- #1

~christina~

Gold Member

- 714

- 0

**[SOLVED] Car Crash and such**

## Homework Statement

At 1320 on the last Friday in September 1989 a frantic call was received at the local police station. There had been a serious automobile accident at the intersection of Main Street and State Street, with injuries involved. Lt. John Henry arrived at the scene 10 minutes after the phone call and found that two cars had collided at the intersection. In one car, the driver was unconscious and in the other car both driver and one passenger were injured.

A sketch of the accident scene is shown below.

**Main Street, a thoroughfare, has a 45 mile per hour speed limit.**

**State Street also has a 45 mile per hour limit, but has a stop sign on either side of the road.**

**Vehicle 2, which weighs 5800 lbs**.,

**skidded for 24 feet**before coming to a

**stop next to the utility pole, marked Dec #20.**

**Vehicle 1, which weighs 2060 lbs**., showed

**no skid marks after the impact and came to a rest next to the house on the corner**. Looking at the impact areas of the cars, it was clear to Lt. Henry that the

**cars impacted at right angles**,

**hitting the front right bumper of vehicle 2 and the front left bumper of vehicle 1.**After impact, they initially were traveling in the same direction. Lt. Henry noted that the weather was clear at 690, and that the roadway was dry. He used a drag sled to determine that the

**coefficient of friction between the tires and road was 0.60.**

__He can't use the drag sled to determine the coefficient of friction between the tires of vehicle 1 as they roll over the roadway and grass, but he knows he can calculate that value.__

John Henry has to use his reconstruction of the accident to

**determine whether the driver of vehicle 2 ran the stop sign and / or if the driver of vehicle 1 was speeding. Assume that the collision occurs in the middle of the intersection.**(a) Find the velocities of the two vehicles just prior to impact. The mks system of units must be used for all calculations.

http://img229.imageshack.us/img229/4585/carcolidenm2.th.png

(b) What is the total energy loss for the cars during the collision?

(c) What is the impulse delivered to car 1 when it is hit by car 2? ; the impulse delivered to car 2 by car 1?

(d) Which vehicle delivers the greater force of impact? Use the appropriate physics principle(s) to support your response.

(e) Lt. Henry measured the skid marks made by both vehicles prior to impact. The skid marks for vehicle 1 were 17 feet in length and for vehicle 2 were 10 feet in length. How fast was each car going just prior to braking?

(f) Which driver do you cite in the accident? Explain your response.

## Homework Equations

[tex]\Delta momentum= P_f- P_i= mv_f- mv_i= I[/tex]

[tex] m_1v_{1i} + m_2v_2i = m_1v_{1f} + m_2v_{2f}[/tex] ==> but since it is at 90 deg before...after would the components of velocity have a angle?

I'm not sure about energy though but I think simply would be

[tex]Kf= Ki - f_k d [/tex]

## The Attempt at a Solution

I think it is a Inelastic collision b/c they initially travel in same direction after collision but I'm not quite sure if it is since they don't really stick to each other but go their own separate ways awhile after the collision.

well the info given:

Cars:

m1= 2060lbs=> 934.40 kg

d1= ?

m2= 5800lbs=> 2,630.84 kg

d2= 24ft=> 7.32m

impact angle= 90 deg

[tex]\mu_{k(between car and road)}= 0.60[/tex] ===> for which car, Not sure since it just says "car"

[tex]\mu_{k(between car and grass)}= ?[/tex] => they say I can find this but I'm not sure how to get it

**a) velocities of cars just prior to impact**

well it would be conservation of momentum...

after the collisions the angle would matter

which would be cos and which would be sin to find the angle I'm not sure about

They stick right after but not later after collison.. => is it inelastic?

I drew a orange line for the x and y-axis after collision angles.

I'm thinking about momentum of system before and momentum of system after

but I don't know the initial velocities..

HELP!

Last edited by a moderator: