# Lesson 1: Newton's First Law- Concept Discussion

Let's look at what the exploration activities you've just completed can show us about how physics works. In the first activity you watched a ball roll across a flat track. You were asked to measure the speed of the ball at the beginning and end of the track, which you could do by simply dividing the distance the ball moves in one or two frames by the time elapsed during those one or two frames. Its important to use just one or two frames at the beginning and end so you can compare the instantaneous speed at those points. If you divide the whole distance the ball travels by the time it takes to travel that distance, you're not really comparing the speed at the beginning and end. You should have found that the ball's speed doesn't change much from the beginning of the track to the end of the track. If the ball's speed is the same at the beginning and end of the track you might think it should have the same speed in the middle of the track. Watching the ball gives no indication that the ball does speed up or slow down, so you would be right. This tells us something important about the way the world works. If an object is moving with a certain speed, then it is inclined to continue moving with that same speed unless something pushes or pulls on it to change the speed. For example if we put an obstacle in front of the ball we know the ball would bounce off, and its speed would change. So it is safe to guess that if we doubled the length of the track then the ball's speed would still be roughly the same as you measured before. At the same time, we know from our life experiences that if the track is made long enough this will stop being the case. If the track were ten times longer then the ball might begin to slow down appreciably, because of friction. The track itself is pulling on the ball, causing its speed to change. So now we've seen Newton's first law of motion: objects will continue in constant velocity motion unless an outside force (push or pull) acts on them to change their velocity.

In the second activity you looked at a coffee cup on the back of a car as the car drove off. Here you saw that even when the car began to move the cup wanted to stay in one place. Its velocity in the horizontal direction didn't change (look at the cup's position with respect to the pole). Ultimately it falls down because gravity exerts a force pulling the cup downward. This tells us another important thing about how things work: constant velocity can mean zero velocity. So if an object is sitting still it will continue to sit still unless something exerts a force to change its velocity. In this case the top of the car and the bottom of the cup are smooth and there is little friction between the cup and the car, so the car doesn't exert an appreciable horizontal force on the cup.

In the third activity you looked at a crash test in which the dummy is not wearing a seatbelt. You were asked to estimate the velocity of the car and dummy before and after the crash. The first thing to note is that the car and dummy move together before the crash so whatever speed one has, the other has the same speed. You can find this speed by estimating a distance traveled in a certain number of frames and then dividing by the time elapsed in those frames. Next note that after the crash the car is not moving, so its speed after the crash is zero. So all that remains is the speed of the dummy after the crash. A careful estimation (or even just qualitative observation) shows that the dummy has roughly the same speed after the crash as it had beforehand, until it collides with the dashboard. The dashboard exerts a force on the dummy, stopping it. Again this shows us Newton's first law. The dummy is moving with constant velocity and continues to do so until something (the dashboard) exerts a force on it to bring it to a stop.

(c) 2008 Kansas State University Physics Education Research Group.
Maintained by Chris Nakamura.