Today we talked about forces and Newton’s 1st and 2nd law. We started with just a block sitting on the table and I asked the kids to describe the forces acting on the block. Then they drew the block on a sheet of paper and labeled the two forces, gravity pulling down and the table pushing up. Then I pushed on the block so it moved across the table and asked them again what forces were acting on the block and we drew another picture, adding the force of me pushing and the force of friction acting against that force. The kids already had a pretty good understanding of friction and the fact that its what slows things down, so they had no problem with Newton’s 1st law, that objects at rest stay at rest and objects in motion want to stay in motion.
We did the lab “Inertia and You!” from Real Science Odyssey Physics where we had lego cars with minifigures sitting on top, but on flat plates, not connected with legos, so the figure was free to slide around. The students guessed what would happen to the lego figure when they gave the car a push, wrote it down and then did the experiment. As expected the minifig got left behind.
Then the kids added a few bricks to give the minifigures a backrest and did the experiment again. This time the car was able to push on the
lego figure and he went along for the ride.
The third part of this experiment involved crashing the car, bringing it to an abrupt stop and observing the motion of the minifigure. We talked about seat belts and airbags and other ways cars are designed to keep us from ending up like the minifigures.
Next the students tied strings to the lego cars and pulled them along on a smooth floor – exerted a constant force on the car. They made observations on the motion/speed of the car. This is Newton’s Lab #4: You’v Got to Keep it Moving!, from Real Science Odyssey Physics.
Lastly we played with the air track. I set up a cart on the track with elastic bumpers so once its put in motion it just keeps moving back and forth along the track. We used the Video Physics app to record the motion and saw that yes, the cart was moving at a constant speed since the red dots marking the carts position moved the same distance in every time interval.
Then I asked them what would happen to the motion of the cart if it had a constant force pulling on it (a string attached to it, going over a pulley and tied to a weight) and most of them answered correctly that the cart would speed up or accelerate
You can see that the cart was barely moving on the left side of the photo since the dots are right on top of each other, but as the cart moves to the right the dots get further and further apart indicating that the cart travels a greater distance with each time interval, its speeding up!
With my older classes I would have the kids take the data and analyse the graphs but these kids are pretty young so my son and I helped them take the data/videos and just talked about the pictures as I did above. But even without using the app we could easily tell the cart was accelerating when there was a constant force acting on it. I asked them what would happen if I doubled the weight on the end of the string, therefore doubling the force and they answered correctly that the acceleration would double. Then I asked, “What if we increase the mass of the cart?”, and they realized that would make it the cart harder to move so the acceleration would decrease. So we figured out Newton’s 2nd Law, F=ma.
One other thing we did in class was play with these air hockey pucks (air powered soccer disk). They’re battery powered and act like a mini hoover craft, shooting air jets out the botoom, so they shoot across the floor with just the smallest of pushes and show that Newton was indeed right, if we can get rid of friction then an object in motion will stay in motion. The kids had a lot of fun playing with these.
Here’s a nice website by BBC with an interactive on forces.