Newton s Second Law of Motion

Transcription

1 Newton s Second Law of Motion Topic Newton s second law of motion describes how acceleration is related to force and mass. Introduction Newton s second law of motion states that the acceleration of an object is directly proportional to the magnitude of the net force acting on it, in the same direction as the net force, and is inversely proportional to the mass of the object. In other words, an object will move in the same direction as the force that is acting on it. The acceleration of the object depends on the strength of the force as well as the mass of the object. A stronger force will cause the object to accelerate more quickly than a weak force, and lighter objects will accelerate more than heavier ones. Newton s second law can be applied to many everyday situations. For example, this law explains why it requires more force to move a bowling ball than it does to move a basketball at the same velocity. In this experiment, you will apply a force to a dynamics cart containing different masses, then create a graph to show how the force applied relates to the mass and acceleration of the cart. Time Required 50 minutes Materials dynamics cart scale or triple-beam balance spring scale that measures in newtons (N) string

2 NEWTON S SECOND LAW OF MOTION 2 balance several standard weights long, level table or floor space science notebook Safety Note Please review and follow the safety guidelines. Procedure 1. Use the scale to find the mass of the dynamics cart in grams (g). Convert the mass of the cart from g to kilograms (kg) with the following formula: mass in kg = mass in g 1,000 Record the mass in kg on the data table. 2. Tie a string to the dynamics cart. Make a loop in the other end of the string and attach it to the spring scale (as in Figure 1). Figure 1 3. Measure the force (N) that is required to pull the empty cart across the table. As you pull, do your best to maintain a constant velocity. Record the force on the data table. 4. Add one weight to the cart. Find the total mass of the cart and weight and record them on the data table.

3 NEWTON S SECOND LAW OF MOTION 3 5. Repeat steps 3 and 4 until you have completed five trials, each with one additional weight. 6. Empty the dynamics cart of all weights. 7. Pull the empty dynamics cart across the table using a constant force. Try to get the spring scale to read, and remain at, 1 N. In your science notebook, describe what happens to the cart as you attempt to maintain 1 N. 8. Next, pull the cart and try to keep the force at 2 N. In your science notebook, describe what happens to the cart. 9. Add several weights to the cart and repeat steps 7 and 8. Data Table Number of weights Mass of cart (kg) Force (N) Analysis 1. Create a graph from your data. Place mass on the x-axis and force on the y-axis. 2. How is the mass of the cart related to the force required to move it? 3. What happened to the cart when you tried to keep the force constant? 4. How did adding mass to the cart affect its acceleration?

4 NEWTON S SECOND LAW OF MOTION 4 What s Going On? Newton s second law of motion picks up where the first law of motion leaves off. According to the first law of motion, objects that are at rest will stay at rest until a force acts on them and objects in motion will stay in motion until they are acted on by a force. Therefore, it takes a force to make an object, such as a dynamic cart, move. More force is needed to accelerate a heavier object than a lighter one. Newton s second law can be summed up with the equation: F = ma in which force (F) is equivalent to the mass (m) of the object multiplied by its acceleration (a). With application of a constant force, an object will continue to accelerate until another force acts on it. Friction and gravity are two forces that slow objects. Massive objects not only require more force to make them accelerate, but they will also accelerate at a slower rate than lighter objects. Want to Know More? See Our Findings.

5 OUR FINDINGS 4.3 NEWTON S SECOND LAW OF MOTION Suggestion for class discussion: Ask students which would take more force to start in motion: a car or a bicycle? Then ask them the reverse: Which would require more force to stop? Help them start thinking about the relationship between the mass of an object and the force required to impact the motion of that object. Analysis 1. Graphs will vary, but they should show a positive linear relationship between the mass of the cart (x-axis) and the force required (y-axis). As the mass increases, the force also increases. 2. The more massive the cart, the more force is required to move it. 3. The cart continued to gain speed as the force was continuously applied. 4. When masses were added to the cart, it did not accelerate as quickly.

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!