1. Forces

2. Force a push or pull acting on an object
typically measured in Newtons (kg•m/s2)
is a vector
can be combined to predict motion net force
Soccer Simulation

3. Types of Forces Contact Forces Non-Contact Forces Applied Normal
Friction
Air Resistance
Tension
Spring
Non-Contact Forces
Gravity
Electromagnetic

4. Applied Force
any push or pull on an object created from another source (person, animal, another object, etc.)

5. Normal Force
the support force exerted on an object directly related to weight (gravity)
consequence of Newton’s 3rd Law
is always perpendicular to the surfaces in contact
Gravity
900
Gravity
900
Box
Box
Friction
Normal
Force
Normal
Force

6. is the force (friction)
exerted by a surface as an object moves across it or attempts to move across it
opposes the motion of an object
depends on the type of surfaces and the normal force (weight)
Types
Kinetic
Static
In which direction
is the force (friction)
vector pointing?
Motion
Friction

7. Friction is the force that opposes motion between two surfaces that are touching each other.
Friction depends on the type of surfaces that are being pressed together and the force at which they are pressed together.
Example: Friction between your shoes and the floor is what allows us to walk! If there was no friction, you would be constantly slipping and falling!

8. You can have rolling, sliding, and static friction.
Static friction is friction between two solid objects that are not moving relative to each other. The static friction force must be overcome by an applied force before an object can move.
Rolling friction is the resistive force that slows down the motion of a rolling ball or wheel. This type of friction is typically a combination of several friction forces at the point of contact between the wheel and the ground or other surface.
Sliding friction is the friction between moving surfaces.

9. What type of friction is this?
NOT MOVING 

10. Air Resistance friction due to air molecules
acts upon objects as they travel through the air
opposes the motion of an object
most noticeable for objects traveling at fast speeds
Examples
Space shuttle re-entry
Meteorite in Freehold
Meteor over Russia

11. Minimizing Air Resistance (Drafting)
used in variety of competitive events (bicycle and car racing, swimming, etc.) to reduce air resistance
Notice how the second biker experiences less air resistance because he is shielded by the first biker.
Image taken from:
Although it does not work
exactly the same way, where
is these seen in nature?

12. Tension
force that is transmitted through a string, rope, cable or wire when it is PULLED tight by forces acting from opposite ends
directed along the length of the wire and PULLS equally on the objects on the opposite ends of the wire

13. Spring
force exerted by a compressed or stretched spring upon any object that is attached to it
for most springs, the magnitude of the force is directly proportional to the amount of stretch or compression of the spring
Images taken from:
If both springs are the same size when not compressed, which spring will apply more force to the ball when released? Explain your reasoning.

14. Gravity natural force of attraction between any two objects factors:
distance – increased distance  less gravitational pull or vice versa
mass – increased mass  more gravitational pull or vice versa
Why does the force of gravity have more of an impact on holding our solar system together compared to holding the parts of an atom together?

15. Electromagnetic Force
force that moving charges exert on one another
results from the repulsion of like charges and the attraction of opposites
Notice how the particles with the same charge move apart and the particles with different charges move together. + + + - - -
Compare and contrast gravitational force and electromagnetic force.

16. Free Body Diagrams visuals that help show net force Practice
use a square and draw all forces acting on the object.
remember size and direction of vector arrows are important!
Practice
Fgrav = 5 N
Ffric = 3 N
Fnorm = 5 N
Fapp = 3 N
What do you think the
symbols w/subscripts
represent?
What is the net
force on this object?

17. You throw a baseball to your friend who is to your left.
What’s the Net Force
Fnorm = 10 N
Fapp = 20 N
Ffric = 5 N
Ffric = 5 N
Fapp = 15 N
Fgrav = 10 N
Fgrav = 10 N
You throw a baseball to your friend who is to your left.
Your dog pulls you down the street on a skateboard in an eastward direction.

18. What’s the Net Force (An Interesting Case)
A skydiver is descending with a constant velocity. Consider air resistance.
The same skydiver is descending after 30 seconds. Consider air resistance.
Fgrav
Ffric
Fgrav
Ffric
What has the skydiver reached in this scenario?

19. Laws
Newton’s 1st Law
Newton’s 2nd Law
Newton’s 3rd Law

20. Newton’s 1st Law
objects at rest remain at rest, and objects in motion remain in motion with the same velocity, unless acted upon by an unbalanced force
also considered the Law of Inertia
How is this illustrated when riding in a car? Can you think of other
experiences where this is illustrated?

21. Inertia
the resistance of an object to a change in the speed or the direction of its motion
directly related to mass

22. Newton’s 2nd Law
the acceleration of an object increases with increased force and decreases with increased mass
the direction in which an object accelerates is the same as the direction of the force
Formula: F = ma (or a = F/m)
Shopping Cart Simulation

23. In this case, the force of the
Centripetal Force
any force that keeps an object moving in a circle
directed toward the center of the circle
In this case, the force of the
ball as it accelerates around the circle is pointing inward, toward the center.

24. Practice Problems - Force
What net force is needed to accelerate a 24 kg dogsled to a rate of 3 m/s2?
2. A 1.5 kg object accelerates across a smooth table at a rate of 0.5 m/s2? What is the unbalanced force applied to it?
72 kg·m/s2
or 72 N
F = ma
F =
(24 kg)
(3 m/s2) =
F =
(1.5 kg)
(0.5 m/s2)
F = ma =
0.75 kg·m/s2
or 0.75 N

25. Newton’s 3rd Law
states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object.
How was this illustrated during the Scooter Games competition? Can you think of other experiences where this is illustrated?

26. Vector a quantity that has both direction and magnitude (size)
drawn as an arrow which shows direction and magnitude (length of arrow)
consists of two parts: tail and head
Head
Tail
Consider the vectors above. Describe the direction and relative magnitude (force) of each ball based on the vector.

27. What is the hockey puck’s net force?
Combining Vectors
can be combined/added to help determine net force
the overall force acting on an object when all of the forces acting on it are combined
What is the hockey puck’s net force?
Gravity = 14 N
Normal Force = 14 N
Gravity = 14 N
Applied Force = 25 N
Friction = 2 N
23 N
Applied Force = 25 N
Friction = 2 N
Normal Force = 14 N