# Momentum, Completely Inelastic Collision

• lfwake2wake
In summary, a friend seeks your help in proving that the other driver was at fault for a traffic accident. After analyzing the scene and using various equations, you are able to determine the speeds of both drivers, given the masses of their cars, the coefficient of kinetic friction, and other factors. You also use the conservation of momentum to solve for the initial velocities of the cars.
lfwake2wake

## Homework Statement

Your friend has just been in a traffic accident and is trying to negotiate with the insurance company of the other driver to pay for fixing his car. he believes that the other car was speeding and therefore the accident was the other driver's fault. He knows that you have a knowledge of physics...(He must not know me very well :)...) and hopes that you can prove his conjecture. He takes you out to the scene of the crash and describes what happened. He was traveling North when he entered the fateful intersection. There was no stop sign, so he looked in both directions and did not see another car approaching. It was a bright, sunny, clear day. When he reached the center of the intersection, his car was struck by the other car which was traveling East. The two cars remained joined together after the collision and skidded to a stop. The speed limit on both roads entering the intersection is 50 mph. From the skid marks still visible on the street, you determine that after the collision the cars skidded 56 feet at an angle of 30 degrees N of E before stopping. (blah blah blah more facts): His car(1) is 2600 lbs and the other drivers car(2) is 2200 lbs (including weight of the drivers). Coefficient of Kinetic Friction= 0.80...I have to find both speeds of the drivers.

m1=Mass of Car 1 = 2600 lbs
m2=Mass Car 2 = 2200 lbs
uk=Coeff. Kinetic Friction = 0.80
dx=56 ft
@=Theta=30 degrees

## Homework Equations

mv=p where p is conserved.

All of Newton's laws of motion... vf=vi +at, dx=vit+1/2at^2, vf^2=vi^2+2ad

KE=1/2mv^2 (even though its a completely inelastic collision, i might use it)

friction=uN

F=ma

F(friction)*d=Work done by friction

## The Attempt at a Solution

Since it is a completely inelastic collision, KE is not conserved; however, momentum is conserved.

Here is the momentum equation:

m1v1(j)+m2v2(i)=(m1+m2)*V

Using dot product rules:

Vx=(m2v2)/(m1+m2)

Vy=(m1v1)/(m1+m2)

V=(Vx^2+Vy^2)^.5

Since F(friction)=uN, F=0.8*(2200+2600)*(g) =====>don't worry about converting, i'll do that.

So here can I solve for (a) using F=ma? I would have:

a=F(friction)/(2200+2600). Then using that, I could solve for t in above question. Is this correct or am I missing something important?

Once I solve for (t) I can solve for V(initial). Then I can substitute for Vx, Vy. When Vx=Driver 1, and Vy=driver 2.

Last edited:
most of it looks right. But why do you want to get t? Just get Vinitial directly. Then get Vx and Vy. then you can solve for v1 and v2.

but what do you mean by this:

Vx=Driver 1, and Vy=driver 2.

? This isn't right. v1 is not Vx. v2 is not Vy.

So I can solve for both velocities.

First of all, I am sorry to hear about your friend's accident. I can provide a logical and evidence-based analysis of the situation, but I cannot prove your friend's conjecture. It is ultimately up to the insurance company and the legal system to determine fault in a car accident.

That being said, let's analyze the situation using the principles of physics. From the given information, we can assume that the collision between the two cars was a completely inelastic collision, meaning that the two cars stuck together and moved as one unit after the collision. This also means that kinetic energy was not conserved, but momentum was conserved.

To solve for the velocities of the two cars, we can use the equation for momentum, as you have correctly stated. However, instead of using the dot product rules, we can use the component method to break down the velocities into their x and y components. This will give us two equations:

m1v1x + m2v2x = (m1 + m2)Vx
m1v1y + m2v2y = (m1 + m2)Vy

We can use the given angle and the fact that the cars skidded 56 feet to solve for Vx and Vy. Then, as you mentioned, we can use the equations of motion to solve for the initial velocities of the two cars. This will give us the speeds at which the cars were traveling before the collision.

However, it is important to note that the coefficient of kinetic friction given (0.80) is for a car on a dry road. Since the accident occurred at an intersection, there may have been different road conditions, such as oil or water on the road, which could affect the friction and ultimately the calculations. It is also important to consider any external forces that may have acted on the cars during the collision, such as the impact of airbags or any other objects inside the cars.

In conclusion, while we can use the principles of physics to analyze the situation and determine the initial speeds of the cars, it is ultimately up to the insurance company and the legal system to determine fault in a car accident. I hope this helps in your understanding of the situation.

## 1. What is momentum in a completely inelastic collision?

Momentum is a measure of an object's motion and is calculated by multiplying its mass by its velocity. In a completely inelastic collision, the objects involved stick together after colliding, so their combined momentum is conserved.

## 2. How is momentum conserved in a completely inelastic collision?

In a completely inelastic collision, the total momentum of the system before the collision is equal to the total momentum after the collision. This is because the objects involved stick together and move with the same velocity after the collision.

## 3. Can momentum be lost in a completely inelastic collision?

Yes, in a completely inelastic collision, some kinetic energy is converted into other forms of energy (such as heat or sound), so the total momentum of the system may be slightly less after the collision. However, the conservation of momentum principle still applies.

## 4. How is the velocity of two objects after a completely inelastic collision calculated?

The velocity of the combined objects after a completely inelastic collision can be calculated by dividing the total momentum of the system by the combined mass of the objects. This is known as the law of conservation of momentum.

## 5. Can momentum be conserved in an elastic collision?

Yes, in an elastic collision, both momentum and kinetic energy are conserved. In this type of collision, the objects involved bounce off each other and there is no loss of energy, so the total momentum before and after the collision remains the same.

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