We can extend the concept of introduced in the previous chapter to analyze forces and changes in experienced by multiple objects during a collision. For example, we could determine the of two cars after a rear-end collision like this one:

Let’s analyze this specific collision seen in the video. We start with our definition for impulse:

(1)

# Momentum

times () is known as the (*p*) , or . The change in momentum (*Δp*) is defined the same way as any other change, final momentum minus the initial momentum: . We can use these definitions to write the **.**

(2)

Equivalently:

(3)

# Conservation of Momentum

The indicates that *if the average force acting on single object or a system of objects is zero, then the* *of the object or system of objects is* *(conserved)*. The previous statement is known as the . The related states that the combined total momentum of all objects in a system must be the same immediately before and immediately after a collision. We can treat both cars in our example collision as a single system as long as we account for the initial momentum and final momentum of both cars.

The initial momentum of the first (stopped) car was zero: . The momentum of the second car was: . Therefore the total initial momentum was

The cars lock together immediately after the collision and only separate later so they have the same final velocity immediately after the collision, we’ll call it . Sticky collisions like this are known as and for such collisions we can treat the objects moving together as a single object that has their combined total . The final is then:

Conserving momentum during the collision tells us to set the initial and final momenta equal:

If we want to solve this equation for the final velocity we divide by the combined mass:

We can look up some data on the cars and find that the length of the Jeep is 3.8 **m**, the mass of the jeep is about 1500 **kg** and a small sports car mass is about 1000 **kg**. ^{[1]}^{[2]} Now let’s estimate some numbers from the video (a quick method is to use the slo-mo feature on a smartphone to film the video as it plays with a running stopwatch also visible in the frame). We see the Jeep covers at least two of its own lengths in about 0.4 **s** so calling its direction the positive, its initial velocity will be:

We are ready to calculate the final velocity:

This is an interesting result, but what’s really cool is that if we estimate the collision interval we can reapply the to calculate the average force applied to each car. From the video the collision time appears to be about 0.5 **s**. Let’s do the Jeep first, just to think ahead, the force on the jeep should be in the negative direction based on our choice of backward and negative.

(4)

We known from that the -23,000 **N **force on the Jeep from the car is paired with a 23,000 **N **force felt by the car from the Jeep. That’s over 5,000 **lbs** of force on each vehicle.

### Reinforcement Exercise

A 48 **kg** soccer player running 5 **m/s** and watching the ball in the air collides with another player of mass 68 **kg** running at 2 **m/s** in the other direction and they tangle together before falling. What is the of the pair immediately after the collision, before hitting the ground?

the average force applied during a time interval multiplied by the time interval

a quantity of speed with a defined direction, the change in speed per unit time, the slope of the position vs. time graph

the value of velocity at the end of the time interval over which motion is being analyzed

a measurement of the amount of matter in an object made by determining its resistance to changes in motion (inertial mass) or the force of gravity applied to it by another known mass from a known distance (gravitational mass). The gravitational mass and an inertial mass appear equal.

the combined effect of mass and velocity, defined as mass multiplied by velocity

the change in momentum experienced by an object is equal to the net impulse applied to the object

not changing, having the same value within a specified interval of time, space, or other physical variable

the momentum of an object or collection of objects (system) remains constant if the net impulse on the system is zero

the combined total momentum of all objects in system immediately prior to a collision be the same as the total momentum of all objects in the system immediately after the collision

collisions in which the colliding objects stick together (explosions are perfectly inelastic collisions in reverse)

for every force applied by an object on a second object, a force equal in size, but opposite in direction, will be applied to the first object by the second object