Fun Physics Workshop

Transcription

1 Name: University of Cape Town Department of Physics Fun Physics Workshop Equipment checklist. Each group should get the following: 2 tins, saucer, beaker, straws, 2 balloons, tube, wooden balance, 2 weights, string, 2 polystyrene cups, 6 nuts, matches, dishwasher, convex lens, candle, wire, Magic tape, 7 blocks, bottle, diver, paper, pens We encourage you to copy and use this worksheet freely. However, please notify us before you use this worksheet with large groups on a regular basis. Andy Buffler and Saalih Allie Physics Education Group Department of Physics University of Cape Town Rondebosch 7701 abuffler@science.uct.ac.za saalih@science.uct.ac.za

2 Bernoulli s Principle Most likely you will not have heard of Bernoulli's Principle but that does not matter. After you have performed the experiments you might be able to figure out what this principle is... Take two cold drink cans and place them upright on a smooth surface. Leave a gap of about 3 cm between the tins. If you now blow hard between the two cans, what do you think will happen? Now try this: Take a piece of paper, hold it on one end with both hands horizontally near your mouth. Blow hard over the top of the paper. What happens and why?

3 More about Bernoulli s Principle Here is another demonstration of Bernoulli's Principle. Fill a cup with water. Cut a drinking straw in half and place one half vertically into the cup. If you use the other half straw to blow hard across the straw in the cup as shown, what do you think will happen? Here is something else that you can try: Place a small coin on the table. Place a saucer with its edge about 10 cm away from the coin. Do you think that it is possible to blow the coin into the saucer? Try it! Now see if you can blow the coin into one of the polystyrene cups.

4 Things with balloons Blow up two balloons so that one balloon is much bigger than the other, and join the two balloons with a piece of tube without allowing air to move from one balloon to the other. When you do allow air to move between the balloons, which of the following will happen and why? (i) the small balloon will become smaller and the bigger balloon will become bigger (ii) the big balloon will become smaller and the small balloon will become bigger (iii) both balloons will end up the same size Now try this Blow up a balloon and let it fly around the room. How is the balloon propelled? Write down an explanation. If balloon full of air was taken into outer space where there is almost a perfect vacuum, will your explanation still work?

5 Things with liquids Tie two weights to pieces of string and balance them as shown in the diagram. Suppose that you take a cup full of water and hold it so that one of them is completely submersed in the water, but not touching the bottom of the cup. Will the balance still be horizontal? Try this at home... Place some ice in a cup and fill the cup with as much water as possible until the cup just starts to overflow. When the ice melts, will the water to run out of the cup?

6 More things with liquids Take a polystyrene cup and make three small holes at different heights using a matchstick, but don t make the holes directly below each other. Fill the cup with water but keep your fingers over the holes so that the water will not run out. Out of which hole will the water shoot the furthest when you remove your fingers? Now try this Take a saucer and fill it partly with water. Float two match sticks on the surface of the water so that they are about 2 cm apart. Take a straw and dip it into some dishwashing liquid and touch the water between the matchsticks with the straw. Explain what you see.

7 Even more things with liquids Experienced gardeners will often tell you not to water plants in strong direct sunlight as it causes brown spots on the leaves. Why do you think this is the case? My wild guess To answer this question take a drop of water and place it over some writing on this page. What do you see? So what do you think causes the brown spots on the leaves? The Cartesian Diver Fill a beaker with water. Gradually draw water into the diver until it just floats in the water with its top just above the surface. Fill the 2 litre plastic bottle to the top with water and transfer the diver from the beaker to the bottle. Be careful not to change the amount of water in the diver while doing this. Screw the cap onto the bottle tightly. Now squeeze the sides of the bottle. Explain what happens:

8 Things with gravity Take a polystyrene cup and make two holes opposite each other towards the bottom of the cup. Fill the cup with water keeping your fingers over the holes so that the water does not run out. If you remove your fingers the water shoots out from the holes. However, if you drop the cup from a height, will the water still shoot from the holes when the cup is in free fall? Now try this... Take a long piece of string and tie on six nuts at regular intervals (about 10 cm apart). Hold the string vertically with the bottom nut near the floor. Now drop the string so that the nuts make a sound as they hit the floor below. What do you hear? Are the nuts hitting the ground at regular intervals? How should you tie the nuts to the string so that they do hit the floor at regular intervals? Try it and see.

9 Centre of gravity Balance a ruler or a metre stick on your outstretched fore-fingers as shown. If you start with both fingers at the ends of the ruler and then slowly push your fingers together, where do you think they will meet? If you now start with one finger at the end of the ruler and other very close to the centre where will your fingers now meet when pushed towards each other? Something else to try: Stand with your heels and back against a wall and try to bend over and touch your toes. Now stand with one shoulder and one foot against a wall and try to lift your other foot without losing your balance. Write down your observations here:

10 Stacking blocks Simple wooden blocks can be stacked so that the top block extends completely past the end of the bottom block. Stack the blocks evenly on top of one another to make a vertical column. Position the stack so that you are facing the long side of the blocks. Start at the top of the stack. Move the top block so that it overhangs the second block as far as possible without falling. Now move the top two blocks to the right as a unit so that they overhang the third block as far as possible without falling. Continue down the stack in this way. How many blocks must you move before the top block is completely beyond the balance point? Notice that you can never move a given block over as far as you moved the previous one. The larger the stack of blocks you are moving, the smaller the distance you can move them before they become unbalanced and topple over. Can you figure out a mathematical pattern in the stacking? Reaction time Reaction time is the time interval between receiving a signal and acting on it - for example the time between seeing a dog in the road and stepping on the brakes of your car. Hold a R10 note so that the mid-point hangs between your friend s fingers. Do you think it will be possible for your friend to catch it by snapping his or her fingers shut when you release it? Try it and see. Now have your friend drop a ruler between your fingers starting with the bottom of the ruler level with your open fingers. Catch it and note the number of centimetres that passed during your reaction time. Now calculate your reaction time using the formula: Reaction time = d where d is in centimeters. Your reaction time = seconds

11 Things with candles Take a piece of wire and bend it into a small loop with a handle. Now take a square piece of paper and fold it in half twice then open it up to form a filter and put it in the loop of wire. If you hold the paper above a lighted candle, what do you expect to happen? Try it and see. What happened (when you did the experiment) Now fold a new piece of paper and put it in the loop, but this time fill it with water. What will happen now if you hold it above a lighted candle?

12 Things with vision The eye s retina receives and reacts to incoming light and sends signals to the brain, allowing you to see. There is, however, a part of the retina that does not give you visual information. This is your eye s blind spot. Hold this page at eye level about an arm s length away. Make sure the cross is on the right. Close your right eye and look directly at the cross with your left eye. Notice that you can also see the dot. Focus on the cross but be aware of the dot as you slowly bring the page towards your face. The dot will disappear and reappear as you bring the page slowly toward your face. Try the activity again, this time rotating the page so that the dot and the cross are not directly across from one another. Are the results the same? As a variation of this activity, draw a straight line across the page from one edge to the other through the centre of the dot and the cross. Notice that when the dot disappears the line appears to be continuous, without the gap where the dot used to be. Your brain automatically fills in the blind spot with simple extrapolation of the image surrounding the blind spot. This is why you do not notice the blind spot in your day-to-day observations of the world. Something else to try: Closing one eye eliminates one of the clues that your brain uses to judge depth. Try holding a pen in each hand and bring the ends together at arm s length while one eye is closed. Open both eyes and try again. One of the clues that your brain uses to judge distance and depth is the very slight difference between what your left eye sees and what your right eye sees. Your brain combines these two views to make a three-dimensional view of the world. Try this experiment again with one eye closed. But this time, move your head from side to side as you bring the pens together. People who have lost one eye learn to perceive depth by comparing the different views they obtain from one eye at two separate times.

13 The pinhole lens Bring this printed page closer and closer to your eye until you cannot clearly focus on it any longer. Poke a single pinhole through a piece of paper and hold the paper in front of your eye and read these instructions through the pinhole. Bring the page closer and closer until it is only a few centimetres away. Then quickly remove the paper and see if you can still read the letters clearly without the benefit of the pinhole. Explain why this happens. A nearsighted person cannot see distant objects clearly without spectacles. Yet such a person can see distant objects clearly through a pinhole. Explain how this is possible. Something else: You may have noticed the round spots of light on the shady ground beneath trees. These are sunballs, images of the sun cast by openings between leaves in the trees that acts as pinholes. The diameter of a sunball depends on its distance from the small opening that produces it. Large sunballs are cast by openings that are relatively high above the ground while small ones are caused by closer pinholes. The interesting point is that the ratio of the diameter of the sunball to its distance from the pinhole is the same as the ratio of the sun s diameter to its distance from the pinhole. Poke a small hole in a piece of card with a sharp pencil. Hold the card in the sunlight and note the circular image that is cast on the floor. This is an image of the sun. Note that its size does not depend on the size of the hole in the card, but only on its distance from the floor. Position the card so that the image exactly covers a 20c or 50c coin or something that can be accurately measured. Carefully measure the distance from the card to the floor. Since the sun is approximately km from the earth, you can estimate the diameter of the sun from: diameter of pinhole diameter of sun = km x distance to pinhole = km

14 Static electricity Pull two strips of plastic adhesive tape from a roll ( Magic Tape works best). Each one should be about cm long. Hold them by their ends and slowly bring them side by side. What happens? One at a time, pass each of the strips of tape lightly between your fingers, then hold the two strips near each other again. Now what happens? Blow up two balloons. Do not rub them against your hair or clothing. Can you create static electricity by rubbing the two electrically neutral balloons together. Discharge your two strips of tape by running each one between your fingers. Hold them near each other to verify that they neither attract or repel each other. Now stick the two strips together, but this time do it with the two sticky sides together. Peel the strips apart and bring them near one another. Are the strips neutral, or do they attract one another? You can investigate the polarity of the strips by rubbing a balloon in your hair (rubber always acquires a negative charge when touched to hair) and holding the balloon near the strip of tape being investigated.