# physics homework

• Sep 13th 2008, 04:12 PM
jbrockoli92
physics homework
PLEEZ HELP!!!!! these are due at 8. i dont need an explanation of the problem unless u want to but thanks if you help :D :D :D

#4 PART 1

Two cars travel in the same direction along a
straight highway, one at a constant speed of
55 mi
/h and the other at 66 mi/h.
Assuming they start at the same point, how
much sooner does the faster car arrive at a
destination 10 mi away? Answer in units of
min.

#4 PART 2

How far must the faster car travel before it
in units of mi.

#6

A tennis ball with a speed of 8 m
/s is thrown
perpendicularly at a wall. After striking the
wall, the ball rebounds in the opposite direc-
tion with a speed of 7 m
/s.
If the ball is in contact with the wall for
0
.012 s, what is the average acceleration of
the ball while it is in contact with the wall?
/s2.

#7

A subway train starting from rest leaves a
station with a constant acceleration. At the
end of 9
.27 s, it is moving at 22.6188 m/s.
What is the train’s displacement in the first
6
.29433 s of motion? Answer in units of m.

#8

Highway safety engineers build soft barriers
so that cars hitting them will slow down at a
safe rate. A person wearing a safety belt can
withstand an acceleration of 300 m
/s2.
How thick should barriers be to safely stop
a car that hits the barriers at 89
.4 km/h?

#9

Two particles are at the same point at the
same time, moving in the same direction. Par-
ticle A has an initial velocity of 7
.9 m/s and
an acceleration of 2
.8 m/s2. Particle B has an
initial velocity of 2
.9 m/s and an acceleration
of 5
.2 m/s2.
At what time will B pass A? Answer in

units of s.
• Sep 14th 2008, 05:33 AM
CaptainBlack
Quote:

Originally Posted by jbrockoli92
PLEEZ HELP!!!!! these are due at 8. i dont need an explanation of the problem unless u want to but thanks if you help :D :D :D

#4 PART 1

Two cars travel in the same direction along a
straight highway, one at a constant speed of
55 mi
/h and the other at 66 mi/[FONT=CMR10]h.
Assuming they start at the same point, how
much sooner does the faster car arrive at a
destination 10 mi away? Answer in units of
min.

Well to travel $10$ miles at $66$ mph takes $10/66$ hours or $(10/66) \times 60$ minutes.

The other car takes $(10/55) \times 60$ minutes.

Therefor the faster car arrives $(10/55) \times 60- (10/66) \times 60$ minutes before the slower.

RonL
• Sep 14th 2008, 05:38 AM
CaptainBlack
Quote:

Originally Posted by jbrockoli92
#4 PART 2
How far must the faster car travel before it
in units of mi.

Generalising from the first part, the faster car arrives at a point $x$ miles from the start point:

$t=(x/55) \times 60 - (x/66) \times 60$

minutes before the slower.

Now put $t=13$ and solve for $x$.

RonL
• Sep 14th 2008, 05:42 AM
CaptainBlack
Quote:

Originally Posted by jbrockoli92
#6
A tennis ball with a speed of 8 m/s is thrown
perpendicularly at a wall. After striking the
wall, the ball rebounds in the opposite direc-
tion with a speed of 7 m/s.
If the ball is in contact with the wall for 0.012 s, what is the average acceleration of
the ball while it is in contact with the wall?