# Safer to lock brakes or not?

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 text book 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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## Answers and Replies

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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 text book 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.

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 text book 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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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?

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^2

Also 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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Doc Al
Mentor
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.

Hurkyl
Staff Emeritus
Gold Member
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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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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Doc Al
Mentor
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.

so we vote that we just don't like zmike's book?