What is the Average Speed and Acceleration of a Car on a Straight Road?

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SUMMARY

The forum discussion centers on calculating the average speed and acceleration of a car accelerating to 45 miles per hour over a 5-second interval. The average speed is determined to be 22.5 miles per hour, while the acceleration is calculated to be 9 miles per hour per second. The speed at 2.5 seconds is also confirmed to be 22.5 miles per hour, and the time when the speedometer reads the average speed as instantaneous speed is established to be 2.5 seconds. The discussion emphasizes the importance of correctly interpreting units and equations in physics problems.

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  • Understanding of basic kinematics and motion equations
  • Familiarity with units of speed and acceleration (miles per hour, miles per hour per second)
  • Ability to manipulate algebraic equations
  • Knowledge of average vs. instantaneous speed concepts
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  • #31
It would be the same?
 
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  • #32
Tonia said:
Would the expression for speed be: v = m = (45 mi/hr - 0 mi/hr)/2.5 sec. = 18 mi/hr/sec.??
No, that's calculating an acceleration, not a speed.
Write out the general equation that relates speed, time and (uniform) acceleration.
You have a value for the acceleration, and a value for the speed, and we're leaving the time as an unknown t. Plug those into the equation.
 
  • #33
Tonia said:
It would be the same?
What would be the same as what?

Edit: Ok, you were replying to this:
Right, so how would the average height (=average speed) compare with the height half way along (=instantaneous speed at half time)?

Yes.
(Please use the Reply button or the quote button so people know what you are answering.)
 
  • #34
The equation would be Vf = Vo + at??
 
  • #35
Tonia said:
The equation would be Vf = Vo + at??
Right, so plug in the values for v0, vf and a that are appropriate to question d).
 
  • #36
45 mi/hr = 0 + 9 mi/hr/sec times t
Solve for t??
 
  • #37
t would be 5.
 
  • #38
Tonia said:
45 mi/hr = 0 + 9 mi/hr/sec times t
Solve for t??
What was the speed you calculated in part a)?
 
  • #39
22.5 mi/hr
 
  • #40
Tonia said:
22.5 mi/hr
Right. That is the speed you are told to use in answering part d).
 
  • #41
so t is 2.5 seconds and that's the answer to d.
 
  • #42
Tonia said:
so t is 2.5 seconds and that's the answer to d.
Yes.
 
  • #43
Okay. thanks for your help!
 
  • #44
Let me ask you one more question just to make sure: for problems a) and c), the answer is the same? 22.5mi/hr??
 
  • #45
Tonia said:
Let me ask you one more question just to make sure: for problems a) and c), the answer is the same? 22.5mi/hr??
Yes.
 
  • #46
Okay, thanks!
 

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