1. Forces & Motion
Chapter 12

2. TUG-O-WAR TIME!!!

3. What is a force A push of pull that acts on an object
Forces can cause a resting object to move, or it can accelerate a moving object by changing the object’s speed or direction

4. Measuring Force Spring Scales
The stretch of a spring scale depend on the weight (type of force) acting on it

5. Units of Force Measured in newtons (N)
1 N = the force that causes a 1 kilogram mass to accelerate at a rate of 1 meter per second squared
1 N = 1 km•m/s2
This unit is named after Sir Isaac Newton ( )
Scientist who explained how force, mass and acceleration are related

6. Force Diagrams
Use arrows to represent the direction and strength of a force (like a vector!)

7. Spring Scale Activity
Choose five objects on your table. Attach a string to your objects if necessary. Use the spring scale to determine the weight (in newtons) of your objects. Draw a force arrow for each object that is to scale relative to each other force arrow.

8. Combining Forces Back to tug-o-war…
You can combine force arrows to show the result of how forces combine
Forces in the same direction add together
Forces in the opposite direction subtract from one another

9. Net Force
The overall force acting on an object after all the forces are combined

10. Balanced Forces Sometimes the net force acting on an object is zero.
Forces that combine to produce a net force of zero
Results in NO CHANGE in an object’s motion

11. Common Example Two people locked in an arm wrestling match
Tug-o-War match with two evenly matched teams!
Two football players pushing against one another at the line of scrimmage

12. Unbalanced Forces
Results when the net force acting on an object is NOT equal to zero
When an unbalanced force acts on an object, the object accelerates

13. = 0 Equal and opposite forces
Combining Forces =
Adding Forces =
Subtracting Forces
= 0 Equal and opposite forces

14. Friction
A force that opposes motion of objects that touch as they move past each other.
Acts at the surface where objects are in contact (includes all solids, liquids, and gases)
Friction is important!
Without friction every surface would be impossibly slippery
Food would slide right off your fork
Walking would be impossible
Cars would slide around with their wheels spinning

15. Four main types of friction
Static Friction
Sliding Friction
Rolling Friction
Fluid Friction

16. Static Friction
The friction force that acts on objects that are not moving
Always acts in a direction opposite to that of the applied force

18. Sliding Friction
A force that opposes the direction of motion of an object as it slides over a surface
LESS than static friction
This means that once an object is moving, less force is needed to keep the object moving than to start it moving

20. Rolling Friction The friction force that acts on rolling objects
When a round object rolls across a flat floor, both the object and the floor are bent slightly out of shape at the point of contact
100 – 1000 times less than static or sliding friction
This is why movers use wheeled dollies to move heavy objects!

22. Fluid Friction Liquids and mixtures of air are known as fluids
Fluid friction results when fluids (like liquids and air) oppose motion of an object
Example, when you stir cake batter you can feel fluid friction
Fluid friction increases as the speed of the object moving through the object increases

23. Air Resistance
Fluid friction acting on an object moving through the air
At higher speeds air resistance is a significant force
For example, swimmers, cyclists and even runners wear slick racing suits to reduce air resistance

24. Suits...
1:00 min

25. Types of Friction Foldable Activity!

26. Gravity A force that acts between any two masses
An attractive force (it pulls objects together)
Unlike friction, gravity can act over large distances (think skydiving!)

27. Gravity (continued)
Earth’s gravity acts downward toward the center of the Earth

28. Falling Objects
Both gravity and air resistance affect the motion of a falling object
Gravity causes objects to accelerate downward
Air resistance acts in the direction opposite to the motion, reducing acceleration

30. Flying Squirrels… As objects fall they accelerate (gain speed)
As speed increases, air resistance increases
If an object falls long enough, the upward force of air resistance eventually will equal the downward force of gravity
Forces are balanced, acceleration is zero and the object continues falling at a constant velocity

31. Terminal Velocity
Constant velocity of a falling object when the force of air resistance equals the force of gravity

32. Projectile Motion
The motion of a falling object (projectile) after it is given an initial forward velocity
The only forces acting on a projectile are gravity and air resistance

34. Projectile Motion (Continued)
The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path