Kinematics of a slingshot

1. Nov 7, 2005

Aka

i have a test tomorow and I am having trouble with these questions can some one pls check over my answers?
1. a slingshop shoots a 12g stone at 36m/s. It goes in an arc and at the top of the arc it is going at 14m/s. How high is it at this point?
12g=0.012kg
36m/s=v1
14m/s=v2
Ek=1/2mv2
=1/2(0.012kg)(36m/s)2
=0.216J
Eg=mgh
0.216=0.012kg(9.8m/s)h
0.216J
0.1176
=1.8m
the hight is 1.8m
2. A child of 27kg climbs to the top of a 5.0m high playground slid
A) what is her potentian energy?
27kg=m
5.0m=h
Eg=mgh
=27kg(9.8m/s)(5.0m)
=1323J
the potential energy is 1323J
B)Her actual speed at the bottom is only 3m/s. According to conservation of energy how fast should se be going?
Ek=1/2mv2
=1/2(27kg)v2
1323/13.5=v2
98=v2
9.9=v
she should be going 9.9m/s
C) the slid is 8.0m long. determine the force of friction acing
d=8.0m
Ff=?
d=(v2+v1/2)t
t=1.6s
a=v2-v1/t
a=9.9m/s-0m/s/1.6s
a=6.2m/s2
F=ma
27kg(6.2m/s2)
F=167.4N
the force of friction is 167.4N
3. A 72kg passenger is in a van moving at 15m/s when it colides with a wall. The frount part of the van collapses 0.50m during the crash. determine the work done on the passenger and the force the seatbelt exerts during the acident.
72kg=m
15m/s=v1
0.50m=d
w=?
Ff=?
Ek=1/2mv2
=1/2(72kg)(15m/s)2
=8100J
...now what??????
4/ A roler coaster has a height of 94.5m above the ground at its highest point. deteremine the maximum speed of the cars as they come down from this high point. the actual speed is 41.1m/s. Calcuate the work done by friction.
Eg=mgh
=1kg(9.8m/s2)(94.5m)
=926.1J
Ek=1/2mv2
926.1=1/2(1kg)v2
926.1/0.5=v2
1852.2=v2
43m/s=v
the maximun speed it 43m/s
i dont know how to calculate the force of friction

Last edited: Nov 7, 2005
2. Nov 7, 2005

cscott

For the first question you forgot to take into account the fact that the projectile still has kinetic energy at the top so [itex]PE_{top} = KE_{bottom} - KE_{top}[/tex]

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