1. Forces Introduction Force Overview What is a Force?
The Four Fundamental Forces
Forces all around us
Normal, Friction and Tension force
Contact vs. Action at a distance force
Measuring force
System and Free-Body Diagrams
Finding Fnet
Force vectors

2. Force Overview ___________
Unbalanced forces result in the acceleration, compression, stretching, or twisting of objects.
Force is measured in Newton’s (kg  m/s2) with the symbol (N).

3. But what is a force?
With the person next to you, come up with a definition of a force.
After watching the video, revise your definition
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5. Check Your Understanding
State an everyday life example in which a force causes an object to
decrease its speed.
become compressed.
become stretched.

6. The Four Fundamentals Forces
There are four forces that act in nature.
Put these forces in order of relative strength (make one of the forces have a strength of 1 and the others as a multiple of that 1).
Provide a multiple for the strengths.
Gravitational
Strong Nuclear
Electromagnetic
Weak Nuclear

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8. Gravitational Force
The force of attraction (not repulsion) between all objects in the universe.
Keeps us from floating away into space.
Keeps the planets in orbit.
Is a relatively weak force unless one or both of the objects is very large.
________

9. Consider This
You probably think the force of gravity is strong but consider this…Your hand and arm are being pulled towards the ground via gravity but it isn’t very difficult to raise them.
It takes very little energy to defy gravity because it is a relatively weak force.
Consider those amusement park rides that spin you quickly against a wall and think about how much more difficult it is to raise your hand while on one.

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11. Finding the Force of Gravity (Fg)
The units for Fg are (kg  m/s2)
In the next few lessons we will be talking about Newton’s 3 laws.
One of these law is Force = Mass x Acceleration
Therefore,
Force of gravity = Mass (kg) x Acceleration due to gravity
The mass must ALWAYS be in kg*
______________________

12. Check Your Understanding
If a person weighs 41.6 kg, what Force of Gravity (Fg) will he produce on the ground?

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14. The Weak Nuclear Force _________
Works at very short distances:  m
Works at the nuclear level
Allows the sun to undergo nuclear fusion
Responsible for radioactive decay

15. Electromagnetic Force
The force caused by electric charges.
Includes electric forces (static electricity) and magnetic forces (such as a compass).
Stronger then the force of gravity  x 1036
Can either be attraction or repulsion.
The forces generally cancel each other out and objects are neutral.
________
Gets weaker quickly as the distance increase (1/x2).

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17. The Strong Nuclear Force
Holds the protons and neutrons together
Stronger then the force of gravity  x 1038
_________

18. Forces All Around Us
What happens when forces are in equilibrium?

19. Consider the pen on your desk
Determine which of the four fundamental forces are acting on it and how.
Gravitational Force:
Weak Force:
Electromagnetic Force:
Strong Nuclear Force:

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21. Everyday Forces
In everyday life we experience the result of the four fundamental forces acting on us and the objects around us.
We have given these interactions special designations.
List as many everyday forces acting on you as you can. Think of names of forces you’ve heard before.

22. Check Your Understanding
If there is a downwards force of gravity (Fg) acting on the pen, why isn’t it falling towards the ground?

23. The Normal Force (FN) _________ It is ALWAYS equal and OPPOSITE.
What would happen if FN > Fg?
__________

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25. Think About It & Try It
If you push an object across the desk, what will happen and why?
Does the type of materials matter? Why?
Does the mass of the object matter? Why?
Once you’ve thought about each with the person(s) beside you, figure out a way to try each.

26. The Force of Friction (Ff)
________
There are two types of friction forces
Kinetic Friction (Fk) – Friction where one object moves over another.
Static Friction (FS) – Friction between two stationary objects.

27. Check Your Understanding
Which direction is friction acting in the following picture?
What kind of friction is acting assuming the car is stationary.

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29. Think About It & Try It
Task: Suspend one object in the air using another object.
How does the object stay in the air?
What causes your muscles to move? Is this the same kind of force? Why or why not?
What are the names of these fibers which allow your body to move?

30. Tension (FT)
________
The rope held between two teams in a tug of war is under tension.
Muscle fibres of our body experience tension when they contract.

31. Contact vs. Action at a Distance Forces
The normal force, friction, and tension are all contact forces, in other words, forces that exist when objects are in direct contact with each other.
__________

32. Common Forces Review Force Symbol Function Fg FN FT Ff FK FS Fair Fapp
Gravity Fg
Force of attraction between all objects
Normal FN
Force perpendicular to the surfaces of objects in contact
Tension FT
Force exerted by materials, such as ropes, fibres, springs, and cables, that can be stretched
Friction Ff
Force that resists motion or attempted motion between objects in contact
Kinetic Friction FK
Force that acts against an object’s motion
Static Friction FS
Force that tends to prevent a stationary object from starting to move
Air Resistance
Fair
Applied Force
Fapp

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34. Measuring Force Let’s look at how you can measure force:
Newton Meters (spring scale)
Force Sensor

35. Measuring Force
What is the relationship between mass and the force of gravity pulling on it?

36. System Diagrams and FBD
A system diagram is a sketch of all of the objects involved in a situation.
_________
Always indicate the +x and +y directions
All force vectors need to be included in a FBD (and labelled) with their lengths proportional to the magnitude of the force.

37. System Diagrams vs. FBD

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39. Check Your Understanding
A child is pushing with a horizontal force against a chair that remains stationary. Draw a FBD of the chair.

40. Check Your Understanding
Draw a FBD for the following: a) Your textbook resting on your desk.

41. Check Your Understanding
Draw a FBD for the following: b) A tennis ball falling through the air from the server’s hand (neglect air resistance).

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43. Check Your Understanding – More Difficult
A child pulls a sleigh up a snow-covered hill at a constant velocity at an angle (Θ) with a force parallel to the hillside. Draw a FBD of the sleigh.
Hint:
Change the axis

44. Analyzing Forces – Finding Fnet
When analyzing a problem involving forces acting on an object, you must find the sum of all the forces acting on that object (called the net force)
This is why it is important to designate a “+” x and y value Fa FN
15o Ff
30o Fg

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46. Check Your Understanding
What is the net force in the following situation?
FN = 250 N
FA = 350 N
Ff = 50 N
Fg = 250 N

47. Check Your Understanding: Simple Fnet Questions
Team Red is pulling on the rope to the right with a force of 5 N and Team Blue is pulling to the left with a force of 7 N. What is the net force?

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49. Check Your Understanding
In hitting a volleyball, a player applies an average force of 9.9 N [33o above the horizontal] for s. The force of gravity on the ball is 2.6 N [down]. Determine the net force on the ball as it is being struck.

50. Steps Draw a FBD for the situation and assign positives.
In hitting a volleyball, a player applies an average force of 9.9 N [33o above the horizontal] for s. The force of gravity on the ball is 2.6 N [down]. Determine the net force on the ball as it is being struck.
Draw a FBD for the situation and assign positives.
Find all the forces in the x and y-directions.
Find the sum of the x and y components.
Create your right-triangle (as we did with 2D-motion).
Find your Fnet , it will be the hypotenuse.
Don’t forget the angle!

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52. 4. Create your right-triangle (as we did with 2D-motion).
In hitting a volleyball, a player applies an average force of 9.9 N [33o above the horizontal] for s. The force of gravity on the ball is 2.6 N [down]. Determine the net force on the ball as it is being struck.
4. Create your right-triangle (as we did with 2D-motion).
5. Find your Fnet , it will be the hypotenuse.
6. Don’t forget the angle and your therefore statement!

53. Check Your Understanding
A boat is secured to a lakeside pier with two horizontal ropes. A wind is blowing offshore. The tensions in the ropes are F1 = 48 N [E 16° N] and F2 = 48 N [E 16° S].
Use a vector diagram to determine the sum of the tension forces in the two ropes (combined x-components).
Assuming that the net horizontal force on the boat is zero, determine the force of the wind on the boat.

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55. Check Your Understanding
Draw a FBD and determine the net force:
A long-jump contestant experiences at the instant of landing a gravitational force of 538 N [down] and a force, applied by the ground to the feet, of 6,382 N [28.3° above the horizontal and towards the jumper].

56. Check Your Understanding
The mass of the cart is 30.6 kg. Find all the forces and the net force. FN
FA = 95 N
22o
Ff = 25 N Fg

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58. Check Your Understanding
Fill in the blanks and determine the net force in the following situation? The person weighs 35.7 kg.
FN = _____ N
Fapp = 455 N
Ff = 150 N
15o
Fg = _____ N