Posted by **Madeline** on Wednesday, December 19, 2012 at 9:38pm.

Two particles move along the x -axis. For 0 is less than or equal to t is less than or equal to 6, the position of particle P at time t is given by p(t)=2cos((pi/4)t), while the position of particle R at time t is given by r(t)=t^3 -6t^2 +9t+3.

1. For 0 is less than or equal to t is less than or equal to 6, find all times t during which particle R is moving to the left. 2. for 0 is less than or equal to t is less than or equal to 6, find all times t during which the two particles travel in opposite directions. 3. Find the acceleration of particles P at time t=3. Is particle P speeding up, slowing down, or doing neither at time t=3? Explain your reasoning. 4. Show that during the interval (1,3), there must be at least one instant when the particle R must have a velocity of -2.

- Calculus -
**Reiny**, Wednesday, December 19, 2012 at 11:26pm
1. p '(t) = -2(π/4) sin(π/4t) , which is the velocity

to move to the left, p '(t) < 0 or negative, so

sin (π/4t) must be positive.

the period of sin(π/4t) is 2π/(π/4) = 8

so for the first 4 seconds the curve is above the x-axis or it is positive

so for 0 ≤ t ≤ 6, the particle is moving to the left for the first 4 seconds, then to the right for the next 2 seconds.

ARGGGG , it asked for particle R and I found it for P, Oh well, maybe we can use the above later on

r ' (t) = 3t^2 - 12t + 9

= 3(t-1)(t-3)

zeros are t = 1 and t = 3

so r'(t) is positive for t < 1 or t > 3 and is negative for 1 < t < 3

make a sketch of both derivatives on the same grid

look at when both curves are on the same side of the x-axis ---> going same direction

when both curves are on opposite sides of the x-axis --> same direction

From 0 to 1, R is moving to the right,

from 1 to 3, R is moving to the left

from 3 to 6 , R is moving to the right again.

So from 0 to 1, they are moving in the opposite directions

From 1 to 3, they are moving in the same direction , (both v's are negative

from 3 to 4 , they are moving in opposites

from 4 to 6 , they are moving in same directions

3. r '' (t) = 6t - 12

r ''(3) = 18-12 which is positive, so r ' (t) or the velocity of R is increasing.

4. when is 3t^2 - 12t + 9 = -2

3t^2 - 12t + 11 = 0

t = (12 ± √12)/6 = appr 2.577 or 1.423

both t = 2.577 and t = 1.423 fall in the interval 1 ≤ t ≤ 3

YEAHHH

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