Newton’s first law This lesson introduces newton’s first law: an object at rest will remain at rest, and an object in motion will remain in motion at constant speed and direction UNLESS acted upon by an unbalanced force. The first part of Newton's first law is intuitive: an object at rest will stay at rest unless acted upon by a net force. The second part of the law is not intuitive: an object in motion will remain in motion unless acted upon by a net force. The lesson creates a paradox by asking how it could be true since all real objects slow down and stop unless a continuous force is applied.

The first law The first law has two parts. This first is obvious from experience.

The first law The first law has two parts. The second part is not so obvious. the second part of Newton’s first law is a conceptual hurdle. Students believe that a force is required to keep an object in motion, and will often invent such forces where they do not exist. Free-body diagrams are excellent for revealing these errors.

The first law In everyday life all moving objects eventually slow down and stop because the net force is NOT zero.

Zero net force To create motion at constant speed in the real world, a constant force must be applied to make the net force zero. The first law is correct (of course) and objects do move at the same speed and in the same direction when the net force is truly zero.

In deep space, far from any source of gravity, objects travel forever in straight lines – precisely obeying the first law.

Understanding the first law These two parts of the first law are really identical. They are both telling you this: This slide summarizes the key idea of the lesson.

The law of inertia Newton’s first law is also known as the law of inertia. Inertia is the tendency of an object to resist changes in the speed or direction of its motion. The inertia of an object is related to its mass. The more massive an object is, the more inertia it has, and the more it will resist having its motion changed. This slide summarizes the key idea of the lesson.

The law of inertia Inertia egg video Inertia Earth video This slide summarizes the key idea of the lesson.

Assessment According to the first law, an object at rest … accelerates inversely proportional to its mass. remains at rest because it has zero acceleration. remains at rest unless acted upon by a net force. remains at rest due to the constant presence of friction. This formative assessment is keyed to the first objective: state the first law and explain its meaning. The answer, C, appears on the next slide.

Assessment According to the first law, an object at rest … accelerates inversely proportional to its mass. remains at rest because it has zero acceleration. remains at rest unless acted upon by a net force. remains at rest due to the constant presence of friction.

Assessment According to the first law, an object in motion remains in motion . . . forever. at the same speed. with acceleration inversely proportional to its mass. at the same speed and direction unless acted upon by a net force. Again, this assessment is keyed to the first objective: state the first law and explain its meaning. The answer, D, appears on the next slide.

Assessment According to the first law, an object in motion remains in motion . . . forever. at the same speed. with acceleration inversely proportional to its mass. at the same speed and direction unless acted upon by a net force.

Assessment At the instant shown, an object is moving upward with a speed of 25 m/s. There are constant forces acting on it, shown on the free-body diagram. Predict the behavior of the object. This assessment is keyed to the second objective: calculate the effect of forces on objects using the law of inertia. The answer appears on the next slide.

Assessment At the instant shown, an object is moving upward with a speed of 25 m/s. There are constant forces acting on it, shown on the free-body diagram. Predict the behavior of the object. The object will move upward at a constant speed of 25 m/s as long as these are the forces acting on it.

Assessment A puck slides across an ice rink. The weight and normal force on the puck cancel out, but the puck gradually slows to a stop. Why doesn’t the puck obey the first law, staying in motion at constant velocity forever? This formative assessment is keyed to the third objective: explain conceptually why moving objects do not always appear to obey the first law. The answer appears on the next slide.

Assessment A puck slides across an ice rink. The weight and normal force on the puck cancel out, but the puck gradually slows to a stop. Why doesn’t the puck obey the first law, staying in motion at constant velocity forever? The NET force on the puck is not zero. There is a small amount of friction that causes the puck to slow down. Students may correctly point out that there is both sliding friction and air friction (drag).