& Collisions Name: Date: Period: AP PHYSICS Lab (Virtual) Weight (Formative) BACKGROUND. Before beginning the lab, let s learn a little about momentum and collisions. Follow the directions given below. From the term bank below, select the terms that fit in the blanks provided below. Each term is only used once and none are left over. rest direction inertia negative moving velocity big vector MOMENTUM. When we think about objects having momentum we think that objects that are moving fast just want to keep on () fast. That is very true but there is a second part to momentum as well. Objects that are really () also want to keep moving too. This is how momentum is different from a very close concept, (3), which we studied in the unit on forces. Actually, Newton s First Law states that objects in motion want to keep in motion, and objects at (4) just want to stay at (5) (same answer as No. 4 above). This law was called the law of (6) (same answer as No. 3 above). You see (7) (same answer as No. 3 above) only depends on mass, while momentum depends on mass and (8). So an object at rest can have (9) (same answer as No. 3 above) but it will have no momentum. Mass is a scalar quantity and so it has magnitude but it doesn t have a (0). This means that () (same answer as No. 3 above) is a scalar quantity., on the other hand, is a () quantity because it has magnitude and direction. So momentum can be positive or (3) depending on which way a moving object is headed.
From the term bank below, select the terms that fit in the blanks provided below. Each term is only used once and none are left over. collisions bug windshield Third mass equal inelastic elastic stick bigger heat forces COLLISIONS. When we talked about Newton s (4) Law, we talked a little about collisions. We talked about a trick question about a bug hitting the (5) of a truck and how the (6) of impact compared between them. Most people say that the (7) experiences the greater force but we know that for every action there is an (8) and opposite reaction. The reason why the bug appears to experience the greater force is not because the force on it is (9), but because it has a much smaller (0) and so it undergoes a greater deceleration. The study of () is just taking a closer look at the relationship between the masses and the velocities of crashing objects before and after they crash. There are two basic kinds of collisions. In the first kind, the collisions are perfectly (), and the two objects bounce off each other with no loss of kinetic energy (in Joules). At right you can see three kinds of (3) (same answer as No. ) collisions. In the top Figure, after the collision, the first ball passes all of its momentum to the second ball. In the middle and bottom Figures, each ball exchanges its momentum with the other ball. In the second scenario, when the two objects collide, some of the kinetic energy turns into (4). This is called an (5) collision because the bounce is not as bouncy as it could be in ideal conditions. Objects in this kind of collision tend to (6) to each other after the collision (see Figure at right). Before collision collision After collision
Run the Collision Lab at at http://phet.colorado.edu/en/simulation/collision-lab (Don t download the app just click the Play button), follow the instructions below, and answer the questions. Make sure you are on the Intro tab and not the advanced tab (at the top). SCENARIO : 00% Elastic collision between balls of equal mass STEP : There is a green menu at right. Make sure the elasticity is set at 00%. There is a red ball and a green ball in the center of the screen. Make their masses equal by adjusting the tabs for mass below them. When you hit the play button below the two balls, the red ball will move towards the green ball and hit it in a perfectly elastic collision.. MULTIPLE CHOICE. Predict which of the following will happen after the red ball hits the green ball a. red ball and green ball will both move to the right at half the red ball s pre-collision speed. b. red ball and green ball will bounce off each other and head in opposite directions with half red ball s pre-collision speed. c. red ball will hit green ball and bounce back with red ball s pre-collision speed. Green ball will not move. d. red ball will stop in its tracks and green ball will move to the right with red ball s pre-collision speed. Run the simulation and see what happens. Were you right or wrong? STEP : Hit the reset button below the balls and the MORE DATA button so that you can see the mass, velocity, and position of the ball s before and after the collision. How did they arrive at the momentum amount in the last column? In other words what is the formula for momentum? (HINT: look at the units of momentum). STEP 3: Rerun the simulation. Fill in the data table below with the mass, velocity, and momentum BEFORE and AFTER the collision. Masses of the balls should be the same. Total (Add Column 3 What do you notice about the total momentum before and after the collision? 3
SCENARIO : 00% Elastic collision between balls of unequal mass STEP : Make sure the elasticity is still set at 00% and make the mass of the red ball double the mass of the green ball. Select an easy whole number velocity.. Predict what will happen after the red ball hits the green ball by scribbling out the wrong answer choices in each answer grouping (EX: slower/same/faster) After the collision, the red ball will move backwards/forwards at a speed that is slower/same/faster than its original speed. The green ball will move backwards/forwards at a speed that is slower/same/faster than the red ball s initial speed.. Run the simulation and see what happens. Were you right or wrong? STEP : Rerun the simulation. Fill in the data table below with the mass, velocity, and momentum BEFORE and AFTER the collision. Remember: the mass of the red ball should be double the mass of the green ball. Total (Add Column What do you notice about the total momentum before and after the collision? Explain the change in velocity of the two balls after the collision. In other words, how does the velocity of the red ball after the collision compare to its velocity before the collision? How does the green ball s velocity after the collision compare to the red ball s velocity before the collision? How do the red ball and green ball s velocities after the collision compare with each other? 4
SCENARIO 3: 00% Elastic collision between balls of unequal mass moving towards each other STEP : Make sure the elasticity is still set at 00% and make the mass of the red ball different from the mass of the green ball. Select an easy whole number initial velocity for both balls making the velocity of the red ball positive and that of the green ball negative.. Predict what will happen after the red ball hits the green ball by scribbling out the wrong answer choices in each answer grouping (EX: slower/same/faster) After the collision, the red ball will move backwards/forwards at a speed that is slower/same/faster than its original speed. The green ball will move backwards/forwards at a speed that is slower/same/faster than the its initial speed. The speeds of the two balls after the collision will be the same/different.. Run the simulation and see what happens. Were you right or wrong? STEP : Rerun the simulation. Fill in the data table below with the mass, velocity, and momentum BEFORE and AFTER the collision. Remember: the mass of the red and green balls should be the same and their initial velocities equal and opposite. Total (Add Column What do you notice about the total momentum before and after the collision? What is the trend with the momentums before and after? MULTIPLE CHOICE. When something happens over and over again without exception, in science it is a: A. Hypothesis B. Law C. Theory D. Fact EUREKA! You just discoverd the (Ans.to MC Question) of Conservation of which states that momentum is conserved before and after a collision! 5
SCENARIO 4: Inelastic collision between balls of equal mass STEP : Change the elasticity to 0% and make the mass of the red ball the same as the mass of the green ball. Select an easy whole number initial velocity for the red ball and make the velocity of the green ball zero.. Predict what will happen after the red ball hits the green ball by scribbling out the wrong answer choices in each answer grouping (EX: slower/same/faster) After the collision, the red ball will move backwards/forwards at a speed that is slower/same/faster than its original speed. The green ball will move backwards/forwards at a speed that is slower/same/faster than the red ball s initial speed. The balls will bounce apart/stick together after the collision.. Run the simulation and see what happens. Were you right or wrong? STEP : Rerun the simulation. Fill in the data table below with the mass, velocity, and momentum BEFORE and AFTER the collision. Remember: the mass of the red and green balls should be the same but only the red ball is moving initially. Total (Add Column Is the total momentum still conserved before and after an inelastic collision? Yes No What do you notice about the velocity of the combined masses (red and green ball) after the collision compared to the velocity of the red ball before the collision? Don t just mention that it is greater/or less than. Finish the last step of the following procedure for finding the final velocity (v f-red/green) of the combined mass m red + m green p in = p out in equals momentum out m in-red v in-red + m in-green v in-green = (m red + m green) v f-red/green Initial momentum or red plus initial momentum of green equals the final momentum of the comb. red/green mass m in-red v in-red + 0 = (m red + m green) v f-red/green the green mass has no initial momentum = v f-red/green solve for v f-red/green 6