Ch 2 Section 7

October 18, 2010HW: PTG #1-5, Due Thursday, Get Grade Report Signed

Learning Objectives: Explain the effects of friction on

moving objects Calculate the coefficient of sliding

friction Explain factors that may change

the amount of friction

Success Criteria Apply the definition of the

coefficient of sliding friction, µ Measure the coefficient of sliding

friction between the soles of athletic shoes and a variety of surfaces

Calculate the effects of frictional forces on the motion of objects

Do Now: Write learning

objectives and success criteria (left side)

WDYS/WDYT (right side)▪ Pg. 210

Agenda: Do Now LO/SC Design Experiment

Investigate #1

Choose a group member’s shoe. This person will need to wear this shoe tomorrow and Wednesday too!

Investigate #2-3

READ ALL OF PG 211-212!!! Design an experiment using the scientific

method Ask a question Hypothesis/Prediction

Perform Experiment Collect Data Analyze DataDraw Conclusion

Experimental Design Diagram

Your question is:

How does changing the surface affect the amount of Force needed to pull a shoe?

By the end of the period you must have: A hypothesis (If…then…because…) Step by step procedures Data table All OK’d by Ms. Law

October 19, 2010

Collect DATA!!! Use your procedures from yesterday

to collect data

Calculate µ

µ=force of friction perpendicular force exerted by

the surface on object µ=Ff

FN

Investigate #4

Complete #4 and record all data in your notebook

Be sure to answer ALL questions!

Investigate #5

Complete #5 and record all data in your notebook

Be sure to answer ALL questions!

October 20, 2010HW: PTG #1-5 Due Thursday

Do Now: Draw a free-body

diagram to show the relationships among the four force acting on the shoe moving at a constant speed.▪ Fpull

▪ Ffloor

▪ Fgravity

▪ Ffriction Agenda:

Do Now LO/SC Physics Talk Quiz

Learning Objectives: Explain the effects of friction on

moving objects Calculate the coefficient of sliding

friction Explain factors that may change

the amount of friction

Success Criteria Apply the definition of the

coefficient of sliding friction, µ Measure the coefficient of sliding

friction between the soles of athletic shoes and a variety of surfaces

Calculate the effects of frictional forces on the motion of objects

Reminder of Newton’s 2nd Law

Newton’s 2nd Law: motion with a constant velocity happens only when there is no net force or forces are balanced.

What does Newton’s 2nd law say?

Physics Talk 2.7

What was the acceleration of the shoe when you pulled it in your experiment?

What does Newton’s 2nd law imply about the force put on the pulled shoe?

The shoe was pulled with constant velocity=no acceleration

All of the forces must add up to zero/balance each other out.

Physics Talk 2.7

One of the forces was your pull on the shoe. How did you measure this force?

What force was pulling the shoe opposite of your pull to balance out the forces?

The force the shoe was pulled with was measured with a spring scale in Newtons.

Friction provided the second force. Friction is a force that resists relative motion between two bodies in contact.

Physics Talk 2.7

If you pulled at a constant speed, then was your pull (force) equal to the frictional force?

YES! Constant speed means balanced forces, no acceleration!

Physics Talk 2.7

What is net force?

Two forces that are equal in magnitude but opposite in direction are net forces

When you add up all the forces, you get the net force

Because they are acting on the same object! Both you pulling and friction were both acting on the shoe!

Physics Talk 2.7

What were the horizontal forces acting on the shoe?

Force of the pull

Force of Friction

Physics Talk 2.7

What were the vertical forces acting on the shoe?

What is another name for the force of the floor pushing up?

Gravity (down) Floor (up)

This is also called the normal force! FN

Normal Force is perpendicular to the surface

Physics Talk 2.7

Draw and label all the forces acting on the shoe

Physics Talk 2.7

How do you calculate coefficient of sliding friction (µ)?

µ=Force of friction Normal force

µ=Ff

FN

The force of friction is equal to the force required to slide the object on the surface with a constant speed

No units Usually a decimal

What do you think now?

Answer the WDYTN question on page 216