Lab10 momentum215

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New Mexico State University *

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1230L

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Physics

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Dec 6, 2023

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pdf

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Momentum Name Section Copyright 2009 by S. Kanim, M. Loverude, & L.Gomez 1 1. Introduction In the last lab, you were introduced to a conserved quantity, energy. Energy is a scalar quantity – it doesn’t depend on direction – and the amount of energy before or after some interaction or process is always conserved. That is, there is as much energy before any event as after, although this energy can change from one form to another (for example, from kinetic to potential). For processes, while the total amount of energy is conserved, some of this energy is transformed into forms that are harder to keep track of. For example, when a child travels down a slide, some of the initial potential energy is converted into kinetic energy, but because some is also converted into heat the final kinetic energy will not equal the initial potential energy. In this lab we will introduce a second conserved quantity, momentum. Momentum is a vector quantity and is equal to the product of an object’s mass and its velocity. During a colli sion, some or all of the momentum from one object may be transferred to another object, but the total of momentum vector will be unchanged or conserved. However, just as with energy, it is possible for momentum to be transferred from objects that we are paying attention to into objects that we are not (i.e., that are not part of our system). So while the total momentum is always conserved, it is possible for the momentum of a group of objects that we are focusing on to change if there is some interaction with objects that are not part of our system. 1.1: Lab Objectives. After completing this lab and the associated homework, you should be able to: 1. Describe the forces on two bodies as they collide. 2. Apply Newton’s third law to reason about the forces in a collis ion. 3. Calculate the momentum of an object, and predict the momentum of other objects based on the law of conservation of momentum. 4. Decide whether momentum is conserved for a system that we have defined. 1.2: Outline of Laboratory Approximate sequence of the lab and homework: 1. Verify Newton’s third law for interacting objects that are moving with varying speed. 2. Use Newton’s third law to make inferences about the change in momentum for colliding objects. 3. Apply momentum ideas to systems of colliding objects. 4. Use momentum ideas to predict velocities of carts after collisions.

Momentum Copyright 2009 by S. Kanim, M. Loverude, & L.Gomez 2 2. Forces between interacting objects In this section, you will perform an analysis of the forces acting as two bodies move together. 2.1: For this experiment, you will be using two sliding carts (with attached force probes). We will call the cart on the left including two of the black 500-gram masses system A and the cart on the right with no added masses system B. Without turning on the data recording program, push the carts horizontally so that they speed up as they move to the right. Note that there is friction between the carts and the track. 2.2: Compare the net force (magnitude and direction) on system A to that on system B while they are speeding up. Explain how you arrived at your comparison. 2.3: Draw separate free- body diagrams for system A and system B. Label each of the forces in your diagrams by identifying: the type of force, the object on which the force is exerted, and the object exerting the force. 2.4: Would you expect the magnitude of the force exerted on system A by system B to be greater than, less than, or equal to the magnitude of the force exerted on system B by system A? Explain. Newton’s Third Law is an expression of the idea that force is an interaction – whenever object A exerts a force on object B, object B exerts a force on object A that has the same magnitude, but is opposite in direction to the force that A exerts on B. These two forces are of the same type (normal, gravitational, magnetic, etc.) and are known as a Newton’s third law force pair. Identify any Newton’s third law force pairs in your diagrams by placing one or more small “ ” symbols through each member of the pair (i.e., mark each member of the first pair as , each member of the second pair as , etc.). What feature of the labeling convention F on , by introduced in the Forces lab might be used to identify the force pair(s)? A B Track Free-body diagram for system A Free-body diagram for system B

Momentum Copyright 2009 by S. Kanim, M. Loverude, & L.Gomez 3 3. Experimental analysis of interacting bodies In this section you will perform an experimental study of the situation that you considered above. With the data recording program running, push on both force probes so that you can identify which probe corresponds to each recording. Notice that a force to the right is recorded as a positive value for each probe – that is, if you push on the pad attached to probe A the force will be negative, but if you push on the pad attached to probe B, the force will be positive. 3.1: Zero the force probes. With the data recording program running, push horizontally so that the carts speed up as they move along the track to the right. Record the values for the measured forces. Which of the forces from your free-body diagrams on page 2 does the force recorded by probe A correspond to? Which of the forces from your free-body diagrams on page 2 does the force recorded by probe B correspond to? Are the values you obtained from the two force probes consistent with your predictions on page 2? If not, resolve the inconsistency. 3.3: Consider the following discussion between 3 students about a person in an elevator that is traveling upward. The elevator is slowing down: Alfonso: Newton’s third law tells us that the force on the person by the elevator is equal and opposite to the person’s weight. These forces are equal in magnitude and opposite in direction. Bettina: Not in this case. The person is not moving at a constant spe ed, so Newton’s third law does not apply. Carlota: Those two forces are not a Newton’s third law pair anyway, because they are acting on the same object, the person. Newton’s third law tells us about the force on the elevator by the person compared to the force on the person by the elevator. These would be equal, but only if the elevator was moving at a constant speed. None of these students is correct. Explain what part of each statement is incorrect, and then explain what (if anything) Newton’s third law tells us for this situation. Does Newton’s third law apply to two forces that act on the same object? A B Track

Momentum Copyright 2009 by S. Kanim, M. Loverude, & L.Gomez 4 Does Newton’s third law apply to an object that is not moving at a constant speed?  Discuss your answers to sections 2 and 3 with your lab instructor before continuing. 4. Force and momentum 4.1: The drawing at right represents a photograph taken of the motion of a toy hovercraft. The hovercraft travels toward a wall at an angle, then strikes the wall and rebounds as shown. It is known that our attached blinkie emits light for 0.1 seconds (one-tenth of a second), and that the floor tiles that can be seen in the background are 10 cm on a side. Based on this drawing: A. Is the speed of the hovercraft at point A : greater than, less than, or equal to the speed of the hovercraft at point B? Explain. B. It is not correct to say that the velocity at point A is the same as the velocity at point B. Why not? C. In the space provided, use the technique of vector subtraction that you learned in a previous lab to find the direction of the change in velocity of the hovercraft during its collision with the wall . 4.2: The momentum p of an object is defined as the product of the object’s mass and its velocity: p m v . Momentum is a vector quantity with direction as well as magnitude. How does the direction of the momentum compare to the direction of the velocity? Hovercraft A B Wall Motion direction Motion direction 10 cm

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