# Finding velocity and displacement with gravity

1. Jul 14, 2009

### kahya

need some help figuring these out!

A stone is dropped from a bridge and falls freely under the influence of gravity.
a) Calculate its velocity after 1.2s

b) Calculate is displacement after 1.2s

2. Jul 14, 2009

### LowlyPion

Welcome to PF.

How would you think to start looking at the problem.

What does gravity do to its velocity?

3. Jul 14, 2009

### kahya

thank you, well since gravity is a form of uniform acceleration, my teacher said i could apply the equation for uniform acceleration to problems involving falling objects. instead of using the variable a for acceleration, i can use the variable g to represent the acceleration due to gravity. with that said i can assume that are resistance remains 9.81 m/s2 throughout the fall.

however i am having problems putting it all together.

4. Jul 14, 2009

### LowlyPion

Last edited by a moderator: May 4, 2017
5. Jul 14, 2009

### kahya

ok thank you so much!

6. Jul 14, 2009

### kahya

A stone is dropped from a bridge and falls freely under the influence of gravity.
a) Calculate its velocity after 1.2s

v(ave) = (vi + vf)/t
v(ave)= (0m/s+ ?)/ 1.2s

what would my final velocity be?

b) Calculate is displacement after 1.2s

7. Jul 14, 2009

### LowlyPion

What is gravity?

How does that affect velocity?

8. Jul 14, 2009

### kahya

im not sure what you mean...isnt gravity the acceleration throughout the fall?
would that be my final velocity?

9. Jul 14, 2009

### LowlyPion

Acceleration increases the velocity 9.8 m/s every second.

After 1 second it's going 9.8 m/s.
After 2 seconds its going 19.6 m/s.

So what do you figure its velocity would be after just 1.2 seconds?

10. Jul 14, 2009

### kahya

v(ave) = (vi + vf)/t
v(ave)= (0m/s+11.76)/ 1.2s
v(ave)= 11.76/ 1.2s
v(ave)= 9.8
?

11. Jul 14, 2009

### LowlyPion

No.

V = g*t

12. Jul 14, 2009

### kahya

v(ave)=9.8*1.2
v(ave)=11.76

?
im confused

13. Jul 14, 2009

### LowlyPion

No. They want V for the answer. Not V average. I'm not sure why you are insisting on trying to use V average here at all.

14. Jul 14, 2009

### majin_andrew

Since acceleration is the amount that the velocity changes each second, then
velocity = initial velocity + acceleration*time

displacement is the integral of velocity with respect to time, so
displacement = initial velocity*time + 1/2 * acceleration * time * time

If you don't know about integrals yet thats ok, but its like the changing velocity always being multipied by time. Its sort of complicated, but that's why there is a 1/2 in front of the acceleration * time, because its like multiplying the average velocity by time.