Unit 4 & 5 - Forces (Newton's Laws)

Unit 4 /5 Forces:

Pre-test with T/F on different sides of classroom:

Learning objective:

Allows teacher to discover students’ pre-conceptions

                         

Intro Forces Discussion:

Introduces different types of forces, free body diagrams and agent-object notation.

Learning objective:

Introduce FBD and agent-object notation

Introduce that Earth pulls on object (gravity is not a thing J, it’s a phenomenon)

Introduce that surfaces also exert a force ( anchor, bridge , target)  

Bowling Ball Intro Activity:

I loved this activity and I will definitely plan to use it next year in my classroom.  In this activity, students used mallets to cause a bowling ball to 1) speed up, 2) move at a constant speed 3) slowdown 4) Make a 90 degree turn and 5) move in a circle.   We also had races.

Learning Objective:

Provide a connection to Newton’s 1^st law.

Post lab:   FBD were drawn for each instance.   This was a great intro to FBD.  Class summarized findings on whiteboards.  Student voted on favorite whiteboards and those were kept at front to revise later (1^st law)

Unbalanced Force –Pulley Demon:

Demo used unbalanced forces to show car accelerating.

Learning objective:

Modify wording on 1^st law to read “unbalanced” force.

Unit 4 Worksheet 1 (no ramps)

Prior to worksheet, we discussed friction and vectors (meter stick with shadow).  After the worksheet we white boarded our FBD .   I may do this activity as a class with white boards instead of on paper.  

Learning Objective:

More practice with FBD

Weight Activity:

Have students pick their own objects to hang from a “force-o-meter” Record mass of object and force.  Students will plot Force on y axis, mass on x axis.  Slope will be approx. 9.8 (ours was 9.76 J). 

Learning Objectives:

Develop Equation for weight F= 10m or F=9.8m (F=mg)

Introduction to ramps (Finish Unit 4 worksheet 1 – with ramps):

Students made motion map of object sliding on a ramp.  Then students were asked to draw a free body diagram, “remember if an object is accelerating, there must be an unbalanced force”.    After responses were drawn and discussed, a whiteboard with a book was shown on a ramp.  Students  were able to see how indent changed with angle.   

Learning Objectives:

Introduction to Free-body diagrams with ramps. (change axis)

Unit 4 Worksheet 3 (at rest or constant speed – prior to 2^nd law) :

Learning Objectives:

Using model to solve problems  - drawing FBD, Solving for weight, breaking forces into components, solving problems on ramps, balancing forces for objects at a constant speed or rest.

Newton 2^nd law lab ( Four labs to choose from )

Prior to lab, each student group pushed a cart with a constant force and observe what happens (accelerates… goes faster and faster).

Part 1: What happens when you push a cart (on a frictionless track) with a constant force (graph F vs. a) Force on y, acceleration on x.

Part2:  Different carts (different masses) with same force, what would happen to acceleration?

Each group picked from 1 of 4 labs.

1)      Use force-o-meter to push/pull with a constant force.  Part 1: change force, Part 2: change mass.  Groups used a motion detector.  May be able to calculate using delta x and time.

2)      Use ramp.  Adjust angle and measure force.  Then remove force-o-meter and release.  We set delta x at 2m and recorded time and calculated acceleration.  Could possibly use motion detector.  Part 1: varied angle to vary force.  Part 2: increase mass, angle will need to be adjusted until force is constant.

3)      Use pulley.   Part 1: As mass is added to hanger, remember to decrease mass to car in order to keep mass of system constant.  Part 2: keep mass of hanger constant and add mass to car.  Remember mass of system is total mass.

4)      Use constant acceleration fan cars.  Part 1: vary the batteries to adjust force.  Part 2: vary mass.

Deploy Unit 5 Worksheet :

Introduce Friction – Bowling ball in net & brainstorm experiments:

Bailey walked, then jogged, then ran  (all at a constant speed) with bowling ball while holing a force sensor attached to the bowling ball.  The force was measured and a prediction was made.

Afterwards, the class was asked to brainstorm items that may affect friction. Different experiments were conducted to discover what might affect friction. 

The class predicted mass, speed, pull angle, surface interaction, surface area may all affect the friction force.   A free body diagram was made

At the conclusion, it was decided that speed and surface area did not have an effect.  Surface angle (ramp angle was inconclusive, but might have an effect).  

The class concluded, that the mass and pull angle, which both affect normal force did have an effect.  In addition, surface interaction also had an effect.    The relationship Friction = muFN was developed.

Learning Objectives:

Students have an understanding of what affects frictional force

Develop equation f= uFn

Intro 3^rd Law:

Use springs to show that force is same everywhere regardless if you pull, are being pulled or are in the middle.  Use 5N force on one side, If two scales on other side, both read 2.5N.

3^rd Law Demo with Logger Pro – I need to see if I have equipment that can duplicate this demo.  Huge emotional response

Two cars on a track (each with force sensor).  Show that forces are equal (opposite in direction) during collision.  Vary situation to address all student misconceptions (mass, ramps, which car hits what, etc).  Make sure all students give ideas and all preconceptions are addressed.  (it’s difficult to set-up so you want to cover it all now).

Learning Objectives:

Address pre-conceptions.

Justify Newton’s 3^rd Law.

3^rd Law & FBD

Laura led a discussion where she drew FDB for a person standing on the ground and asked if it represented the 3^rd law since the Feo and Fgo were equal and opposite.     Students recognized that two free body diagrams were needed since the first only had one object not two.  I realized how agent-object notation really helped address this common pre-conception.

A FBD was also drawn for the earth and students recognized the small acceleration due to the large mass of the earth.   This also addresses a concept that is difficult for student to accept.

Learning objective:

Address common issues- two FBD for 3^rd law and that unbalanced force on the earth.

Deploy Unit 4 & Unit 5 Worksheet 4 for both.