Tuesday, Dec 2, 2008 - Block 4B

Content Objectives
Student will be able to:
Define what is meant by force
Define Impulse
Define Momentum
Explain with examples: Impulse = Change in Momentum
State and define the two main types of collisions

We talked about speed - how fast you are going.
Does direction matter? yes. If you combine speed with direction
Students were able to define force as a push or a pull
Gave examples of balanced forces - no change in motion
If the forces are unbalanced, there is a change in motion
We describe changes in motion by acceleration = rate of change in velocity = change in velocity/time
The acceleration of gravity at the surface of the Earth is about 9.8 m/s/s which we approximate as 10 m/s/s. We found this from the Picket Fence Lab

If you drop an object and it falls for 1 second, what is its speed? 10 m/s
after 2 seconds? 20 m/s
after 5 seconds? 50 m/s

Why does the speed increase? The force acted for a longer period of time.
If you want to really increase the speed of an object, you apply the biggest force for the longest time.
Force * Time = Impulse It is a measure of how you change an objects motion.

If I throw a tennis ball at you, it is easy to catch. It is easy to stop it.
If I throw a bowling ball, it is much harder to stop. It has something more because of its mass. For the same speed, it is easier to stop the object with the smaller mass.
However, would you rather stop a bowling ball I throw to you or a bullet fired from a gun? In this case, even though it has less mass, the bullet has something more because of its speed. There are two things involved, mass and speed. In addition, direction matters. It does make a difference if the gun is aimed at you or away from you. Instead of speed, we use velocity.

Momentum = mass * velocity It is a measure of how hard it is to stop. A sports team on a winning streak has a lot of momentum - hard to stop. When you catch a bowling ball, you make the sound, "ooomph". This is the technical term. When you see a really massive object, you say, "OOO that's really massive". The "mph" is how fast you are going.

Impulse = change in momentum
Force * time = m vf - m vi

For the same change in momentum, you can have very different forces depending on the time of collision. Several examples: jumping into a net or sidewalk, catching a baseball, stopping an egg.

Went outside and did egg toss lab with sheet. Later tossed between partners.

There are two main types of collisions, elastic - things bounce, inelastic - things stick. Demonstrate with air track.

Show elastic collisions with Newton's Cradle.

What will happen when objects of different masses collide elastically. Demonstrate golf ball hitting bowling ball, bowling ball hitting golf ball. Both coming together. Competition with bowling ball and golf ball.

Test on Monday next week on motion and momentum.