Determine the Velocity of a Softball

Transcription

1 Determine the Velocity of a Softball Topic The time, distance, and direction that an object travels, along with the pull of gravity, determine the velocity of that object. Introduction The velocity of an object is different from its speed. Velocity is a vector quantity, which expresses both the speed and the direction of an object. One of the essential differences between speed and velocity is that speed refers only to the distance traveled over time. Velocity not only expresses the distance that is traveled during a certain period of time, but it also tells the direction of the object. Velocity describes a change in location, or displacement, over a period of time. If you travel several miles and end up at your starting point, your velocity is zero. During a professional baseball game, balls are thrown at high velocities. In order to calculate a baseball s velocity, you measure the distance the ball travels and the amount of time it took to get there, or the vertical displacement of the ball. Since the ball moves both vertically and horizontally, you must calculate the X-displacement (the distance and time it moved horizontally) and the Y-displacement (the distance and time it moved vertically). For the Y-displacement, you factor in the force of gravity (9.8 meters/second [m/sec]), which causes the ball to accelerate toward the ground. In this experiment, you will determine the velocity of a softball that is thrown through the air. Time Required 45 minutes Materials softball stopwatch

2 DETERMINE THE VELOCITY OF A SOFTBALL 2 measuring tape flags or wickets to mark positions calculator paper pencil access to an outdoor area or gymnasium science notebook Safety Note Please review and follow the safety guidelines. Procedure 1. Work in groups of four students. Select one person from your group to be the softball thrower, one to be the timer, one to be in charge of measuring, and one to record the data. 2. The measurer finds the distance from the ground to the thrower s point of release. (The thrower may take a couple of practice throws to identify the point of release). The recorder writes this in the science notebook as the Y-distance. 3. The measurer marks a starting position on the floor or ground with a flag. 4. The thrower stands at the starting position and pitches the softball forward. The timer starts the stopwatch at the moment that the ball leaves the thrower s hand and stops it when the ball hits the ground. The recorder writes the time in sec in the science notebook. 5. The measurer uses a flag to mark the exact location that the ball hits the ground or floor. This location is the horizontal endpoint. 6. The measurer uses a measuring tape to determine distance that the ball traveled from the start point to the end point in meters (m). The recorder writes the X-distance in the science notebook. 7. To find the X-displacement, the recorder divides the X-distance by the time (m/sec).

3 DETERMINE THE VELOCITY OF A SOFTBALL 3 8. To find the Y-displacement, the recorder multiplies the Y-distance by the pull of gravity, which is 9.8 m/sec. 9. In order to find the total velocity of the softball, the recorder uses the Pythagorean theorem: (X-displacement) 2 + (Y-displacement) 2 = V The recorder writes the calculations in the science notebook. Analysis 1. What was the total velocity of the softball? 2. Why was it necessary to calculate both the X- and the Y- displacement? 3. Why was it necessary to consider the pull of gravity on the softball as part of the calculation? 4. In what situations would it be more appropriate to use velocity than speed? What s Going On? When thrown into the air, a softball moves not only horizontally but also vertically. This is because there are two major forces acting on the ball when it is in the air. The first force, placed on the ball by the person throwing it, causes the ball to move horizontally at a certain speed. The second force acting on the ball is gravity. Gravity pulls objects toward the earth at approximately 9.8 m/sec. These 2 forces act in a 90-degree angle of each other, much like 2 sides of a right triangle (see Figure 1). The velocity is the equivalent of the hypotenuse of this right triangle. Therefore, in order to calculate the velocity, one can simply use the Pythagorean theorem: a 2 + b 2 = c 2, or = a 2 + b 2 = c.

4 DETERMINE THE VELOCITY OF A SOFTBALL 4 Figure 1 In our everyday lives, we often express movement in terms of speed. When we describe how fast a car is moving, we will typically tell its speed, not its velocity. However, velocity is very important in many areas of life. Velocity is often used when flying and landing airplanes, since aviators need to be aware of specific angles and directions of flight. Velocity is also commonly used to calculate the launch of rockets and other projectiles, such as those that would be launched in warfare. Want to Know More? See Our Findings.

5 OUR FINDINGS 4.8 DETERMINE THE VELOCITY OF A SOFTBALL Suggestion for class discussion: Ask students if they know how fast a great pitcher can throw a baseball. During games, pitches reach velocities of 90 to 100 miles per hour. Explain that a radar gun (the type used to detect the speed of a moving car) can be used to find the velocity of a baseball. In this experiment, students will be using a paper and pencil technique to find velocity. Analysis 1. Answers will vary depending on student results. 2. The ball moved both horizontally and vertically. 3. Gravity is a force that is acting on the ball to make it fall to the ground. 4. Answers will vary but could include launching a projectile or flying a plane.

6 SAFETY PRECAUTIONS Review Before Starting Any Experiment Each experiment includes special safety precautions that are relevant to that particular project. These do not include all the basic safety precautions that are necessary whenever you are working on a scientific experiment. For this reason, it is absolutely necessary that you read and remain mindful of the General Safety Precautions that follow. Experimental science can be dangerous, and good laboratory procedure always includes following basic safety rules. Things can happen very quickly while you are performing an experiment. Materials can spill, break, or even catch fire. There will be no time after the fact to protect yourself. Always prepare for unexpected dangers by following the basic safety guidelines during the entire experiment, whether or not something seems dangerous to you at a given moment. We have been quite sparing in prescribing safety precautions for the individual experiments. For one reason, we want you to take very seriously every safety precaution that is printed in this book. If you see it written here, you can be sure that it is here because it is absolutely critical. Read the safety precautions here and at the beginning of each experiment before performing each lab activity. It is difficult to remember a long set of general rules. By rereading these general precautions every time you set up an experiment, you will be reminding yourself that lab safety is critically important. In addition, use your good judgment and pay close attention when performing potentially dangerous procedures. Just because the book does not say Be careful with hot liquids or Don t cut yourself with a knife does not mean that you can be careless when boiling water or using a knife to punch holes in plastic bottles. Notes in the text are special precautions to which you must pay special attention. GENERAL SAFETY PRECAUTIONS Accidents caused by carelessness, haste, insufficient knowledge, or taking an unnecessary risk can be avoided by practicing safety procedures and being alert while conducting experiments. Be sure to

7 SAFETY PRECAUTIONS 2 check the individual experiments in this book for additional safety regulations and adult supervision requirements. If you will be working in a lab, do not work alone. When you are working off-site, keep in groups with a minimum of three students per groups, and follow school rules and state legal requirements for the number of supervisors required. Ask an adult supervisor with basic training in first aid to carry a small first-aid kit. Make sure everyone knows where this person will be during the experiment. PREPARING Clear all surfaces before beginning experiments. Read the instructions before you start. Know the hazards of the experiments and anticipate dangers. PROTECTING YOURSELF Follow the directions step by step. Do only one experiment at a time. Locate exits, fire blanket and extinguisher, master gas and electricity shut-offs, eyewash, and first-aid kit. Make sure there is adequate ventilation. Do not horseplay. Keep floor and workspace neat, clean, and dry. Clean up spills immediately. If glassware breaks, do not clean it up; ask for teacher assistance. Tie back long hair. Never eat, drink, or smoke in the laboratory or workspace. Do not eat or drink any substances tested unless expressly permitted to do so by a knowledgeable adult. USING EQUIPMENT WITH CARE Set up apparatus far from the edge of the desk. Use knives or other sharp-pointed instruments with care.

8 SAFETY PRECAUTIONS 3 Pull plugs, not cords, when removing electrical plugs. Clean glassware before and after use. Clean up broken glassware immediately. Do not touch metal conductors. Check glassware for scratches, cracks, and sharp edges. Do not use reflected sunlight to illuminate your microscope. Use alcohol-filled thermometers, not mercury-filled thermometers. USING CHEMICALS Never taste or inhale chemicals. Label all bottles and apparatus containing chemicals. Read labels carefully. Avoid chemical contact with skin and eyes (wear safety glasses, lab apron, and gloves). Do not touch chemical solutions. Wash hands before and after using solutions. Wipe up spills thoroughly. HEATING SUBSTANCES Wear safety glasses, apron, and gloves when boiling water. Keep your face away from test tubes and beakers. Use test tubes, beakers, and other glassware made of Pyrex glass. Never leave apparatus unattended. Use safety tongs and heat-resistant gloves. If your laboratory does not have heat-proof workbenches, put your Bunsen burner on a heat-proof mat before lighting it. Take care when lighting your Bunsen burner; light it with the airhole closed, and use a Bunsen burner lighter in preference to wooden matches.

9 SAFETY PRECAUTIONS 4 Turn off hot plates, Bunsen burners, and gas when you are done. Have a fire extinguisher on hand. Keep flammable substances away from flames and other sources of heat. FINISHING UP Thoroughly clean your work area and any glassware used. Wash your hands. Be careful not to return chemicals or contaminated reagents to the wrong containers. Do not dispose of materials in the sink unless instructed to do so. Clean up all residues and put them in proper containers for disposal. Dispose of all chemicals according to all local, state, and federal laws. BE SAFETY CONSCIOUS AT ALL TIMES!