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# physic math

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Felipe drives his car at a velocity of 28 m/s. He applies the brake, which slows the vehicle down at a rate of 6.4 m/s2 and causes it to slow to a stop. How long does it take for the car to stop? Round your answer to the nearest tenth.

Oct 25, 2021

#1
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Felipe drives his car at a velocity of 28 m/s. He applies the brake, which slows the vehicle down at a rate of 6.4 m/s2 and causes it to slow to a stop. How long does it take for the car to stop? Round your answer to the nearest tenth.

What we want is for the velocity to equal 0 m/s.

(Initial velocity) minus (acceleration times time) = 0

28 m/sec – [(6.4 m/sec2) • (t sec)]  =  0

Let's subtract (6.4 m/sec2) • (t) from both sides.

28 m/sec  =  (6.4 m/sec2) • (t)

Divide both sides by (6.4 m/sec2)     This is going to be tricky.

I'm going to call sec2 sec-sec so I can cancel one of them

Dividing by a term is the same as multiplying by the inverse of that term.

28    meter         1    sec-sec

–––  –––––   •   –––  ––––––

1      sec          6.4    meter

Cancel units

28   meter         1    sec-sec           28 sec

––– –––––   •   ––– ––––––    =    ––––––           =   4.375 sec, rounds to  4.8 sec

1     sec          6.4   meter               6.4

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Nov 3, 2021
#2
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Oh fudge, I made a mistake in my explanation.

On line 4, that should be "Let's add (6.4 m/sec2) • (t) to both sides."

I did it right, I just said it wrong.  Why don't I find theses things before it's too late to edit....

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Guest Nov 3, 2021