Newton’s First Law

1. The law of inertia.

Explain how the law of inertia plays a role in each of the following. Many of these are available here for you to try out.

a. Pulling a tablecloth out from underneath the dishes (what we have is a sheet of paper and a bottle).

b. Tightening the head on a hammer.

c. Balancing a broom upside down.

d. Shaking the dust from a rug.

e. Seat belts in a car.

2. Natural State of Motion

Galileo convinced himself that the natural state of motion was to move with constant velocity...that is, constant speed and constant direction. He did this in part by observing that a ball rolled down an incline will keep moving until it comes back up to its original height. We have two ways of exploring that here:

a. Track. Roll a ball down the incline at one end and see if it rises to the same height at the other. Try this several times, with different initial heights. What do you find? If you don't find the initial and final heights to be equal, can you explain why this is?

b. Pendulum. Note that the pendulum will also rise back to its original height...even if a peg changes the "length" of the pendulum in the middle.

3. Coin and feathers. Galileo argued (correctly) that bodies fall at the same rate independently of their mass. This is only true if gravity is the only force acting on the bodies, which isn’t always the case. Air resistance slows down many objects, such as paper and feathers. Look at the demonstration device we have and carefully turn the tube upside down so the coin and feathers fall. With air in the tube, they fall at very different rates. Then turn the vacuum pump on and see if things change. Describe your observations.

4. Free body diagrams.

We represent forces by arrows pointing in the direction they act. A free body diagram is just a diagram of a given body, showing all the forces that act on that body. This is often useful in analyzing the motion of a body. For instance, here is a book lying on a table:

And here is a free body diagram for the book. Note that I've only drawn the book, not the table. I have labeled the two forces, and tried to start the arrows where the forces act.

support

gravity

Draw free body diagrams for the following situations.

a. A mass hanging from a string.

b. A baseball being hit by a bat.

c. A baseball flying through the air after being hit by a bat.

d. The moon in its orbit.