Newton’s first law Pg. 19 in NB 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.

Objectives Physics terms at rest inertia inertial reference frame State Newton’s first law and explain its meaning. Calculate the effect of forces on objects using the law of inertia. Explain conceptually why moving objects do not always appear to obey the first law. at rest inertia inertial reference frame The lesson objectives describe what a student should know and be able to do.

The first law Part 1 The first law has two parts. This first is obvious from experience. An object at rest stays at rest

The first law Part 2 The second part is not so obvious. An object in motion stays in motion with the SAME speed & direction unless acted on by an outside force. (i.e. keeps doing what it was doing as long as Fnet = 0N) 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.

First law examples Give an example of an object obeying part 1 of Newton’s 1st Law Give an example of an object obeying part 2 of Newton’s 2nd Law Allow students time to suggest answers.

Quick Reminder What does net force (Fnet ) refer to? If the net force (Fnet) acting on an object is 0N (zero) what happens?

Reality In real life would this ball keep rolling the same speed & direction forever? YES or NO

So what does that mean? If the ball slows down, is Fnet = 0N? YES or NO What forces are acting on the ball? So what’s the force causing the ball to slow down? Excellent question knowledge detective! Let’s investigate

Lets analyze the impact of the forces on the ball Is the ball moving vertically or horizontally? Does Fnet in the vertical equal zero? Yes or No Does Fnet in the horizontal equal zero? Yes or No Does Weight act vertically or horizontally? So does the weight slow the ball down? Yes or No What other force acts in the vertical? Does this force slow the ball down? Yes or No

What forces act on the ball horizontally? The vertical forces on the ball aren’t slowing it down so now we’ll look at the horizontal forces to determine the mystery force! What direction is the ball rolling? LEFT or RIGHT Is there anything pushing the ball to keep it rolling that way? YES or NO So if the ball is slowing down, what direction do you think the missing force is going? LEFT or RIGHT SOOOO the mystery force going OPPOSITE THE ball’s motion is….. FRICTION!

What forces act on the ball? Friction occurs when two surfaces that are NOT perfectly smooth move against each other. The force of friction is always in the OPPOSITE DIRECTION of the way the object is moving.

Zero net force To create motion at constant speed in the real world, a constant force must be applied to oppose the force of friction & make the net force zero.

Zero net force So even though the 1st law is true, the net force on an object is very rarely zero. Can you think of an example of zero net force on a moving object?

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

In a diffuse gas the atoms and molecules move according to the first law. The molecules experience forces when they collide with each other or the container walls.

Understanding the first law So going back to where we started, the first law tells us that: This slide summarizes the key idea of the lesson. If 𝑭 𝒏𝒆𝒕 =𝟎 then 𝒂=𝟎

Understanding the first law The first law goes both ways: This slide summarizes the key idea of the lesson. If 𝑭 𝒏𝒆𝒕 =𝟎 then you know 𝒂=𝟎 If 𝒂=𝟎 then you know 𝑭 𝒏𝒆𝒕 =𝟎

Understanding the first law Motion ONLY changes through the action of a net force. If the net force is zero – there can be NO changes in motion. (i.e. NO acceleration)! 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 = laziness Matter (what makes up mass) is lazy & won’t change its motion unless its FORCED to. So that means 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.

Homework #1 (for review) If something is accelerating is its velocity (speed) changing or staying the same? If something has a nonzero acceleration is its velocity (speed) changing or staying the same? What’s a force? Is force scalar or vector? Does force care about direction? When you stand on the floor, why don’t you sink through it? What 2 things do you need to know to find an object’s weight?

Homework #2 2. 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.

Homework # 3 3. 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.

Homework # 4 4. 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.

Homework # 5 5. 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.