General Physics I: Day 7Newton’s Laws, Kinematics Review & Mini-Exam

Catalog of Forces

Weight () is the force of gravity and is different than an object’s mass. Always down!

Spring forces (). Springs will push out when compressed or pull back when stretched. Lots of other things behave the same way (trampolines!).

Tension forces (). Forces from ropes/cables/strings/wires/chains, etc. Pull only.

Normal force (). Force caused by contact with a surface. Always perpendicular to the surface.

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Catalog of Forces

Friction (). Sometimes present objects touch. Parallel to the surface.Static: Surfaces do not slide against each other. Kinetic: They do slide.

Drag () is similar to friction, caused by air, water or other fluids. Only used if explicitly discussed

Thrust (). Contact force created by expelling gas out the back of a rocket or other engine.

Electric and magnetic forces. The next long range force you will study… but not until Physics II!

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Make a quick list of the forces acting on you right now. How many are there?

A) 1

B) 2

C) 3-4

D) 5-6

E) 7+

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Free-Body Diagrams

FBDs let you visualize how forces are interacting.

Creating a free-body diagram (FBD):

1. Each FBD is for a single “thing”

2. Represent the object with a drawing or a dot.

3. Draw all the forces on the object as arrows.(don't draw anything else on the FBD)

4. Label each force with an appropriate name.

5. Larger forces → longer arrows.

6. For each force you should know: Object exerting it, object it acts on, what kind of force.

Draw the arrows outward from the

center of the object.

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FBDs – Do’s and Don’ts

FBDs are not drawings of the situation, they won’t look like the full picture… they are idealized diagrams all about force.

Before or after you draw the FBD, draw an arrow (off to the side) that represents the net force.

Once your FBD is done you are ready to use Newton’s 2nd Law!

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Consider the following situations for a ball:1. Held at rest in your hand.2. Falling downward after being dropped.3. Moving upward just after being thrown upward.

Which situation matches the force diagram shown?A) Only 1B) Only 2C) Only 3D) Both 1 & 3E) All three

Ignore Air Resistance

Motion With Constant Acceleration

We can handle constant acceleration (or averaged) in one dimension. Four equations apply.

For two dimensions, each component must be treated separately using the same tools.

For projectile motion:

• Must split into horizontal & vertical parts.

• and (is constant!)

• Do not mix horizontal & vertical quantities!

• You will usually need to tackle both

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Sample Problem

A boy throws a water balloon at and angle of 23° above horizontal and hits his friend who is 3.6 meters away. It is in the air for 1.1 seconds.

How far did the balloon fall on its way?

What was the horizontal speed as it hit the friend?

Other Qs:

At what speed was the ball thrown?

At what angle did it hit the friend?

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Circlular & Relative Motion

Moving in a circle requires centripetal acceleration

Relative motion

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2

c

va

r

/ / /P A P B B Av v v

Quiz/Exam Four Question TypesTrue or False: Choose T or FMultiple choice:

Choose one, no work needed… but you should draw/calculate something

Short answer:Answer in (readable) sentences and/or diagrams.Your explanation counts more than your answer.

Problems:Traditional physics problems with occasional conceptual pieces.

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Coming up…Right now → Quiz!

Thursday (9/11) → 4.4 – 4.6

Warm-Up due Wednesday by 10:00 PM

Homework #4 due Tuesday the 16th

Homework #5 due Saturday the 27th

Exam 1 is on Tuesday the 30th

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