Physical Science Forces and Motion Study Guide ** YOU MUST ALSO USE THE NOTES PROVIDED IN CLASS TO PREPARE FOR THE TEST **

Transcription

1 Physical Science Forces and Motion Study Guide ** YOU MUST ALSO USE THE NOTES PROVIDED IN CLASS TO PREPARE FOR THE TEST ** 1. What is a force? A push or a pull on an object. Forces have size and direction. 2. What is the SI unit for force? The SI unit of measurement for force is the newton, abbreviated N. 3. What do we mean by the term net force? The net force is the force that results from combining all the forces acting on an object. 4. How do you find net force if the forces are going in the same direction? Draw a force diagram. If forces acting on an object are going in the same direction, add the measurements of the forces together. The object will move in the direction of the forces. The nurse is PUSHING with 35 N of force. The orderly is PULLING with 25 35N 25N N of force. Together, they are putting 60 N of force on the bed. The bed moves in the direction of the forces. 5. How do you find net force if the forces are going in the opposite direction? Draw a force diagram. If forces acting on an object are going in the opposite directions, subtract the smaller value from the larger value. The object will move in the direction of the larger force. 3N 4N The baby chicks are fighting over the worm. One chick on the left is pulling with 3N of force and the chick on the right is pulling with 4N. Since the chick on the right is pulling with more force, he will get the worm. The net force on the worm is 1 N to the right.

2 6. What are balanced forces? Draw a force diagram. Balanced forces occur when the net force on an object is zero. Balanced forces produce NO change in motion. 5N 5N 5N 5N 5N 7. What are unbalanced forces? Unbalanced forces occur when the net force on an object is not zero. The object moves, because one or more of the forces is (are) greater than the others. Unbalanced forces produce a change in motion. 8. Which produces motion balanced forces, or unbalanced forces? Unbalanced forces produce motion. 9. Two teams are playing tug of war. If Team A is winning, describe what must be happening, using the terms net force and either balanced forces or unbalanced forces. Draw a force diagram. If one of the teams is winning, there must be unbalanced forces at work, because otherwise, there would be no movement at all. Team A must be exerting more force than Team B. Team A Team B For instance, if the girl on Team A was pulling with 40 N of force, and the boy with 35N of force, there would be 75 N of force altogether on the rope, in that direction. If they are winning, it means that Team B is putting less than 75 N of force on the rope in the opposite direction.

3 10. List five other directional terms for force (examples: forward, north). to the right, backward, to the left, upward or downward, south, northeast, west 11. What is gravity? A force of attraction between two objects that is there just because there is matter there. 12. What is the Law of Universal Gravitation? Who came up with it? Sir Issac Newton is said to have determined that gravity is caused by the masses of objects. The story goes that he was sitting under an apple tree when an apple fell and conked him on the head. He started wondering why apples always fell down and never up. The Law of Universal Gravitation says that the larger the masses of the objects, and the closer together they are, the stronger the gravitational force between them. 13. Which of these sets of objects have the greater gravitational attraction? a. and b. and OR These do! They have more mass!

4 14. Which of these sets of objects have the greater gravitational attraction? and OR and These do! They are closer together! 15. What is weight? Weight is the measure of the force on an object due to gravity. 16. Are weight and mass the same? Explain. No. Mass is the amount of matter in an object. Weight is a measure of how much gravity is pulling on an object. In this picture, there are two astronauts and a flag. The astronauts and the flag are each made up of a certain amount of matter, and that doesn t change just because they are on the moon. Their weights, however, will be different when they are on the moon than when they are on earth. Because the moon is smaller than earth, it doesn t have as strong of a gravitational pull. So their weight will be less than it would be on earth.

5 17. WRITE the following terms in the correct boxes, to tell if they are true about weight, or about mass: 18. What is friction? Friction is a force that opposes (works against) motion between two surfaces that are touching. 19. What causes friction? Draw a picture of what causes friction. Friction is caused by the hills and valleys of one surface sticking against the hills and valleys of the other surface.

6 20. Label each picture as illustrating sliding friction, rolling friction, fluid friction and static friction: (Make sure you understand what each means!) rolling friction sliding friction fluid friction static friction 21. Name three ways to reduce friction. Using lubricant (a slippery liquid that goes between the surfaces, like grease on wheels) Making the surfaces smoother Use wheels that create rolling friction instead of sliding friction.

7 22. Name two ways to increase friction. All surfaces are rough there are no surfaces that are completely smooth. But, some surfaces are smoother than others. Ice, for instance, is smoother than asphalt, so when it snows, there is less friction on the roads, making it harder for cars to stop. Putting sand on icy roads makes the surface less smooth, and increases friction so that cars can drive more safely. You can also increase friction by switching from rolling friction to sliding friction. 23. What is air resistance? Air resistance is a form of fluid friction. It opposes motion. It is caused by the invisible air particles that exist all around us in the atmosphere pushing against the falling object. The amount of air resistance acting on an object depends on the size and shape of the object. Air resistance would affect a flat sheet of paper more than a crumpled one. 24. What characteristics cause some things to be more affected by air resistance than others? Objects that are flat, thin, and have a small mass are most affected by air resistance. Things like leaves and tissue paper are highly affected by air resistance. You can see this when you watch them fall. They fall less quickly than other things, because the air particles push up and oppose the force of gravity.

8 25. Look at these two parachutes. Which would make the parachute come down more slowly? Why? The first parachute will come down more slowly because it has more air resistance. There are more air particles underneath the wide swath of material, pushing up against the motion caused by gravity. 26.What is motion? Motion is when an object changes its position (location) over time. 27. What two things does speed depend upon? the amount of distance the object covered, and the amount of time it took for the object to cover that distance 28. What is the formula for average speed? Show two other ways to arrange this formula, so that you can find distance, or time, by using the formula. d S = t speed = distance divided by the amount of time it took to cover that distance distance = speed times times time = distance divided by speed

9 29. A car travels 300 km in 6 hours. What is its average speed? Show your work. d S = t The car s average speed was 50 kilometers per hour. S = 300 km 6 hr S = 50 km/hr 30. A bicyclist has an average speed of 10 km per hour. She rode for 4 hours. How far did she ride? Show your work. d = 10 X 4 d = 40 km She rode 40 kilometers over the 4 hours, at 10 km/hr. 31. An airplane travelled an average of 275 km per hour. It went from Washington, DC to Orlando, Florida -- a distance of about 1300 km. How long did it take the plane to get to Orlando? Show your work. Round to the nearest hundredth km t = 275 km/hr It took the plane 4.72 hours to go from Washington DC to Orlando at that speed. t = repeating or 4.73 hours rounded to the nearest hundredth

10 32. What is velocity? the speed of an object in a particular direction You can say the cheetah s speed is 15 m/s. If you want to talk about the cheetah s velocity, you d say 15 m/s north. 33. If I say The plane has a velocity of 265 mph, is the statement correct or not? Why or why not? It is not correct. Velocity is not the same as speed. What the statement above tells us is the speed. In order to be correct, you would have to tell the direction the plane was moving. In other words, put on the direction the plane in moving ( the plane has a velocity of 265 mph west. 34. Calculate the velocity of a car that travelled 60 miles to the south, in 1.5 hours. Speed = 60 miles Speed = 40 miles/hour Velocity = 40 miles/hour south 1.5 hours Remember, velocity is speed in a certain direction. 35. If an airplane makes a left turn in the air, while maintaining its speed, has it changed its velocity? Why or why not? Yes. Velocity is speed in a particular direction. So, if you change the speed or the direction, you ve made a change in velocity. 36. What is acceleration? Acceleration is a change in velocity either a speed change, or a direction change, or both. 37. Is the following statement true or false: My mother s car accelerated as it turned the corner at 30 mph.? Explain your answer. This is a true statement. To accelerate, the object needs to change speed, change direction, or both. In this example, the car changed direction as it went around the corner, so it did accelerate.

11 38. Draw a picture of something that is negatively accelerating and explain what is happening. It means the object slowed down. Negative acceleration is slowing down. When an airplane comes in for a landing, it is slowing down, or having negative acceleration. 39. What is another word for negative acceleration? Negative acceleration is also known as deceleration. It means slowing down

12 40. Albert, because he ran fastest: 100 meters in 12 second = 8.3 m/s Charlie. You can tell by the flat line between 8 and 13 seconds. 5 seconds 14 seconds; the race was 100 meters in distance, and the graph shows it took him 14 seconds to go that distance As stated above, Albert s average speed was 8.3 m/s. Bob s average speed was 7.1 m/s. Charlie s average speed was 5.9 m/s

13 41. The graph below shows the TOTAL distance two cars drove over time (a time/distance graph). One of them had a constant speed and the other is accelerating. Which one (A or B) shows the car with the constant speed and which one shows the car accelerating? How can you tell? Car A is going at a constant speed. You can tell because the line showing its speed over time is straight. Car B is not going at a constant speed, it is accelerating. At first it was going slow, then, at about 10 seconds, it sped up very quickly.