Why does momentum change

In both parts of this example, the magnitude of momentum can be calculated directly from the definition of momentum:. Although the ball has greater velocity, the player has a much greater mass. Therefore, the momentum of the player is about 86 times greater than the momentum of the football. What was the average force exerted on the 0.

As noted above, when mass is constant, the change in momentum is given by. To determine the change in momentum, substitute the values for mass and the initial and final velocities into the equation above. In this case, using momentum was a shortcut. Use the Check Your Understanding questions to assess whether students master the learning objectives of this section.

If students are struggling with a specific objective, the assessment will help identify which objective is causing the problem and direct students to the relevant content. As an Amazon Associate we earn from qualifying purchases. Want to cite, share, or modify this book? This book is Creative Commons Attribution License 4. Changes were made to the original material, including updates to art, structure, and other content updates. Skip to Content Go to accessibility page.

Physics 8. My highlights. Table of contents. Chapter Review. Test Prep. Teacher Support The learning objectives in this section will help your students master the following standards: 6 Science concepts.

The student knows that changes occur within a physical system and applies the laws of conservation of energy and momentum. The student is expected to: C calculate the mechanical energy of, power generated within, impulse applied to, and momentum of a physical system.

Teacher Support [OL] [AL] Explain that a large, fast-moving object has greater momentum than a smaller, slower object. Hand Movement and Impulse In this activity you will experiment with different types of hand motions to gain an intuitive understanding of the relationship between force, time, and impulse. What are some other examples of motions that impulse affects?

Figure 8. Bending your knees increases the time of the impact, thus decreasing the force. Bending your knees decreases the time of the impact, thus decreasing the force. Bending your knees increases the time of the impact, thus increasing the force. Inertia is the property of mass that resists change. Therefore, it is safe to say that as the mass of an object increases so does its inertia.

Weight is the measurement of resting inertia and momentum is the measure of inertia at a certain velocity. We all know that at the same forward velocity it would be harder to stop a rolling car that a rolling bike. Common sense tells us that the mass of the car makes it more difficult to stop. Here are some simple rules for momentum..

At constant velocity the momentum of an object remains constant but if that object comes in contact with another object there is a change in momentum acceleration or deceleration that is related to the time of contact. This relationship is called impulse. The way that the knowledge of impulse becomes useful is in the application of time. The longer it takes to change the momentum, the less force is exerted on an object and vice-a-versa.

The answer to this question entails considering a sufficiently large system. It is always possible to find a larger system in which total momentum is constant, even if momentum changes for components of the system. If a football player runs into the goalpost in the end zone, there will be a force on him that causes him to bounce backward.

However, the Earth also recoils —conserving momentum—because of the force applied to it through the goalpost. Because Earth is many orders of magnitude more massive than the player, its recoil is immeasurably small and can be neglected in any practical sense, but it is real nevertheless. Consider what happens if the masses of two colliding objects are more similar than the masses of a football player and Earth—for example, one car bumping into another, as shown in Figure 1. Both cars are coasting in the same direction when the lead car labeled m 2 is bumped by the trailing car labeled m 1.

The only unbalanced force on each car is the force of the collision. Assume that the effects due to friction are negligible. Car 1 slows down as a result of the collision, losing some momentum, while car 2 speeds up and gains some momentum. We shall now show that the total momentum of the two-car system remains constant. Figure 1. The momentum of each car is changed, but the total momentum ptot of the two cars is the same before and after the collision if you assume friction is negligible. Intuitively, it seems obvious that the collision time is the same for both cars, but it is only true for objects traveling at ordinary speeds.

This assumption must be modified for objects travelling near the speed of light, without affecting the result that momentum is conserved. Because the changes in momentum add to zero, the total momentum of the two-car system is constant.

We often use primes to denote the final state. This result—that momentum is conserved—has validity far beyond the preceding one-dimensional case. It can be similarly shown that total momentum is conserved for any isolated system, with any number of objects in it. The total momentum can be shown to be the momentum of the center of mass of the system. We have noted that the three length dimensions in nature— x , y , and z —are independent, and it is interesting to note that momentum can be conserved in different ways along each dimension.

For example, during projectile motion and where air resistance is negligible, momentum is conserved in the horizontal direction because horizontal forces are zero and momentum is unchanged. But along the vertical direction, the net vertical force is not zero and the momentum of the projectile is not conserved. See Figure 2. However, if the momentum of the projectile-Earth system is considered in the vertical direction, we find that the total momentum is conserved.

Figure 2. The forces causing the separation are internal to the system, so that the net external horizontal force F x—net is still zero. The vertical component of the momentum is not conserved, because the net vertical force F y—net is not zero. In the vertical direction, the space probe-Earth system needs to be considered and we find that the total momentum is conserved.

The center of mass of the space probe takes the same path it would if the separation did not occur. The conservation of momentum principle can be applied to systems as different as a comet striking Earth and a gas containing huge numbers of atoms and molecules. Conservation of momentum is violated only when the net external force is not zero.

But another larger system can always be considered in which momentum is conserved by simply including the source of the external force.

For example, in the collision of two cars considered above, the two-car system conserves momentum while each one-car system does not. Hold a tennis ball side by side and in contact with a basketball. Drop the balls together. Be careful!