Acceleration due to gravity?

Question

From the top of a cliff, a person uses a slingshot to fire a pebble straight downward. the initial speed of the pebble is 9.0m/s. a.) What is the acceleration of the pebble during the downward motion?Is it deceletating?explain. b.) after .50 sec, how far beneath the cliff is the pebble.

A wrecking ball is hangng at rest feom a crane when suddenly the cable breaks. the time it takes for the ball to fall halfwya to the ground is 1.2 sec. What is the time it takes for the ball from rest all the wasy tio the ground.

Does wight affect gravity?, does heit do? what are external factors that might affect gravity?

Answer 1

nyphdin is right except for the last bit.

We are talking about gravity experienced at the earth's surface.   It is NOT an exact constant.

Gravity gets less with height according to Newton's inverse square law.  (It also gets less if you descend into the earth until at the center it is zero)  

Other factors affecting it are different density of rocks, nearby masses - mountains and such.  Airborne gravimetric surveys - measuning small changes in g - are used for detecting mineral deposits.

Answer 2

I can't give thumbs up, yet; but the answer above (first answer) is great.  LIsten to it.

Answer 3

The pebble is accelerating downward at 9.8 m/s^2 - the accelereation due to gravity.  It's speed will increase by 9.8 m/s for every second it travels.

It travels a distance y according to:

y = 1/2gt^2 +v0 t + y0    v0 = initial speed, t = time, g = 9.8 m/s^2 and y0 = initial position.

Set y0 = 0.  Let down be the positive direction.  Then if t = 0.5 s

y = 1/2*(9.8 m/s^2)*(0.5 s)^2 + (9 m/s)*(0.5 s) = 5.725 m beneath the cliff

For the crane problem, use the above equation.  Let y = distance to ground.  In this case y0 = 0 and v0 = 0

y = 1/2gt^2  ---> let t = 1.2 s, then   y/2 =1/2 g(1.2 s)^2

or y = g*(1.2 s)^2 = 1.44 * 9.8 m =14.11 m

Weight does not affect gravity - weight is the force exerted by gravity on a mass.  For these types of problems, height does not change teh force of gravity - g is always 9.8 m/s^2.  Gravity is not altered by externalforces.  The resulting motion , like a ball falling, can be altered by things like friction or air resistance.