Hi there!

Assuming you're asking about the solar system, there are nine planets. But, if you're wanting to know about the universe, there are trillions and trillions of planets (most of which we haven't yet discovered).

Hope this helps!

5 0

3 years ago

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g An arrow is shot straight up in the air at an initial speed of 15.5 m/s. After how much time is the arrow moving downward at a

Sati [7]

Answer:

2.11 seconds

Explanation:

We use the kinematic equation for the velocity in a constantly accelerated motion under the acceleration of gravity (g):

$v_f=v_i-g*t\\-5.2 = 15.5 - 9.8\,t\\9.8 \,t= 15.5 + 5.2\\t = 20.7/9.8\\t = 2.11 \,\,sec$

5 0

3 years ago

An unknown radioactive sample is observed to decrease in activity by a factor of two in a one hour period. What is its half-life

elena55 [62]

Answer:

The half-life is $t_{1/2} = 1.005 h$

Explanation:

Using the decay equation we have:

$A=A_{0}e^{-\lambda t}$

Where:

λ is the decay constant
A(0) the initial activity
A is the activity at time t

We know the activity decrease by a factor of two in a one hour period (t = 1 h), it means that $A = \frac{A_{0}}{2}$

$\frac{A_{0}}{2}=A_{0}e^{-\lambda*1 h}$

$0.5=e^{-\lambda*1 h}$

Taking the natural logarithm on each side we have:

$ln(0.5)=-\lambda$

$\lambda=0.69 h^{-1}$

Now, the relationship between the decay constant λ and the half-life t(1/2) is:

$\lambda = \frac{ln(2)}{t_{1/2}}$

$t_{1/2} = \frac{ln(2)}{\lambda}$

$t_{1/2} = \frac{ln(2)}{0.69}$

$t_{1/2} = 1.005 h$

I hope it helps you!

6 0

3 years ago