Physics 11 Significant Figures Doubts

[studenta]

1) Which is right?

65m/s x 3.2 s = 208 m

or

65m/s x 3.2 s = 2.1 x 10^1 m2) is 210 - 3 s.f. or 2 s.f. ?
3) my teacher uses the question as a guide for s.f., is that correct?

e.g. A projectile is fired with an initial speed of 1.12 s at a height of 34 m .

( after few calculation steps)

d = 7.32 m/s x 2.18 s
= 15.96m
= 16 m ( because 34 m in the question is 2 s.f.)

 

[SammyS]

208 m → 2.1 × 10² m

 

[studenta]

208 m → 2.1 × 10² m

SO 2.1 × 10^2 m is the correct answer instead of 208 m? right?

 

[QuarkCharmer]

65m/s x 3.2 s = 208 m

This is not correct. When you multiply significant digits, you use the number of significant digits from the term with the least. If 65 has 2 SD, and 3.2 also has 2 SD, then your answer should be 2 as well. You simply need to take that 208 to 2 SD, by rounding the 8 up to the next decimal place (it is > 5 afterall).

For this one:
65m/s x 3.2 s = 2.1 x 10^2 m (square it)
This is actually correct, but I don't know why you wouldn't just say 210

2) is 210 - 3 s.f. or 2 s.f. ?

It looks like your problem is in determining placeholder digits and such. 210 is 2 significant figures, but "210." is 3 significant digits (note the decimal place).

 

[studenta]

Thanks.
What about the 3rd questions? Should we determine sig.figs according to questions?