Newton’s First Law

What do you think? Imagine the following two situations: Pushing a puck across an air hockey table Pushing a book across a lab table What should your finger do in each case to maintain a constant speed for the object as it moves across the table or desk? (Choose from below.) A quick push or force, then release the object Maintain a constant force as you push the object Increase or decrease the force as you push the object Explain your choice for the puck and the book. When asking students to express their ideas, you might try one of the following methods. (1) You could ask them to write their answers in their notebook and then discuss them. (2) You could ask them to first write their ideas and then share them with a small group of 3 or 4 students. At that time you can have each group present their consensus idea. This can be facilitated with the use of whiteboards for the groups. The most important aspect of eliciting student’s ideas is the acceptance of all ideas as valid. Do not correct or judge them. You might want to ask questions to help clarify their answers. You do not want to discourage students from thinking about these questions and just waiting for the correct answer from the teacher. Thank them for sharing their ideas. Misconceptions are common and can be dealt with if they are first expressed in writing and orally. Answers will vary. If you have an air track or table or a low-friction cart, you should have the students try their ideas. Hopefully they will see that the continuous push is only needed if there is an opposing force of friction. If not, you can come back to these questions after working through Newton’s 1st law. Students might insist that you need to keep pushing the air hockey puck because it will slow down ever so slightly due to the slight amount of friction. Ask them questions about why this is true. Be very accepting of all answers because there may be a wide variety of beliefs.

Newton’s First Law An object at rest remains at rest, and an object in motion remains in motion at constant velocity (that is, constant speed in a straight line) unless the object experiences a net external force

Called the law of inertia Inertia Newton’s First Law Called the law of inertia Inertia Tendency of an object not to accelerate Mass is a measure of inertia More mass produces more resistance to a change in velocity Students may choose the moving tennis ball if they confuse inertia (mass) with momentum (mass times velocity). Emphasize that inertia depends only on mass, and so the baseball has a greater inertia in both cases.

Newton’s First Law Which object in each pair has more inertia? A baseball at rest or a tennis ball at rest Answer: the baseball A tennis ball moving at 125 mi/h or a baseball at rest Students may choose the moving tennis ball if they confuse inertia (mass) with momentum (mass times velocity). Emphasize that inertia depends only on mass, and so the baseball has a greater inertia in both cases.

Net Force: the Sum of the Forces This car is moving with a constant velocity. Fforward = road pushing the tires Fresistance = force caused by friction and air Forces are balanced Velocity is constant because the net force (Fnet) is zero. Ask students how to increase the speed of the car. Answer: Increase the forward force (accelerator) or decrease the resistance force (make the car more aerodynamic). Ask students how to decrease the speed of the car. Answer: Increase the resistance force (the brakes) or decrease the forward force (accelerator). This will provide a nice introduction to Newton’s 2nd Law.

Equilibrium The state in which the net force is zero. All forces are balanced. Object is at rest or travels with constant velocity. In the diagram, the bob on the fishing line is in equilibrium. The forces cancel each other. If either force changes, acceleration will occur. After reviewing this slide, return to the previous slide and ask students if the car is in equilibrium.

Practice Problem Derek leaves his physics book on top of a drafting table that inclines at a 35o angle. Draw a free-body diagram showing the forces acting on the book. Find the net force acting on the book.