Newton’s First and Second Laws of Motion, Inertia, Mass, Volume, Weight, centripetal force and rotational inertia. Stations 1-7

Station 1: Cup and pennies with card 1. How does the law of inertia apply to this activity? What type of equilibrium does this illustrate? 2. Use the 1st and 2nd laws of motion to explain why the penny did not move away with the card when the card was flicked away? 3. Predict what would happen if the index card was replaced with sandpaper? How could the 1st and 2nd laws of motion to explain.

Station 1 1. The penny at rest stays at rest due to its inertia. This illustrates static equilibrium. The penny resists any change in velocity unless it receives an unbalanced force (1st law). Acceleration of the penny did not occur because the net force acting horizontally on the penny was essentially equal to zero (2nd law). Sandpaper would increase the friction force on the penny, which could provide an unbalanced force greater than zero (2nd law) that could cause the penny to be accelerated and overcome inertia (1st law).

34. Tim practices a demonstration before doing it for Sunday dinner 34. Tim practices a demonstration before doing it for Sunday dinner. What concept is he illustrating, and why is he careful not to pull the tablecloth slightly upward? He is demonstrating the law of inertia (objects at rest stay at rest unless receiving an unbalanced force). Pulling upward would lift the plates and cause them to be unbalanced.

Station 2: Air Track Questions 1. What force is removed from the track when the positive air supply is turned on? 2. Describe the motion of the sled in one direction when it is pushed with the air supply turned on. 3. How do Newton’s first and second laws of motion apply to the air track and sled? 4. As more mass is added to the sled, what happens to the sled’s inertia? 5. If the air track was one mile long, describe the sled’s motion after receiving a push. Would it require a force to keep moving? What type of equilibrium does this demostrate?

Station 2 Friction is removed. The sled on the air track demonstrates that an object in motion will continue moving with constant velocity until an unbalanced force changes the velocity. Objects in motion stay at a constant velocity unless acted upon by an unbalanced force (1st law). The sled does not receive an unbalanced force until making impact with the bumper (2nd law). As more mass is added to the sled, the sled’s inertia increases. The sled would continue to move at a constant velocity and requires no force to keep it moving, due to its inertia. This is dynamic equilibrium.

36. To pull a wagon across a lawn at a constant velocity, you have to exert a steady force. Does this contradict Newton’s first law? No. The key is net force. Your steady force is balancing the rolling friction, which means that the net force = 0, which means dynamic equilibrium, which means constant velocity, which means no acceleration. Removing friction would allow the wagon to continue moving at the same velocity without any more pull force.

Station 3: Penny with Hanger What type of motion is illustrated when the penny is rotated in a circle using the hanger? What is keeping the penny on the hanger? How does this activity illustrate Newton’s first law of motion? What would the penny do if the hanger was removed? Use the first law of motion to explain.

Station 3 Acceleration is illustrated by the rotating penny. It takes an unbalanced force to cause the penny to turn in a circle or to keep it on the hanger. This unbalanced force is called centripetal force, and keeps the penny turning. 3. If the hanger was removed, the penny would take a straight path due to its inertia.

8. When you whirl a can at the end of a string in a circular path, what is the direction of the force that acts on the can? The force is directed inward towards the axis of rotation. This inward-directed force is called centripetal force.

42. Can an object move along a curved path if no force acts on it? No. An object moving along a curved path is accelerating and requires an unbalanced force to cause it to turn. This type of force is referred to as centripetal force.

Station 4 Which ball has the most mass? Which ball has the greatest volume? Which ball has the greatest weight? Which ball is the most dense? Rank the balls in order from greatest to least inertia. Predict whether a solid disk or a hollow disk with roll down a ramp faster. Test to verify and explain why.

Station 4 Most mass= bowling ball Greatest volume= bowling ball Greatest weight = bowling ball Most dense = bowling ball Most Inertia to least= bowling ball, tennis ball, styrofoam ball The solid disk rolls faster down the ramp because it has less rotational inertia. The greater the rotational inertia, the greater the resistance to rotation. Rotational inertia increases as the mass of the object is distributed further from the center of rotation.

32. In an orbiting spacecraft, you are handed two identical closed boxes, one filled with sand and the other filled with feathers. How can you tell which is which without opening the boxes? The one that is easier to shake back and forth is the one with less mass (less inertia, less resistance to changes in motion)

Mass remains unchanged. Volume is reduced. 37. When a junked car is crushed into a compact cube, does its mass change? Its volume? Its weight? Mass remains unchanged. Volume is reduced. Weight remains unchanged (no change in position within the gravitational field so no change in gravitational force)

38. If an elephant was chasing you, its enormous mass would be very threatening. But if you zig-zagged, the elephant’s mass would be to your advantage. Why? Because the elephant’s mass is greater than yours, so is its inertia. Therefore, the elephant would have more difficulty switching directions than you because its inertia is greater.

40. Which has more mass, a 2-kg fluffy pillow or a 3-kg small piece of iron? More volume? Why are your answers different? The iron has more mass (greater) and the pillow has more volume (takes up more space. The answers differ because mass and volume are completely different concepts.

20. Beginning from rest, a solid disk, a solid ball and a hollow disk race down an incline. What happens? The solid ball moves fastest, followed by the solid ring and the hollow disk. The solid ball has the least amount of rotational inertia because its mass is distributed closest to the center or axis of rotation.

31. Consider two rotating bicycle wheels, one filled with air and the other with water. Which would be more difficult to stop rotating? Explain. The bicycle wheel filled with water would be the more difficult wheel to stop rotating because it has the greater amount of rotational inertia or resistance to change its rotational motion.

Station 5 What happened to the cart and clay figure during the test? Why did the clay figure continue to move after the cart hit the wood block? Use Newton’s First and Second laws of Motion to explain.

Station 5 1. Both the cart and the clay figure move at the same speed in the same direction until the cart slams into the wood block. Then the clay figure moved forward after the cart stopped. 2. The cart receives an unbalanced force, which causes the cart to decelerate rapidly (2nd law), but the clay figure does not, so it continues to move at the same speed and in the same direction due to its inertia until it receives an unbalanced force (1st law).

33. Many auto passengers suffer neck injuries when struck by cars from behind. How does NL of I apply? Why headrests? The body is accelerated forward with the seat, but the head remains (behind) at its current velocity until an unbalanced force pulls it forward (which is the neck, which can cause whiplash if forceful enough). Headrests provide the unbalanced force needed to accelerate the head with the body.

35. Suppose you place a ball in the middle of a wagon that is at rest and then abruptly pull the wagon forward. Describe the motion of the ball relative to the ground. Except for some change in motion due to friction between the wagon and ball, the will be no motion of the ball relative to the ground; but relative to the wagon, the ball will appear to move toward the back.

43. The head of a hammer is loose and you wish to tighten it by banging it against the top of a workbench. Why is it better to hold the hammer with the handle down as shown rather than with the head down? The handle stops when it hits the bench, but the relatively massive head tends to keep moving towards the handle and tightens.

Station 6 Weight Calculation Practice (show work, use sig figs) Measure the mass of the block Convert mass to kilograms Calculate the weight of the block in pounds. Convert the weight from pounds to newtons. Multiply the mass (in kg) by acceleration rate due to gravity (on Earth) to find Fgrav (the weight of the object in newtons).

Station 6 1. Mass of wood block: 217.2 g 2. Kilograms-- 0.2172 kg 3. Weight of wood block: convert using 1 kg = 2.2 lb 0.2172 kg x 2.2 lb = 0.48 lb Convert pounds to newtons 1 lb = 4.45 N 0.48 lb x 4.45 N = 2.1 N Calculate weight using w = mg w = 0.2172 kg x 9.8 m/s2 = 2.1 N

41. Is it more accurate to say that a dieting person loses mass or loses weight? It is more accurate to say that a dieting person is losing mass (the amount of matter that composes the person). A reduction in mass leads to a reduction in weight. It is possible to change weight (only) if the person moves further from the Earth’s surface or to another place (such as the moon).

Station 7 The baseball, although encountering a small amount of air resistance across the diamond, continues to move without a force, due to its inertia. Only an unbalanced force with change its horizontal velocity. Tendency for an object to resist acceleration is inertia. In the plane= 0 km/h, outside observer = 925 km/h The coin keeps the same velocity in every situation. It is the passenger that changes speed and direction. This makes it appear to the passenger that the coin moved, but the coin maintains its velocity due to its inertia; it’s the passenger that moved.

31. A space probe can be carried by a rocket into outer space 31. A space probe can be carried by a rocket into outer space. Your friend asks what kind of force keeps the probe moving after it is released from the rocket and on its own. What is your answer? Nothing keeps the probe moving. With no propelling force it continues moving in a straight line—moving of its own inertia.