Motion

Unit 1: Motion Chapter 1: Describing the Physical Universe 1.1 The Science of Physics 1.2 Distance, Time, and Measurement 1.3 Speed

1.1 Investigation: Measuring Time Key Question: How do we measure and describe time? Objectives: Use electronic timing equipment and photogates. Use units of time in calculations and conversions. Correctly apply the terms accuracy, precision, and resolution to scientific instruments and measurements.

The Science of Physics Physics is a type of science that studies matter and energy. Everything in the universe is believed to be either matter or energy.

The Science of Physics Matter is anything that has mass and takes up space. Energy is a measure of a system’s ability to change or create change in other systems.

The Science of Physics A natural law is a rule that describes an action or set of actions in the universe. Sometimes a natural law, like Newton’s second law of motion, can be expressed by a mathematical equation. We do not know all of the natural laws, so there is a lot left for students like you to discover!

Systems and variables A variable is a factor that affects the behavior of the system. When you are trying to find out how a system works, you look for relationships between the important variables of the system.

The scientific method Learning by chance is one way to learn. The scientific method is a much more dependable way to learn and gather information. Key parts include: the hypothesis, a tentative, testable statement that tries to explain a set of scientific observations. the experiment, a situation specifically set up to test or investigate a hypothesis.

The scientific method 1. Scientists observe nature, then develop or revise hypotheses about how things work. 2. The hypotheses are tested against evidence collected from observations and experiments. 3. Any hypothesis that correctly accounts for all evidence from the observations and experiments is a potentially-correct theory. 4. A theory is continually tested by collecting new and different evidence. Even one single piece of evidence that does not agree with a theory will force scientists to return to the first step.

Scientific theories In science, the word theory is used differently than in everyday use. A scientific theory is a comprehensive, well-tested description of how and why a process in nature works the way it does.

Scientific theories The purpose of scientific research is to do experiments which show that existing theories do not give the right prediction. A theory that correctly explains 1,000 experiments but fails to explain the 1,001st cannot be wholly complete.

Scientific theories It is important to be able to distinguish between pseudoscience and science. “The word pseudo means fake,” says professor Coker, of the Physics Department at the University of Texas. Beware of “science” you find on the Internet. Some of it is correct, but much of it is not.

Investigation systems Experiments on systems usually have a question associated with them. An example would be “How does the steepness of a ramp affect the speed of a ball?”

Investigation systems The variable causing the change in the system is called the independent variable. The angle of the ramp is the independent variable in this example.

Investigation systems The variable that may show the effect of those changes is called the dependent variable. The speed of the ball is the dependent variable.

Investigation systems In an ideal experiment, you change only one variable at a time. You keep all of the other variables the same. A variable that is kept the same is called a control variable. What variables should be controlled in this system?

Scientific evidence There are exacting rules defining what counts as scientific evidence. Scientific evidence can include numbers, tables, graphs, words, pictures, sound recordings, or other information. Scientific evidence must also be objective and repeatable.

Models In physics, a model links the variables in a system through cause-and-effect relationships. Our car‑and‑track system links height and speed to the idea of energy. This conceptual model is known as the law of conservation of energy, a natural law of physics.

Unit 1: Motion Chapter 1: Describing the Physical Universe 1.1 The Science of Physics 1.2 Distance, Time, and Measurement 1.3 Speed

1.2 Investigation: Speed Key Question: Can you predict the speed of the car as it moves down the track? Objectives: Predict what happens to a car’s speed as it travels down a track. Create and interpret a speed vs. position graph. Use a graph to make a prediction that can be quantitatively tested. Calculate the percent error between a measurement and a prediction.

Measurement A measurement is a precise value that tells how much. A measurement communicates a quantity, and a unit. For example, 2 meters is a measurement.

Distance Distance is the amount of separation between two unique points. Distance is measured in units of length. Commonly used units of length include: inches, miles, centimeters, meters, and kilometers.

Two systems of measurement The English System is used for everyday measurements in the United States. During the 1800s, a new system of measurement— the Metric System—was developed in France and was quickly adopted. In 1960, the Metric System was revised and simplified, and a new name was adopted—the International System of Units, or SI.

Two systems of measurement Almost all fields of science worldwide use SI units because they are so much easier to work with. In SI, factors of 10 are easier to calculate mathematically.

SI prefixes Today, the United States is the only industrialized nation that has not switched completely to SI.

Measuring time A quantity of time is called a time interval. Most problems in physics measure time in seconds, so you may need to convert from hours or minutes.

Time scales in physics The second (s) is the basic unit of time in both the SI and English systems. In many experiments, you will observe how things change with time.

Accuracy, precision, and resolution The words accuracy and precision also have different meanings in science than every day use. Accuracy is how close a measurement is to its accepted or “true” value. Precision describes how close together several repeated measurements or events are to one another.

Accuracy, precision, and resolution Resolution is another important term to understand when you are working with measured quantities. Resolution is a reference to the smallest interval that can be measured.

Working with measurements All measurements involve a degree of uncertainty. It is impossible to make a measurement of the exact true value of anything, except when counting. Significant digits are the meaningful digits in a measured quantity. Mathematic answers involving measured quantities should have no more significant digits than the starting measurement with the least number of significant digits.

Unit 1: Motion Chapter 1: Describing the Physical Universe 1.1 The Science of Physics 1.2 Distance, Time, and Measurement 1.3 Speed

1.3 Investigation: Experiments and Variables Key Questions: How do you design a valid experiment? Objectives: Set up an experiment. Explain the difference between control and experimental variables. Discuss why conducting multiple experimental trials is better than gathering only one set of data.

Speed To understand the concept of speed, use the bicycle example below:

Speed Think about two questions: How many meters does each bicycle move each second? Does the bicycle move the same number of meters every second?

Speed Think about two questions: How many meters does each bicycle move each second? Ans: Bike #1= 1 meter , Bike #2 = 3 meters Does each bicycle move the same number of meters every second? Ans: yes

Speed The speed of a bicycle is the distance it travels divided by the time it takes. At 1 m/s, bike #1 travels 1 meter per second. At 3 m/s, bike #2 travels 3 meters per second. Constant speed means the same distance is traveled every second. Each bicycle is moving at constant speed, but their speeds are not the same.

Calculating speed To calculate the speed of an object, you need to know two things: the distance traveled by the object, and the time it took to travel the distance.

Calculating speed Speed is also a ratio of distance to time. The word per means “for every” or “for each.” You can also think of per as meaning “divided by.”

Relationships between variables There are three ways to arrange the letters, or variables, that relate distance, time, and speed. You can solve for any one of the three variables if you know the other two.

How to solve physics problems Learning physics will make you a better problem solver, a skill is important in all careers. The technique for solving problems in this book has four steps: Identify what the problem is asking, and what variables need to be in the answer. Identify the information you are given. Identify any relationships between the information you are asked to find and what is given. Combine the relationships with what you know and what you are to find.

Looking for: … the speed in meters/second. Calculating speed An airplane flies 450 meters in 3 seconds. What is its speed in meters per second? Looking for: … the speed in meters/second. Given: …the distance (450 m) and the time (3 s) Relationships: Use a version of the speed equation: v = d ÷ t Solution: v = 450 m ÷ 3 s = 150 m/s

Scientific Method and Serendipity Serendipity is a term used to describe an event that happens by accident and results in an unexpected discovery. It is through education and a strong sense of curiosity, tempered with a bit of creativity (and yes, sometimes a little luck), that people can make great scientific discoveries.