# Speedometer physics problem

1. Sep 24, 2008

### missie

Let's say a rock had a speedometer on it so that you could measure how fast it was going at any moment. By how much would the speed reading on the speedometer increase with each second that it fell?

can you explain?

i also learned today that g = 9.8 for most things right?

2. Sep 24, 2008

### PhanthomJay

Re: Speedometer

g, the acceleration of gravity, applies at or near the earth's surface, for all objects. What did you learn about the relationship between speed, acceleration, and time?

3. Sep 24, 2008

### missie

Re: Speedometer

that no matter what time or how long it stays at g = 9.8 m/s

4. Sep 24, 2008

### PhanthomJay

Re: Speedometer

What I mean is, I know you have studied the kinematic equations of motion for free fall, you know, the ones like vf = vo + ????

5. Sep 24, 2008

### missie

Re: Speedometer

vf = vo + at ? and the teacher said for free fall it's vf = gt

6. Sep 24, 2008

### PhanthomJay

Re: Speedometer

well, that's if vo is zero (if it starts from rest). In general, vf=vo +gt, or rearranging, vf-vo =gt, that is , $$\Delta v = gt$$. So what's the velocity change each second? Don't forget units.

7. Sep 24, 2008

### missie

Re: Speedometer

how will i know the velocity change every second when i can't solve that because it doesn't tell time. It tells me to answer with words and that its not a math problem.

8. Sep 24, 2008

### Mattowander

Re: Speedometer

Think about what is actually happening. When we say that g = 9.8 m/s^2 , what is that saying? Acceleration refers to the change in velocity over time.

An object in free fall starting from an initial speed of zero falls at a velocity of 9.8 m/s after ONE second. How fast do you think it's going after two seconds?

9. Sep 24, 2008

### missie

Re: Speedometer

should it be 9.8 x 2 = 19.6m/s at 2 second?

10. Sep 24, 2008

### Mattowander

Re: Speedometer

You're one the right track yes.

What the acceleration 9.8m/s^2 means is that every second, the velocity increases by 9.8m/s. Now do you understand how that works?

11. Sep 24, 2008

### missie

Re: Speedometer

yes i understand now thank you so much ;]

12. Sep 24, 2008

### Mattowander

Re: Speedometer

No problem :)