Find the value of ##T## and distance of particle in the first ##4## seconds

  • #1
chwala
Gold Member
2,675
352
Homework Statement
See attached. (question with solution)
Relevant Equations
Mechanics
1711620715468.png


solution is here;

1711620751293.png


I just need to understand this part ##14.7 = -14.7 =9.8T##... why initial velocity upwards is a negative value? or i am interpreting it wrongly.

...........
In my reasoning,

##v=u+at##
##0=14.7 + (-9.8)t##
## t_1=1.5##

in reverse direction, from top to start point ##T##,

##14.7=0+9.8t##
##t_2=1.5##

##T=1.5+1.5=3##seconds

For second part i have the equation,

particle moving up vertically,

##v^2=u^2+2as##

##0 = 14.7^2 + (2× -9.8s)##
##216.09=19.6s##
##s=11.025##m

and for particle moving downwards,

##v=u+at##
##v=0 + 9.8× 2.5##
##v=24.5##

##24.5^2=0+19.6s##

##s=30.625##m

thus ##s_{total} = 11.025+30.625=41.65≅41.7 ##m
 
Last edited:
Physics news on Phys.org
  • #2
chwala said:
I just need to understand this part 14.7=−14.7=9.8T... why initial velocity upwards is a negative value?
It's ##14.7=-14.7+9.8T##.
Assuming these are values substituted into ##v=v_o+at## we have ##v=+14.7##, ##v_o=-14.7##, and ##a=+9.8##. Evidently the author has chosen the downward direction to be positive, thus the initial upward velocity is negative.
 
  • Like
Likes MatinSAR and chwala
  • #3
… and physics does not care which direction you consider positive. The author chose down as the positive direction, you chose up. Both conventions give the same result as long as you are consistent.
 
  • Like
Likes MatinSAR, chwala and PhDeezNutz

Similar threads

  • Introductory Physics Homework Help
Replies
16
Views
350
  • Introductory Physics Homework Help
Replies
15
Views
397
  • Introductory Physics Homework Help
Replies
7
Views
5K
  • Introductory Physics Homework Help
Replies
11
Views
289
  • Introductory Physics Homework Help
2
Replies
38
Views
2K
  • Introductory Physics Homework Help
Replies
2
Views
1K
  • Introductory Physics Homework Help
Replies
3
Views
791
  • Introductory Physics Homework Help
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
5
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
885
Back
Top