1. You need:
Clean paper (2) / pencil
HW: ALL of the Net Force WS (even and odd)
Warm Up:
List the four basic types of friction.
I CAN: recognize and define balanced and unbalanced forces.
Nov. 16, 2017

2. How can we describe motion?
Motion can be described by:
DISTANCE (how far did it travel?)
TIME (how long did it travel?)
SPEED (how fast did it travel?)
DIRECTION (which way did it go?)
ACCELERATION (does the motion change?)

3. Speed vs. Velocity
SPEED – tells you have fast or slow something is moving (changing position).
Example = 25 km/h
VELOCITY – tells you speed AND DIRECTION! (changing position in a certain direction)
Example = 25 km/h EAST

4. Acceleration Acceleration is the CHANGE in speed.
Acc = Speed (final) – Speed (initial)
total time

5. Introduction to
FORCES

6. A FORCE is a PUSH or PULL in a particular DIRECTION.
FORCES
When you ride a bike, your foot PUSHES against the pedal. The push makes the wheels of the bike move.
When you drop something, it is PULLED to the ground by gravity.
A FORCE is a PUSH or PULL in a particular DIRECTION.

7. FORCES FORCES AFFECT HOW OBJECTS MOVE.
Forces can affect motion in the following ways: They can make objects:
START MOVING
MOVE FASTER
MOVE SLOWER
STOP MOVING
CHANGE DIRECTION
CHANGE SHAPE
BIG SCIENCE IDEA

8. Forces cause ACCELERATION.
Since forces cause changes in SPEED or DIRECTION of an object, we can say that forces change VELOCITY, so….
Forces cause ACCELERATION.

9. FORCES 1) Strength of the Forces 2) Direction of the Forces
More than one force can act on an object at one time. What happens to the object when forces act depends on 2 things:
1) Strength of the Forces
2) Direction of the Forces

10. FORCES Forces may WORK TOGETHER or OPPOSE each other.
When 2 or more forces act on an object, the forces combine to form a net force.
Forces may WORK TOGETHER
or OPPOSE each other.

11. FORCES
If the forces cancel each other out, and do not cause the object to move, the forces are said to be BALANCED.
If the forces don’t cancel each other out – 1 force is stronger than the others – the forces are UNBALANCED and will cause a CHANGE IN MOTION.

12. MEASURING FORCE The strength of a force is measured in NEWTONS.
The symbol is (N).
We use a SPRING SCALE to measure force.

13. Shoe lab
Record the amount of force at the moment the shoe first moves!
Record your data!

14. COMBINING FORCES = 5 N right + 10 N right 5 N right
Two forces in the same direction can add together to produce a larger net force.
5 N
right + =
10 N
right
5 N
right

15. COMBINING FORCES = 5 N right - 5 N left 10 N left
Two forces in opposite directions can subtract to produce a smaller net force in the direction of the larger force.
5 N right - =
5 N left
10 N left

16. COMBINING FORCES 0 N = - 5 N right 5 N left
Two forces may cancel each other out (if equal and opposite) to produce NO NET FORCE.
5 N right
5 N left - =
0 N
(No Net Force)

17. 50 N
NET FORCE
200 N
150 N
Two movers are trying to move a heavy box. One mover pushes to the right with a force of 150 N. The other mover pushes to the left with a force of 200 N.
a) Draw & label the forces on the diagram.
b) What is the net force? N LEFT
c) Will the box move? YES
d) If yes, in what direction? LEFT

18. 14) Two movers are trying to move a heavy chair
14) Two movers are trying to move a heavy chair. One mover PULLS to the left with a force of 200 N. The other mover PUSHES to the left with a force of 200 N.
a) Draw & label the forces on the diagram.
b) What is the net force?
400 N LEFT
c) Will the chair move?
YES
d) If yes, in what direction?
LEFT
400 N NET FORCE
200 N
200 N

19. MIKE
Four children are fighting over the same toy. Mike is pulling North with a 50 N force, Justin is pulling East with a 40 N force, Chantal is pulling South with a 50 N force, and Tykera is pulling West a 30 N force.
a) Draw & label the forces on the diagram.
b) Is there a net force on the toy?
YES = 10 N EAST
c) In which direction will the toy move?
EAST
d) Who gets the toy?
JUSTIN
50 N
TYKERA
JUSTIN
30 N
40 N
50 N
10 N
Net Force
CHANTAL

20. Net Force Practice (WS)
You should have done all 17!
We worked on ½ in class (either even or odd).
I asked that you finish the other half for homework. So if you had a question, it should have been asked in class, right?

21. FRICTION

22. The force that slows the ball to a stop is FRICTION.
What will happen when the ball is released?
When the ball reaches the bottom of the slope, will it keep moving forever?
The force that slows the ball to a stop is FRICTION.
Since the ball stops, there must be a force acting to slow the ball down.

23. What is Friction?
Friction is a force that two surfaces exert on each other when they rub against each other.
The direction of the friction force is always OPPOSITE to the direction of the motion.
Direction of Motion
Friction Force

24. Direction of Intended Movement
Types of Friction
Static Friction opposes the motion of an object that is at rest
To make the object move, you have to exert a force larger than the force of static friction.
Direction of Intended Movement
Static Friction Force

25. Sliding Friction Force
Types of Friction
Sliding friction occurs when two solids slide over each other.
Sliding friction makes car brakes work and stops athletes from slipping.
Direction of Slide
Sliding Friction Force

26. Types of Friction
Rolling friction occurs when an object rolls across a surface.
Rolling friction is easier to overcome than sliding friction for the same materials.
Direction of Motion
Friction Force

27. Types of Friction
Fluid friction occurs when a solid object moves through a liquid or gas.
Air resistance is a type of fluid friction.
Direction of Motion
Friction Force

28. Which type of friction is slowing down the object in each situation?
You are slipping down a waterslide at Emerald Pointe.
You use a lot of force to slide a desk across the floor.
You’re riding a skateboard down the street and it slowly rolls to a stop.
You try to push the couch, but can’t seem to move it.
FLUID FRICTION
SLIDING FRICTION
ROLLING FRICTION
STATIC FRICTION

29. Friction Thought Questions
Why would you add oil to a rusty bike chain?
Why would you add sand to an icy driveway or road?
Why is it easier to move heavy furniture using a handcart rather than pushing it?
Why would a shoe company be interested in studying friction?
What would happen if we repeated the tug-of-war and one team had only socks on?

30. Try this twist!
Use the spring scale to pull the shoe across the table.
This time, try different surfaces. Have one person hold down the wax paper as someone pulls the shoe across.
Write your observations!
Remember to record the amount of force needed to start motion!

31. Review of Friction Forces

32. Review - What is Friction?
Friction is a force that two surfaces exert on each other when they rub against each other.
The direction of the friction force is always OPPOSITE to the direction of the motion.
It SLOWS down moving objects!
Direction of Motion
Friction Force

33. Conclusions Questions
Which surface material created the MOST frictional force?
Which surface material created the LEAST frictional force?
Give an example of a situation where we use a certain surface material to:
a) REDUCE friction
b) INCREASE friction

34. & AIR RESISTANCE The physics of falling
GRAVITY
& AIR RESISTANCE
The physics of falling
I think 2nd period stopped at Gravity and Air resistance on

35. The Force of Gravity
Gravity is the force that pulls all objects down to the earth.
Rain falls from the sky down to earth…
If you drop a book, it falls to the ground…
If you trip, you’ll fall down…

36. Universal Gravitation
Actually in science, gravity is a force of attraction that acts between ALL objects (the earth, you, the desk, a book)
The force of gravity is much STRONGER for LARGER objects (more mass).

37. Universal Gravitation
Because the Earth is by far, the largest and closest object around, it has the greatest force of attraction...
So, no matter where you are on earth, all things fall to the ground due to gravity…

38. What is “free fall”?
When gravity is the ONLY force acting on an object, it is in free fall.
In that case, gravity is an UNBALANCED FORCE which causes the object to accelerate.

39. Acceleration due to Gravity
Calculate the acceleration of an object in free fall.
A = Final speed – initial speed
time
A = 50 m/s – 0 m/s
5 s
Acceleration = 10 m/s2
3rd and 4th made it AT LEAST this far – but not 2nd.

40. Objects in Free Fall Do all objects fall at the same rate?
If we dropped a bowling ball and a tennis ball from the same height, which would land first?

41. Objects in Free Fall Do all objects fall at the same rate?
ALL objects in free fall travel at the same rate, regardless of mass!
In free fall, heavy objects and light objects fall at the same rate!

42. So, which will land first?
WHY does the penny land first?
Remember the force that opposes motion (slows things down)? FRICTION!
Falling objects experience friction with the air called AIR RESISTANCE that slows them down.

43. Air Resistance
The larger the object (more surface area), the more air resistance.
That’s why parachutes work! The upward force of the air acting on the LARGE parachute slows you down as you fall.

44. Air Resistance Draw a diagram showing the forces…
Gravity
Draw a diagram showing the forces…
Downward force of gravity is same on both.
Upward force of air resistance is greater on the feather.
The net force (down) is greater on the penny.
Net Force on Penny
Net Force on Feather

45. Without air resistance, all objects would fall at the same rate…
Galileo Drops the Ball
Hammer and Feather Drop on the Moon

46. That’s why astronauts weigh less on the moon!
Gravity on the moon?
The force of gravity is much weaker on the moon because…
It is much farther away from earth.
The moon is much smaller than earth.
That’s why astronauts weigh less on the moon!

47. Gravity Review
Gravity is the force that pulls all objects down to the earth.
When gravity is the ONLY force acting, ALL objects accelerate at a rate of 10 m/s2.
Mass doesn’t matter – in free fall, heavy objects and light objects fall at the same rate!

48. Air Resistance Review
Some objects take longer to fall – they are slowed down by FRICTION with the air called AIR RESISTANCE.
The larger the surface area, the greater the force of air resistance pushing up.
Without air resistance, all objects would fall at the same rate…

49. Draw a free body diagram of the sky diver and label ALL the forces.
Gravity = 1000 N
Air Resistance = 800 N
What is the net force?
Air Resistance = 800 N
Net Force = 200 N
Gravity = 1000 N