Solving Kinematics Problems: Maximum Height, Time, and Velocity Calculations

[BigCountry]

Good Morning,

Just checking my work, your help would be much appreciated.

A bullet is fired vertically into the air at a speed of 512 m/s.

a) To what maximum height will the bullet does the bullet go?

b) How much time passes before the bullet stops rising?

c) What is the velocity of the bullet after 60 seconds?

My Answers:

a)

Vi = 0, Vf = 512, g = -9.8, find d

Vf^2 = Vi^2 + 2ad
512^2 = 2(-9.8)d
262144 = (-19.6)d
262144/(-19.6) = d
13375 m = d




b)

Vi = 0, Vf = 512, g = -9.8, find t

Vf = Vi + at
512 = 0 + (-9.8)t
512/(-9.8) = t
52.24 Secs = t

c)

Vi = 0, g = -9.8, t = 8, find Vf

Vf = Vi + at
Vf = 9.8 * 8
Vf = 78.4 or 78 m/s^2

Thank You

 

[Galileo]

All correct.
Better not use t=8s in c), but the more accurate 7.76s. Makes a difference of 2m, but who's counting.

 

[Doc Al]

My Answers:

a)

Vi = 0, Vf = 512, g = -9.8, find d

Vf^2 = Vi^2 + 2ad
512^2 = 2(-9.8)d
262144 = (-19.6)d
262144/(-19.6) = d
13375 m = d

I'd say that Vi = 512 m/s, Vf = 0. (This way you won't have to ignore any strange minus signs. ) Otherwise, looks good.

b)

Vi = 0, Vf = 512, g = -9.8, find t

Vf = Vi + at
512 = 0 + (-9.8)t
512/(-9.8) = t
52.24 Secs = t

Again: Vi = 512 m/s, Vf = 0. Otherwise, looks good.

c)

Vi = 0, g = -9.8, t = 8, find Vf

Vf = Vi + at
Vf = 9.8 * 8
Vf = 78.4 or 78 m/s^2

Huh? t = 60 s, not 8. (Again: Vi = 512 m/s) Note: Signs count!

(Note: Now I see what you did for question c: You started from the top. Good! But take Galileo's advice about accuracy. Note that the velocity is negative.)

 

[BigCountry]

Thanks Galileo!

Much appreciated!