# Car tires

questions

1. why are car tires moving at 30 m/s experience four times greater forces than when moving at 15 m/s (or half the speed)?
2. What is meant by average velocity?
3. How can you tell if the speed or velocity is constant?
4. How do you change from one unit to another?
5. what is the difference between positive and negative acceleration?

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It would be a shame to just give you the answers. You'd learn a lot more if you think about the problems. Post what you think is correct.... and we can check. 1. the car tires in 30 m/s experienced greater force because of its motion. It is moving fast than in 15 m/s.
2. average velocity could be expressed as: velocity/2
3. if the time travelled is the same
4. by using some conversion tables,then you can change into another unit
5. I have no idea

king_2005 said:
1. the car tires in 30 m/s experienced greater force because of its motion. It is moving fast than in 15 m/s.
Yes, but why exactly 4 times greater ? Do you know any formulas involving velocity and force on rotating objects ?

mrjeffy321
king_2005 said:
2. Average velocity could be expressed as: velocity/2
3. If the time traveled is the same
4. by using some conversion tables,then you can change into another unit
5. I have no idea
2. the average velocity is described as the change in position over the change in time. For example, if you went 10 meters in 10 seconds, the average velocity is 1 m/s. This does not mean that at every point in between t = 0 and t = 10 that you had a velocity of 1 m/s, it just means that on average your velocity was 10 meters / second.

3. If your speed/velocity is constant, you are not accelerating. Which means that the net force acting on you is zero.
You might also say that if the distance traveled in one interval of time is equal to the distance travel in another interval of the same length of time, then the average velocity is the same.

andrevdh
Homework Helper
What if you consider the tyre to rotate about the point of contact with the road and calculate the centripetal acceleration of the middle of the tyre with respect to this point?

andrevdh
Homework Helper
5. The sign is used to indicate direction of the component. When one attempts such one dimensional (motion in aline) problems, you choose a coordinate system (direction of positive motion or x-axis or y-axis). Positive kinematic quantities therefore points in the one way, while negative quantites are in the opposite direction. If the velocity is positive and the acceleration is negative (or the other way round), the object is decelerating. If both velocity and acceleration have the same signs, then the object is accelerating.