# 4 October Science 7 Motion, Inertia, Force, Acceleration–Newton’s Laws

Read, Explain, Question, Summarize and Reflect

Newton’s 1st Law of Motion

is about a property of matter called inertia. If you realize that the cart could keep moving in a straight line with constant speed with no one pushing or pulling it (see graph below), you have understood a great deal about the concept of inertia. If you realize why it is important to wear a seatbelt in a car or airplane (or any moving vehicle), you have understood a great deal about the concept of inertia. Newton’s First Law of Motion is stated in several different ways. I suggest you examine some of these different ways and think about what they mean and how this law establishes a starting point for describing and understanding motion. You might find treatments like this <https://web.phys.ksu.edu/fascination/chapter7.pdf> useful.

Find as many different ways that people state Newton’s First Law of Motion as you can. Write a conclusion of your own.

Think about the times when an airplane pilot turns off the seat belt sign. Those are the times when an airplane is moving at a constant speed and is not changing directions. During those times you are in an inertial frame of reference. That means you can do any experiments with motion and you will get the same results you would if the plane were sitting still on the ground! Understanding what frame of reference means is fascinating and fun.

Watch these classic films and see if you can figure out what is happening: <https://www.youtube.com/watch?v=aRDOqiqBUQY>. What does this mean, “All motion is relative?”

Galileo, the important and famous Italian scientist, died the year Newton was born. Galileo’s work on relative motion and on acceleration set the stage for Newton to derive his incredible system for describing and explaining motion. Newton’s is the system we are examining in Science 7. (Later you will learn about how Newton’s system is incorporated in a more comprehensive system of describing and explaining motion developed by the famous modern scientist, Albert Einstein).

What happens when an unbalanced force is applied to an object? We first looked at this situation with the cart when we pulled it with a constant force. The cart changed its speed. You could think of how each successive instant of force was like a kick that ADDED to the existing inertial motion, thus the speed picked up. This link between force and acceleration is what Newton described in his Second Law of Motion. This law is often expressed as F = ma. What this expression means is that if a force of a particular size (magnitude) is applied to a mass, an acceleration occurs of a certain size. If the size of the force increases on the same mass, the acceleration increases proportionally. If a bigger mass is to be accelerated at the same rate as the smaller mass, a proportionally greater forces is needed to be applied to the greater mass.

Of great importance is the fact that mass determined by inertia (through the F = ma relationship) is equivalent to mass determined by acceleration due to gravity (Fgravity = mg (the rate of acceleration due to gravity).

This equivalence is important in resolving the puzzle of why two objects of different mass dropped from the same height at the same time reach the ground at the same time. See if you can use the concepts of inertia and gravitational force to explain the results of this puzzle that confounds many people.

How can we determine the value of acceleration?

• Acceleration = change in speed / change in time
• remember that speed is change in distance / change in time
• thus acceleration could be expressed as change in distance / change in time / change in time
• acceleration is the rate of a rate!!!

Before we have used the idea of average speed = change in position/change in time.

• We need new concepts to think about changing speed or acceleration
• Instantaneous speed and Instantaneous velocity. (Velocity is a speed with the direction of movement indicated)
• Changing velocity includes speeding up, slowing down, changing directions. All of these situations are considered acceleration
• To find acceleration we would need to know instantaneous velocities (how might this be done?)

We will see how this is important in a few upcoming examples.

Let’s try to get some times for a lump of clay falling from different heights.

We’ll do a few trials for each height and take the median times. Then we’ll find the average speeds for each height using the median times. We’ll plot a distance vs. graph using mean times.

We will needed everyone’s cooperation in timing, videoing, measuring, retrieving the clay, etc.

If anyone has a video app that includes a running timer in the background, please let us know. We can use the building as a kind of ruler.

See the video captures from another year. Let’s get better images and video this year!

Inertia refers both to constant speed and straight line motion. When both speed and direction are specified, the term is velocity. If speed and direction are constant, the velocity is constant. If the velocity changes, there must be a force.

Why is gravity considered a force? How do you think Newton related the fall of an apple to the orbit of the moon?