Safer: Braking with or without Skidding?

In summary, the conversation discusses the concept of skidding and whether it is safer to slam the brakes and skid to a stop or to slam the brakes as hard as possible to avoid skidding. A previous question is mentioned which calculated the rate of deceleration for a car with just applied force or just kinetic friction, and it was found that the car slowed down quicker when skidding. The textbook mentions anti-lock brakes being safer, but the question is raised about whether the car decelerates slower when not skidding. The conversation concludes that it is safer to not let the car slide across the ground, and the reason for the car decelerating faster when skidding is unclear. The textbook also mentions Egyptian's use of rolling
  • #1
zmike
139
0
Safer to skid or not?

I think I got the question right, I am just not sure if I had covered all aspects of the problem (unfortunately, my textbook doesn't have answers).

Homework Statement



Is it safer to slam the brakes (therefore locking the wheel) and skidding to a stop OR is it safer to slam the brakes as hard as possible so that the car doesn't skid? (on regular road conditions)

The Attempt at a Solution



From a previous question, I calculated the rate of deceleration of a car with JUST applied force or JUST kinetic friction. Strangely the one with just kinetic friction (skidding) slowed down quicker. Therefore slamming the brakes and skidding should stop the car faster (also sliding friction is greater than rolling friction).

EDIT: In the next chapter, my textbook says anti-lock brakes are safer to stop a car with? but wouldn't the car decelerate slower than skidding?

thanks
 
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  • #2
zmike said:
Is it safer to slam the brakes (therefore locking the wheel) and skidding to a stop OR is it safer to slam the brakes as hard as possible so that the car doesn't skid? (on regular road conditions)

Is that the exact wording of the problem?

From a previous question, I calculated the rate of deceleration of a car with JUST applied force or JUST kinetic friction. Strangely the one with just kinetic friction (skidding) slowed down quicker. Therefore slamming the brakes and skidding should stop the car faster (also sliding friction is greater than rolling friction).

Key idea about skidding:
When a car skids, i.e. when the wheels lock, the wheels slide across the ground. Thus the friction is [kinetic/static].
As long as the wheels roll without skidding, the bit of wheel touching the ground is stationary. Thus the friction is...

EDIT: In the next chapter, my textbook says anti-lock brakes are safer to stop a car with? but wouldn't the car decelerate slower than skidding?

I believe you just answered your own question: a car decelerates more slowly when it doesn't skid. Decelerating more slowly is
(a) safer or (b) more dangerous. :wink:
 
  • #3
I think I understand it now.

When a car skids, i.e. when the wheels lock, the wheels slide across the ground. Thus the friction is [kinetic].
As long as the wheels roll without skidding, the bit of wheel touching the ground is stationary. Thus the friction is...[static?]

So if this is right, then it would be safer to not let the car slide across the ground since static friction is greater than kinetic friction. Thus no skidding is safer.

The strange thing is my textbook gives rolling logs as an example of kinetic friction while wikipedia says it's static. Also, why was did the car deccelerate faster (in the previous question) when it was skidding?
 
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  • #4
I believe you got it!

No clue why the textbook would give rolling logs as example of kinetic friction. Dragging logs is kinetic friction; perfectly rolling logs should be static friction.

As for why the car would decelerate faster when it skids...
decelerates faster than what?
Could you please post the two problems?
 
  • #5
Here's the previous questions (in 2 parts)

1. a) A brake test on dry asphalt (u=1.07). Initial velocity at 26.8 m/s, car can stop (no skidding) after 39.3 m. The mass is 1580 kg. Find the deceleration rate.

My answer: 9.14 m/s^2 (in negative direction)

b) The same situation as 1a except there is skidding now.

My Answer: 10.5 m/s^2Also the textbook was talking about Egyptians using rolling logs to transport large stones and since kinetic friction should always be lower than static friction, the textbooks stated that it was kinetic?

If static friction is greater than sliding friction then why do people rollerblade on the road as opposed to sliding? - this one is difficult to explain
 
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  • #6
zmike said:
Here's the previous questions (in 2 parts)

1. a) A brake test on dry asphalt (u=1.07). Initial velocity at 26.8 m/s, car can stop (no skidding) after 39.3 m. The mass is 1580 kg. Find the deceleration rate.

My answer: 9.14 m/s^2 (in negative direction)
No reason to think that this is the maximum acceleration, just that there is no skidding. Is "u=1.07" the coefficient of kinetic or static friction?

b) The same situation as 1a except there is skidding now.

My Answer: 10.5 m/s^2
Are you assuming that "u=1.07" is the coefficient of kinetic friction?

Note: Just because the coefficient of kinetic friction is always smaller than the coefficient of static friction does not mean that kinetic friction is always smaller than static friction. Kinetic friction is always smaller than the maximum static friction--that's why you can brake faster if you apply the brakes as strongly as possible without locking the brakes.
 
  • #7
I don't know if I like this question -- unless I'm totally wrong, the comparison is between:

(1) The friction you get as your tires slide across the ground.

(2) The friction you get as your brake pads clamp to the rotor. (plus a little bit more from the wheels rolling across the ground)
 
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  • #8
Well, the brakes questions told me to referr to the previous q (the brake test) so I don't understand what it wants me to figure out. The questions appear related but the equations don't match with the brakes question.
 
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  • #9
What matters is whether the tires slip or not. If they don't slip, you can use the coefficient of static friction (between tire and road) to calculate the maximum acceleration; if they do slip, you must use the coefficient of kinetic friction.

It's not clear what that problem is describing.
 
  • #10
so we vote that we just don't like zmike's book?
(please, please, pretty please...)
 
  • #11
thanks, I understand it now except for one thing, if the object rolling has static friction why does it move faster on roads than sliding objects. By the way, the text I have is Nelson Physics 11.
 
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1. Is it safer to lock brakes or not?

The answer to this question is that it depends on the situation. In some cases, it may be safer to lock the brakes, such as when driving on wet or slippery roads. This allows the vehicle to come to a controlled stop and prevents the wheels from spinning out of control. However, in other situations, it may be safer to not lock the brakes, such as when driving on dry roads with good traction. This allows the driver to maintain control of the vehicle and steer away from potential hazards.

2. What causes brakes to lock?

Brakes can lock due to various reasons, such as excessive force applied to the brake pedal, damaged brake components, or driving on slippery surfaces. When the brakes lock, the wheels stop rotating and the vehicle may skid, making it difficult for the driver to control the vehicle.

3. Can locking brakes cause accidents?

Yes, locking brakes can cause accidents. When the brakes lock, the wheels stop rotating and the vehicle may skid, which can make it difficult for the driver to maintain control. This can lead to accidents, especially if the vehicle is traveling at high speeds or there are other hazards on the road.

4. How can I prevent my brakes from locking?

To prevent brakes from locking, it is important to maintain your vehicle's brakes and tires in good condition. This includes regular maintenance and replacing worn out brake pads and tires. Additionally, it is important to drive cautiously and adjust your speed according to road conditions, especially in wet or slippery conditions.

5. What should I do if my brakes lock while driving?

If your brakes lock while driving, it is important to remain calm and avoid panicking. Do not release the brake pedal suddenly, as this can cause the vehicle to skid. Instead, gradually release the brake pedal and steer towards a safe area. If possible, try to pump the brakes to regain control of the vehicle. If you are unable to regain control, try to steer towards a soft surface, such as grass or gravel, to slow down the vehicle. Always maintain a safe following distance and avoid sudden braking to prevent your brakes from locking while driving.

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