Essentials of Physics- PHYS 101

Lecture 4

Class business-- Solar observations

Joke of the day.

About that ILD on Monday.....

The relationship between acceleration and force.

Class Business-- Solar Observations

Please get an acrylic half-dome for solar observations. Solar observations are to be recorded for 2 different, sunny days. Try to complete the first day's observations as soon as possible. Separate your observations by at least two weeks, if not more. Use the same dome for both sets of observations. Mark the domes with different colored pens to denote each day.

Observations, including the completed instruction sheet and dome, are due at the beginning of Lab 6, on May 6th or 7th.

Here is the PDF file of the instruction sheet.

Movietime!

• "Absolutely riveting!," Rosie (the) Riveter, Steel Structure Journal.
• "A fascinating juxtaposition of concepts and graphics with, well... ideas and graphs," Sir Oliver Stone-Lb, Royalty Weekly.
• "A plaintive dynamic cart defying a pushy professor while retaining its pride and dignity-- a lifelike triumph of reality and algebra over style and ignorance," Prof. Livelybrooks, The Extremely August Journal of Physics and Movie Review.

The relationship between acceleration and force.

Let me guess:

• they're brother and sister.
• they've been carrying on for years now... its scandalous!
• they're frosty just now, acceleration was seen in public snuggling up to mass.

In physics, a "relationship" refers to how one varies when the other is changed. Say we double the total force acting on an object. What will happen to its acceleration? Will it half, stay the same, double, quadruple, or something else?

Here is a graph showing various possible relationships between acceleration and force:

Which of the above mathematical relationships do you think will apply to force and acceleration? Why?

Now that we have predicted (hypothesized) a particular relationship, how do we test it?

Thinking about it another way, if the following velocity-time graph is observed for a certain force acting on a cart, how will that graph change when the force is doubled?

We can test this using our old friends the cart, track, motion detector and, introducing the fabulous hanging mass

Problem-solving tricks and techniques

Problems solving problems?

1) You can always multiply by 1

Example: 1.6 km = 1 mile. 1.6 km / 1 mile = 1. To convert miles to km, I multiply by 1:

8.0 miles x (1.6 km / 1 mile) = 12.2 km

2) Multiply both sides of an equation by the same thing (number):

Say we know that SF = m a

Say we also know a and SF. How do I solve for m?

Multiply both sides of equation, above by the same thing (1/a):

SF (1/a) = m a (1/a) => m = SF/a

More on the relationship between a, m, and F

 

Last week we observed that the acceleration of a cart doubled when the force pulling it was doubled.   More generally we found that acceleration is proportional to the net force.

a µ F

 

What about mass?

 

We know mass gives an object inertia which resists a change in motion.  Another way to say this is more mass makes an object harder to accelerate.

What is the exact relationship between acceleration and mass?

In lab you kept the pulling force constant but changed the mass of the car.  What you found (hopefully) was that if the mass of the car doubled the acceleration was reduced by half.

 

a µ 1/m

 

'acceleration is inversely proportional to mass'

put all this together to get-- Newton's Second Law

Put this together with a µ F and what do we get?

a = F_net/m  more commonly written as:   F_net = ma !

How does gravity work?

Let's observe the effect of "gravity" on a softball with the following experimental setup:

Question: What happens when the board is pulled out from under the softball?

Answer: Duh, let me guess.....

Question: So, HOW is the ball moving towards the ground?

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