Vertical motion of a bullet

[Glenboro]

Homework Statement

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

1) To what maximum height does the bullet go?

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

3) What is the velocity of the bullet after 60.0 s?[/B]

Homework Equations

1) V2²=V1²+2ad
2) a= (V2-V1)/Δt
3) V2=V1+aΔt

The Attempt at a Solution

1)512^2 = 0^2 + (2)(9.8)(d)
262144 = (19.6)d
262144/19.6 = d
D = 13374. 694m (My teachers says it has too many digit, so should I change to 1.33 X 10^4m ?)

2) T= (0 m/s - 512m/s)/9.8 m/s^2
T = 52.24 s

3) Vf = 512 m/s + (-9.8m/s^2)(60)
Vf = 512 - 588
V2 = 76 m/s^2 downward or opposition direction ( my teacher mentioned about significant digits, so should answer must written as 76.0 m/s^2 ?)

[/B]

 

[jbriggs444]

1)512^2 = 0^2 + (2)(9.8)(d)
262144 = (19.6)d
262144/19.6 = d
D = 13374. 694m (My teachers says it has too many digit, so should I change to 1.33 X 10^4m ?)

What figures in the original equation are not exactly known? I count two.

2) T= (0 m/s - 512m/s)/9.8 m/s^2
T = 52.24 s

That's four significant figures in the result. Again, I count two inputs that are not precisely known.

3) Vf = 512 m/s + (-9.8m/s^2)(60)
Vf = 512 - 588
V2 = 76 m/s^2 downward or opposition direction ( my teacher mentioned about significant digits, so should answer must written as 76.0 m/s^2 ?)

This time the significant figures calculation is trickier. You need to take it step by step. What is the rule for significant figures when adding or subtracting?

 

[Addem]

In part three, the measurement for the gravitational constant is 9.8 using two significant digits. See what you get when you incorporate that.

 

[Glenboro]

In part three, the measurement for the gravitational constant is 9.8 using two significant digits. See what you get when you incorporate that.

I might be incorrect but answer for part 3 should be v2 = 76.0 m/s^2 since it is using two significant digits :)

 

[jbriggs444]

I might be incorrect but answer for part 3 should be v2 = 76.0 m/s^2 since it is using two significant digits :)

How did you conclude that the answer should have two significant digits? It should not. How do you figure that 76.0 m/s² has two significant digits? It does not.

 

[Glenboro]

How did you conclude that the answer should have two significant digits? It should not. How do you figure that 76.0 m/s² has two significant digits? It does not.

I will spend about an hour studying about significant digits, and will come back in hour and reply to you.

 

[PeroK]

If you take $g = 9.8 ms^{-2}$ then that means it could be anywhere between $9.75 ms^{-2}$ and $9.85 ms^{-2}$. You have the same with $v$ between $511.5 ms^{-1}$ and $512.5 ms^{-1}$

If you repeat the calculation with those two figures, it will give you an idea of how accurate your answer is. Try maximum $v$ with minimum $g$ and vice versa.

This should show you why you have to cut down the number of digits.

 

[Glenboro]

If you take $g = 9.8 ms^{-2}$ then that means it could be anywhere between $9.75 ms^{-2}$ and $9.85 ms^{-2}$. You have the same with $v$ between $511.5 ms^{-1}$ and $512.5 ms^{-1}$

If you repeat the calculation with those two figures, it will give you an idea of how accurate your answer is. Try maximum $v$ with minimum $g$ and vice versa.

This should show you why you have to cut down the number of digits.

Thank you for clear explanation, I got an idea how all these digits should working. Thank you

 

[SteamKing]

Homework Statement

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

1) To what maximum height does the bullet go?

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

3) What is the velocity of the bullet after 60.0 s?[/B]

3) Vf = 512 m/s + (-9.8m/s^2)(60)
Vf = 512 - 588
V2 = 76 m/s^2 downward or opposition direction ( my teacher mentioned about significant digits, so should answer must written as 76.0 m/s^2 ?)

What are the units of velocity? Units are just as important as quibbling over significant figures.

 

[Glenboro]

What are the units of velocity? Units are just as important as quibbling over significant figures.

I usually write down all units, I was just in hurry when I was making a thread. However, I will be more careful to write units everytime