Physical Science Notes: Motion and Speed

Physical Science Notes: Motion and Speed Critical Thinking Question: Suppose you are standing on a sidewalk and your friend rides past you on his skateboard. Which one of you is moving? A. How can you tell if something is moving? 1. You see it move a. Does this mean if you don’t see it move, it hasn’t moved - No!!!! 2. If you have a reference point you can determine if something has moved a. Reference Point – the background or object that is used for comparison - So you do not need to see something move to determine it moved A___________________________________A The A has moved from one point to another. The reference point is the first A Example: You are on a plane. How do you know you are moving? - This cannot be determined without a reference point!!!!! Back to the critical thinking question. Who is moving? Answer: Depends what the reference point is!!!! If the reference point is the earth, then your friend is moving. However if you use your friend as the reference point you are moving!!! Something to think about: Is there any one frame of reference that is better than the other? Answer: NO!!!!!!! Everything in the universe is moving. There are no fixed reference points that makes one better than the other. Most often, people use the earth as the reference point B. We know we moved. How far and fast did we move? 1. What we are dealing with here is motion a. Definition: Change in position in a certain amount of time 2. If you want to know the distance traveled, what must you have in order to determine this? a. Reference Point!!!!! - If you have a reference point how can you determine the distance? - Measure 1. With a ruler or tape measure 2. By time - On Earth distance is usually measure in length. However the distances on Earth are relatively short compared to the rest of the universe. The distances are so great that it is easier and more reasonable to measure in time. An example is a light year (1 light year = 5,880,000,000,000 miles. At 60 mph it would take 98 billion hours or around 4 billion days or 11 million years to get there). 3. Speed – rate of change in position or how fast you moved. a. This is all relative to the distance you traveled The relationship is written: D = V * T - Where V = Speed, D = Distance, and T = Time b. Calculating Speed: V = D/T Where V = Speed, D = Distance, and T = Time - Example: If you traveled 10 miles in 0.3 hours, how fast was your speed? V = D/T V = ? D = 10 miles T = 0.3 hours V = 10 miles/0.3 hours V = 33 m/h or mph -Example: If you traveled 100 miles in 45 minutes, how fast was your speed in miles/hour? V = D/T - V =? D = 100 miles T = 45 minutes Problem!!! T is in minutes while the question asks for T in hours. You have to convert minutes to hours. Conversion: 45 minutes X 1 hour/60minutes T = .75 hours V = 100miles/.75 hours V = 133 miles/hour - Example: A car is speeding along at 75 miles/hour. How far did the car go in 2 hours? D=VxT D = ? V = 75 miles/hours T = 2 hours D = 75miles/hour x 2 hours D = 150 miles - Example: A runner is running at a speed of 8 miles/hour for 10 miles. How long did the runner run for? T = D/V T = ? D = 10 miles V = 8 miles/hour T = 10 miles/8miles/hour T = 1.25 hours c. There are 3 types of speed 1. Constant Speed – A speed that does not vary 2. Average Speed – Distance divided by time 3. Instantaneous Speed – The rate of motion at any given moment Important Note: Something you must all accept: Time and reference points are all relative. This is all based on Einstein’s Theory of Relativity which is beyond the realm of this class. An example in regards to speed: You are on a train watching two people play ping pong. You calculate that the ping pong ball is traveling at 10 miles per hour. However if you are standing at the train terminal and the same train travels past you at 40 mph how fast is the ping pong traveling (Answer 50 mph…..you must take into account both the speed the ball is being hit at and the speed of the train). Which speed is correct? Both. It is all determined by the reference point which as we learned there is no absolute reference point so each answer is valid?