Friction, Gravity, and Elastic Forces

Previously, in physical science…

• You learned about forces and combining forces. Now, you continue to combine forces and identify two or more forces acting on an object.

Std 2d Students know how to…

• Identify separately the two or more forces that are acting on a single static object, including gravity, elastic forces due to tension or compression in matter, and friction.

1. What is friction?The force that two surfaces exert on each

other when they rub against each other.

Byproduct of friction…Heat!

2. What direction does friction act?

• Acts in direction opposite to the objects in motion.

3. How do you show friction as a force?• An arrow opposite of the object’s

motion

                                                     

              

Friction force

Motion

4. What determines the strength of friction?The types of surfaces (smooth or rough)and how hard the surfaces push together

5. What are 4 types of friction?

a. Static friction: acts on object NOT moving:

b. Sliding FrictionTwo solid surfaces slide over each other

ex: book on desk

c. Rolling FrictionObject rolls across a surface

ex: ball bearings

d. Fluid Friction• Solid object moves through a fluid

ex: oil, water, air

(what do you use in your car to reduce friction?)

• What are two ways in which friction can be useful?

• allows objects to remain in place!

• allows moving objects to stop to avoid colliding with other objects!

6. What is gravity?

• The force that pulls objects toward each other

                       

7. How do you show gravity as a force on Earth?• An arrow toward the center of Earth

Gravity

8. What is weight?

• The gravitational force exerted on an object

http://youtube.com/watch?v=L55jImEhNZ0

9. What is the acceleration due to Earth’s gravity?• 9.8 m/s2

What is your weight (on Earth)?

Weight = Mass × 9.8 m/s2

Weight = ?

10. What is free fall?

• When the force of gravity alone causes an object to accelerate downward.

• Causes an object to accelerate downward at 9.8 m/s2

11. What is air resistance?• Type of fluid friction as an object

moves through air.• Depends on speed and surface area

Air resistance

Air resistance

Free fall:

Gravity

Air resistance

Net Force

accelerate downward

Free fall:

Terminal velocity:

Gravity

Air resistance

0 net force

Move at constant

speed

Terminal velocity:

Gravity

Air resistance

Net Force

Let’s drop coffee filters (again)!

Leave one coffee filter openCrumple the other coffee filterDrop them at the same time, what happens? Draw a diagram of your observationsDraw in your force arrows, including gravity

and air resistance.

Gravity

Fluid friction

Gravity

Rolling friction

Air resistance

12. When is matter elastic?

• When it returns to its original shape after it is squeezed or stretched.

…all matter has some degree of elasticity…

13. What are 2 types of elastic forces?• Compression: when an elastic

force squeezes or pushes matter together.ex: couch cushion

• Tension: when an elastic force stretches or pulls matter apartex: guitar string

Sometimes the tension forces can be great…like when catching a marlin…

Compression forces squeeze together…like the shocks on a car

Think of compression on a sponge…or a Twinkie