a## is opposite to the direction of motion when in deceleration.

What is missing from your force diagram is the inertia, i.e. ##ma##. The inertia is always opposite the direction of the acceleration ##a##.

Since the sum of the forces in the x-direction (horizontal) must be equal to ##ma## and the only force acting on the car is the braking (friction) force, then it must be equal and opposite to the inertia ##ma##.

The question doesn't mention the driving force coming from the engine and is irrelevant to find the answer of this particular problem.

## 1. How does braking affect the friction force on a car?

When a car is braking, the friction force between the tires and the road increases. This is because the brakes create a force that opposes the motion of the wheels, resulting in a larger friction force between the tires and the road surface.

## 2. Why is friction force important in braking?

Friction force is essential in braking because it is what allows a car to slow down and come to a stop. Without sufficient friction force, the brakes would not be able to effectively slow down the car, leading to potential accidents.

## 3. How does the weight of a car impact its braking and friction force?

The weight of a car plays a significant role in the amount of friction force needed for braking. A heavier car will require more friction force to come to a stop compared to a lighter car. This is because the weight of the car affects the amount of force needed to overcome its inertia and stop its motion.

## 4. Can weather conditions affect the friction force during braking?

Yes, weather conditions can have a significant impact on the friction force during braking. For example, wet or icy roads can decrease the friction between the tires and the road surface, making it more challenging for the car to stop. This is why it's essential to adjust driving habits in different weather conditions.

## 5. How can I improve the braking and friction force of my car?

To improve the braking and friction force of your car, you can ensure that your tires are properly inflated and have good tread. This will maximize the surface area of the tires in contact with the road, increasing the friction force. Regular maintenance of the brakes and using quality brake pads can also improve the car's braking performance.