# Physics 11 Speed and acceleraton

• anOldMan
In summary, the question is asking for the distance required for a car traveling at 60.0km/hr to come to a complete stop with a deceleration rate of -8.0m/s^2. The correct answer is 173.6m, which can be found using the equation v_f^2 = v_i^2 + 2ad. The reason the previous attempt was wrong is because the average velocity was incorrectly calculated.
anOldMan
[SOLVED] Physics 11 Speed and acceleraton

## Homework Statement

The brakes on a car permit it to decelerate at the rate of -8.0m/s^2. How much distance is required to stop this car when it is traveling 60.0km/hr?

A=V/T
D=RT

## The Attempt at a Solution

I solved it this way
60km/h = 16.667m/s
A=V/T 16.667m/s / -.80m/s^2 = 20.834 seconds
D=RT 16.667m/s x 20.834seconds = 347.24m

This is wrong and I do not understand why. Am I doing it right or is the answer wrong?

The reason your answer is wrong is because you assumed the car maintained its 60.0 km/hr pace for the entire 20.834 seconds. It did not, because it was slowing down.

Since its deceleration is constant, you are justified in saying that the car traveled for 20.834 seconds at an average velocity of (16.667 + 0)/2 m/s, and you will come up with the right answer.

Another way to do this problem that avoids this potential pitfall is to just use the equation $v_f^2 = v_i^2 + 2ad$.

Adding to what Tedjn said, it was probably just a typo but make sure you use -0.8 ms^-2 for the acceleration or the answer is a tenth of what it's supposed to be. Your speed looks right.

Last edited:

## 1. What is the difference between speed and acceleration?

Speed is the rate at which an object moves, while acceleration is the rate at which an object's velocity changes. In other words, acceleration is the change in an object's speed over time.

## 2. How is speed calculated?

Speed can be calculated by dividing the distance traveled by the time it takes to travel that distance. The formula is: speed = distance / time.

## 3. What units are used to measure speed?

The most common units used to measure speed are meters per second (m/s) or kilometers per hour (km/h). Other units such as miles per hour (mph) and feet per second (ft/s) may also be used.

## 4. How does acceleration affect an object's motion?

Acceleration causes an object's velocity to change, either by increasing or decreasing its speed or changing its direction. This ultimately affects an object's motion by causing it to speed up, slow down, or change direction.

## 5. Can an object have a constant speed but still be accelerating?

Yes, an object can have a constant speed but still be accelerating if its direction of motion is changing. This is because acceleration takes into account both the object's speed and direction of motion.

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