1. Forces
Unit 4

2. The Science of Physics
As we study forces, we are continuing to gain a deeper understanding of physics. **Physics is the study of matter and how energy affects matter. For example, forces require energy to move matter.

3. What is Force? A force is simply a push or a pull.
The unit used to express force is the Newton (N).
Forces can change the acceleration of an object. Forces do not always cause an object to move.
For example: If a person sits on a chair, they exert force downward, but this does not make the chair move. The reason—the chair exerts a force back on the person.
Just because a force acts on an object does not mean that motion will occur.

4. Determining Force
Often, more than one force is acting on an object. The combination of all the forces acting on the object is called the net force.
Rule 2: Forces going in the same direction get added.
Rule 1: Forces going in opposite directions get subtracted. The direction goes towards the larger force.

5. Checkpoint:
Draw a picture to represent this problem: A box experiences a force of 5N to the right and a force of 10N to the left.
Use your picture to determine the net force on the object in the space below. Be sure to round your answer to the nearest 0.1 and put a unit and direction.

6. What is the difference between balanced and unbalanced forces?

7. Balanced Forces:
Balanced forces are when the forces on an object produce a net force of zero.
Balanced forces DO NOT CHANGE THE POSITION of an object.
If a balanced force is applied, a moving object stays moving and a stationary object stays stationary. Objects all around us are experiencing balanced forces.

8. Example 1: A light hanging from the ceiling does not move because the force of gravity pulling down is equal to the force of the wire pulling upward.

9. Example 2: The picture below demonstrates balanced forces.
Net Force = ____________

10. Unbalanced Forces:
Unbalanced forces are when the net force on an object is not zero Newtons.
Unbalanced forces on an object produce a change in motion.
Unbalanced forces can start motion, stop motion
or change position of motion.

11. Example 1: When a soccer player kicks a soccer ball.

12. Example 2: The picture below demonstrates unbalanced forces
Example 2: The picture below demonstrates unbalanced forces. What is the net force?
Net Force = ___________

13. Checkpoint:
According to the graphic organizer below, identify one similarity and one difference between balanced and unbalanced forces.

14. Understanding the Unit “Newton”
The science community wanted to give Isaac Newton credit for his work in the area of physics and forces. They decided to name the unit for force after him! But what is a Newton? 1.0 N = 1.0 kg•m/s2 + direction Illustrate the picture below to increase your understanding of the unit.
1 m/s
2 m/s
3 m/s
1.0 kg
1 s
2 s
3 s

15. Friction
Unit 4

16. What is Friction?
Friction is a force that opposes motion between two surfaces that are in contact. The word “opposes” means to go against or in the opposite direction.

17. Friction occurs because objects are not totally smooth.
The surface of every object has hills and valleys. When the two surfaces are in contact, the hills and valleys of one surface stick to the hills and valleys of the other surface.
When the surfaces rub together they cause moving objects to slow down or eventually stop.
To measure friction on a surface we can use a spring scale.

18. Effects of Force on Friction
If the force pushing down on a surface increases than the amount of friction increases. Objects with a larger mass exert a larger downward force.
On the graph below, draw a line that would represent this effect.

19. Effects of Rougher Surfaces on Friction
Rougher surfaces have more microscopic hills and valleys than smooth surfaces. Therefore, if the surface is rougher there will be more friction.
On the graph below, draw a line that would represent this effect.

20. Checkpoint:
Let’s think about sports! Compare the friction of a soccer ball rolling on the grass and a bowling ball rolling on the grass. Next, compare the friction between the puck and ground in deck hockey versus ice hockey.

21. What is the difference between kinetic and static friction?

22. Kinetic Friction
The word kinetic means “moving’. Kinetic friction occurs between two surfaces. There are 3 types of kinetic friction;
Sliding kinetic friction
Rolling kinetic friction
Fluid kinetic friction

23. Sliding Kinetic Friction
Sliding friction occurs when two objects slide past each other. This is the strongest or most difficult to overcome. Example: Pushing a chair cross the room or using a pencil to write.

24. Rolling Kinetic Friction
Rolling friction must have wheels, a ball or ball bearings. It is weaker than sliding friction but stronger than fluid friction. Example: Most forms of transportation such as a car or a skateboard.

25. Fluid Kinetic Friction
Fluid friction occurs when an object is moving through a liquid or a gas. It is the weakest or least difficult to overcome. Example: Swimming or skydiving

26. Static Friction
The word static means “ stationary”. Static friction occurs between and object and the surface it is on when the object is not in motion. As soon as the object starts moving, static friction turns into kinetic friction.
***In order to put an object in motion, the force applied to the object must be greater than the force of the static friction.

27. Checkpoint:
How is riding a bike an example of all three types of kinetic friction: sliding, rolling and fluid? How can you use a bike to show static friction?

28. Is Friction Helpful or Harmful?
Friction can be both helpful AND harmful
Think about a car—
Helpful:
Friction allows the car to push against the ground so that the car can move.
Friction also enables the brakes to stop the car from moving.
Harmful:
When parts of the engine rub together the friction creates too much **heat** and can cause engine parts to overheat.
Sometimes it is necessary to increase or decrease the amount of friction present.

29. How can we Reduce Friction?
To reduce the amount of friction we can add a lubricant.
Lubricants help make the surface smooth
Examples of lubricants are wax, motor oil or grease.

30. Friction can also be reduced by changing sliding kinetic friction to rolling friction by adding wheels or ball bearings.
Example: In rollerblades, there are ball bearings between the wheels and axle to create less friction so that the rollerblader can move more easily.
By smoothing out a surface, friction can also be reduced.
Example: Using sandpaper on a park bench.

31. How can we Increase Friction?
To increase the amount of friction, you can make the surface more rough.
Example: Salt is added to icy roads so that cars do not slip when driving or wearing batting gloves when playing baseball for a better grip on the bat.
Increasing surface area is another way to increase the friction.
Example:
Increasing mass will cause an increase in friction as well because it is adding more force pushing the surfaces together.
Example: Pushing down on sandpaper harder when trying to sand a piece of wood.