1. Chapter 3 Section 1
Forces
IPC

2. Types of Friction Foldable
Make a foldable to help you keep track of the types of friction
3 Headings on cover flaps:
Static Friction
Sliding Friction
Rolling Friction

3. What is force and what does it do?
Force: a push or a pull
A force can cause the motion of an object to change

4. Net Force Net Force: the sum of all forces acting on an object
As we move forward keep in mind the rules for adding forces

5. Balanced and Unbalanced Forces
Which is balanced? Which is unbalanced? Why?
Balanced forces do not cause a change in motion.
The net force will be the difference between the two forces if they are in opposite directions.

6. Friction
Friction: the force that opposes the sliding motion of two surfaces that are touching each other
What causes friction?
Surfaces that appear smooth actually have many bumps and dips

7. Types of Friction
Static friction: frictional force that prevents two surfaces from sliding past each other, caused by microwelds that form between two surfaces
These microwelds are stronger when pushed together with larger force
This means that when you are pushing an object and it is not moving the force opposing you is static friction

8. Types of Friction cont.
Sliding friction: frictional force that opposes the motion of two surfaces sliding past each other
Think about the resistance you feel when trying to slide your shoes along carpet
The force of sliding friction is usually smaller than the force of static friction!
This is why once you get an object moving it is easier to keep it moving.

9. Types of Friction cont.
Rolling friction: friction between an object that is rolling in one direction and the surface of another object
Rolling friction, aka traction, is very useful and is the reason your tires move on your vehicles
In icy weather sand is used to increase the friction with vehicle tires or people’s shoes

10. Gravity
Gravity: an attractive force between any two objects that depends on the masses of the objects and the distance between them
Gravity is one of four fundamental forces
Newton’s Law of Universal Gravitation: the relationship between the gravitational force between two objects, the object’s masses, and the distance between them
F = G×m1m2 d2
G = universal gravitational constant (6.67×10-11 m3/kg×s2)
m1 and m2= the masses of the two objects
d= the distance between the centers of the two masses
In the 1660’s, British scientist Sir Isaac Newton used data on the motions of planets to write the law of universal gravitation.

11. More About Gravity
Why do I not feel the gravity of the textbook on my desk?
While the distance from the book to me is very close, the mass of the book is not enough for me to feel it’s gravitational pull
The range of gravity
According to Newton’s law of universal gravitation, the gravitational force between two objects decreases rapidly as the distance between the masses increases
However, no matter how far apart the gravitational force between two objects is never zero
Gravitational fields
A field is a region of space that has a physical quantity (such as force) at every point.
All objects are surrounded by a gravitational field that becomes weaker as distance from the object increases

12. Weight
Weight: the gravitational force exerted on an object results in the object’s weight
Weight Equation:
Weight (N) = mass (kg) × gravitational strength (N/kg)
Fg = mg
Do you think the weight of a plane is the same on earth’s surface as it is in earth’s atmosphere? Why?

13. Weight and Mass Weight and mass are NOT the same Weight is a force
Mass is a measure of the amount of matter an object contains
Weight on earth and weight on the moon are not the same… Why?
Is the mass on earth the same as the mass on the moon? Why?

14. Force Activity In groups of 3-4, model one of the following forces
Balanced/unbalanced forces
Static/sliding friction
Gravity
Weight (on and off earth)
Be able to justify your reasoning and explain the finer details behind your chosen force

15. Homework
Complete the handout and turn it in next class time

16. Wrap Up Are mass and weight the same thing?
What happens to gravitational force the further you get from an object?
What are three types of friction?
What is net force? Why is it useful?
Have a Great Day!