When the Normal Force is not Normal

As we learned about friction in class, we were introduced to the concept of the normal force. Many times, the normal force is exactly equal to the force of gravity on the object. However, this is not always the case. Can you think of any situations where the normal force is not simply equal to the object's weight? Leave comments with your ideas so that each idea is in its own comment.

Alternatively, do you have any questions about either the normal force or friction?

The Curveball

Photo by Sean Winters

I've been watching a lot of baseball on TV lately. I think it's amazing how professional pitchers can cause the path of their pitches to curve (or deviate from a straight-line path) radically. Here are some questions for discussion:

1. Many baseball terms exist to described pitches that curve. For example, curveball, screwball, slider, and cutter. What are the distinguishing characteristics of these pitches? Are there other terms that belong on this list?
2. What causes a pitch to curve (or break)?
3. Does the shape of a baseball affect how much it breaks? If so, what shape ball would break the most?
4. Does the speed of the ball affect how much it breaks?
5. Thinking about 3 and 4, what kind of ball could you use to demonstrate various breaking pitches? Ideally, the ball would be easy to throw and it would very noticeably break.

Thinking about Free Fall

We've been talking about free fall in physics class. Consider the following questions:

1. What are some examples of object that are in free fall (or at least "nearly" in free fall)?
2. Can an object be in free fall and have a velocity of zero? Explain.
3. Suppose the acceleration of gravity on Planet X is 20 m/s/s. How long would it take an object to fall to the ground from a height of 100 meters on Planet X? (Don't just give a numeric answer. Explain your reasoning.)
4. The acceleration of gravity on the Moon is about 1/6 as much as Earth's. Suppose an astronaut throws a ball up vertically with an initial velocity of 20 m/s. How high above the Moon's surface will the ball rise?  (Don't just give a numeric answer. Explain your reasoning.)

Select one of the above questions and leave your answer as a comment. You can also comment about what another student has said. Please don't continue to comment on a discussion that has been "solved".