# Can somone help me (kinematics problems)

1. Oct 24, 2004

### Stripe15

I have a big packet of questions due for physics. I did around 5 of them, but these 7 really frustrate me because I dont know how to do them. Any help would be appreciated.

Some forumlas I know off the top of my head.
FORMULAS: d=1/2gt*2 v= g X t avg v= d/t g= v-v0/t
v= v0 + gt displacement= v1 x t + 1/2gt*2
v2-v0*2 = 2g x displacement
*2 = squared

1. A motorboat traveling on a straight course slows down uniformly from 75 km/h to 40 km/h in a distance of 50m. What is the required acceleration

2. A rifle bullet with a muzzle speed of 330 m/s is fired directly into a special dense material that stops the bullet in cm. Assuming the deceleration is constant, what is the magnitude?

3. The speed limit in a school zone is 40 km/h. A driver traveling this speed sees a child run into the road 13m ahead of his car. He applies the brakes and the car decelerates at a uniform rate of 8.0 m/s2. IF the driver's reaction time is .25s will the car stop before hitting the child. (There seems to be two parts, one about reaction time distance and braking distance.)

4 A boy throws a stone straight upward with an initial speed of 15 m/s on the surface of the Earth (acceleration is 9.80 m/s2). What maximum hieght above the starting point will the stone reach before falling back down.

5. A spring-loaded gun shoots a 0.0050-kg bullet vertically upward with an inital velocity of 21 m/s. What is the hieght of the bullet 3.0 s after firing? At what time is the bullet 12m above the muzzle of the gun.

6. The world trade center and the empire state building in New York City have hieghts of 417m, and 381 m, respectively. If objects were dropped from the top of each, what would be the difference in time in their reaching the ground? What would be the time of fall for Chicago's Sears tower, at 443 m?

7. A ball is projected vertically downward at an inital speed of 14 m/s from a hieght of 65m above the ground. How far does the stone travel in 2.0s? What is its velocity just before it hits the ground?

2. Oct 24, 2004

### HallsofIvy

Staff Emeritus
First, these should be in the homework section.

Second, show us what you have tried yourself.