1. Chapter 13
Forces and Motion

2. What is motion?
You can look around you and see that there are things in motion:
Ex: (Your teacher walking across the room;
your friend writing with a pencil).
Even if you don’t see anything moving, motion is still occurring all around you.
(Ex: air particles moving, the Earth circling
the sun, blood traveling in your body)

3. What is Motion?
An object is considered to be in motion if it is constantly changing position.

4. Relative Motion Something in motion changes position
The position of the object is described relative to another object.
The object that appears to stay in one place is called the reference point.

5. Something to think about….
Suppose you look out of the window and see a truck parked next to a tree. A few hours later, you look out the window and see that the truck is parked further down the street about 200 yards away from the tree.
What is your reference point in this scenario?
Tree
Street

6. Speed Depends on Distance and Time
When you are moving, your position is changing. How quickly it changes depends on the speed.
Speed is the rate at which an object moves
Speed depends on 2 things: distance traveled & the time taken to travel that distance

7. Speed cont’d The SI unit for speed is m/s, or meters per second
Other ways to calculate speed include:
kilometers per hour (km/h)
feet per second (ft/s)
miles per hour (mph)
Average speed- Average speed equals the total distance divided by the total time.

8. = Speed Depends on Distance and Time, continued
Determining Average Speed- Average speed equals the total distance divided by the total time.
average speed =
total distance
total time

10. Answers to problems; #1) 2 m/s 4) 8,000 m/h
#2) 5 km/h 5) 7 hrs
#3) 315 km/h

11. Riddle me this…..
Imagine that two birds leave the same tree at the same time. They both fly at 10km/h for 5 min, 12 km/h for 8 min, and 5km/h for 10 min. Why didn’t they end up at the same place?

12. Direction matters…. The birds went in different directions!!!
Their speeds were the same, but they had different velocities.

13. Velocity: Direction Matters!!
The speed of an object in a particular direction is called velocity.
BE CAREFUL! Do not confuse speed & velocity; they do not mean the same thing!
So, what is the difference between speed and velocity?
Velocity MUST include the “reference direction” that the object is moving!

14. Example of Speed vs. Velocity
Speed: A car traveling 60 mph
Velocity: A car traveling 60 mph east
Direction is added to the object’s velocity. EX: When you use a compass to determine the direction in which you are walking 5 mph.

15. How do these thrill rides show velocity?
They are changing direction and/or speed.

16. Bellwork 2/22/17
If a car travels 300 miles in 6 hours, what is the average speed?
If a bird flies at an average speed of 20 kilometers per hour for 4 hours, how far will the bird travel?

17. Acceleration
The rate at which velocity changes over time is called acceleration.
** An object accelerates if its speed, direction, or both change.

18. Did acceleration occur?
Scenario 1:You are riding your bike at 9km/h. Ten minutes later, your speed is 6km/h.
(Yes!! Acceleration occurred because speed decreased)
Scenario 2:You ride your bike around the block at a constant speed of 11km/h.
(Yes!! Acceleration occurred because direction changed)
Scenario 3:You ride your bike in a straight line at a constant speed of 10km/h.
No!! Acceleration did not occur because neither speed nor direction changed.)

19. Are you accelerating if…
1. You pick up the pace walking?
2. A car turns on a race track?
3. You slow down to a steady walk?
4. An object is moving in a circle?
What about you standing still?
Yes!
Yes!
Yes!
Yes!
Yes! Your are traveling in a circle as the earth rotates!

20. Acceleration?
Yes…direction is changing

21. Acceleration according to Tim and Moby
Brainpop- Acceleration

22. Force – push or pull
a force is a push or a pull and always acts in a certain direction
Force is measured in newtons(N)
Ex. If you push something, the force is in the direction of the push.
A force can change the acceleration of an object. This acceleration can be a change in the speed or direction of the object.

23. Mass vs. Weight
Mass is the amount of matter in an object, so will it ever change?
Mass is measured with a balance scale
Weight, however, is the measure of the amount of gravitational force exerted on an object.
Weight is measured with a spring scale

24. Mass vs. Weight Weight is a force measured in Newtons.
Weight Is not the same as mass!
- Mass is measured in grams/kilograms.
Even if the mass of an object doesn’t change its weight will change if its distance from Earth changes.

25. Mass versus Weight (copy chart)
Mass is…..
A measure of the amount of matter in an object.
Always constant for an object no matter where the object is in the universe.
Measured using a balance scale in grams (G)
Weight is…..
A measure of the gravitational force on an object.
Varies depending on where the object is in relation to the Earth
Measured using a spring scale in Newtons (N)
Same chart as page 185 in your book

26. Something to think about…
Does this astronaut have the same mass on Earth as he does on the moon?
Does he have the same weight?
Yes, same mass…….No, different weight, because there is no gravity/ farther away from earth so the gravitational force decreases

27. Newton’s Laws of Motion
I. Law of Inertia
II. F=ma
III. Action-Reaction

28. Newton’s Laws of Motion
1st Law – An object at rest will stay at rest, and an object in motion will stay in motion at a constant velocity, unless acted upon by a force.
2nd Law – Force equals mass times acceleration.
3rd Law – For every action there is an equal and opposite reaction.

29. 1st Law
Once airborne, unless acted on by a force (such as gravity and air resistance), the golf ball would never stop!

30. 1st Law
Or unless acted upon by a force, this golf ball would just sit on the tee forever.

31. 1st Law
Inertia is the tendency of an object to resist changes in its velocity: whether in motion or motionless.
Basically, what that means if that an object will “keep doing what it was doing” unless acted on by a force.
These pumpkins will not move unless acted on by a force.

32. Friction and Newton’s First law:
When you push your desk, it will quickly stop because of the force we know as “Friction”.
The friction between the desk and the floor works against the motion of the desk.
Friction also causes a rolling ball to slow down and stop.

33. Mass and Inertia:
Mass is a measure of inertia. An object that has a small mass will have less inertia than an object that has a larger mass.
Changing the motion of the smaller object will also be easier.
For example, inertia makes it harder to accelerate a car than to accelerate a bicycle. It also makes it easier to stop a moving bicycle than a car moving at the same speed.

34. More Examples from Real Life
A powerful locomotive begins to pull a long line of boxcars that were sitting at rest. Since the boxcars are so massive, they have a great deal of inertia and it takes a large force to change their motion. Once they are moving, it takes a large force to stop them.
On your way to school, a bug flies into your windshield. Since the bug is so small, it has very little inertia and exerts a very small force on your car (so small that you don’t even feel it).

35. Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hour is stopped by the brick wall, your body keeps moving at 80 km/hour.

36. If objects in motion tend to stay in motion, why don’t moving objects keep moving forever?
Things don’t keep moving forever because there’s almost always a force acting upon it.
A book sliding across a table slows down and stops because of the force of friction.
If you throw a ball upwards it will eventually slow down and fall because of the force of gravity.

37. 2nd Law
F = m x a

38. 2nd Law
The acceleration of an object depends on the net force acting on the object and the object’s mass (F= ma)

39. What does F = ma mean?
- Is it harder to throw a basketball or a baseball?
- Why??
-Force is measured in Newtons.
A basketball, of course! It has more mass. If you throw both balls with the same force, the speed of the baseball will be greater.
To get the basketball to have the same acceleration as the baseball, you must double the force you apply.
So….if the force applied is the same, as mass increases, the acceleration of an object decreases.

41. 2nd Law (F = m x a)
How much force is needed to accelerate a 1400 kilogram car 2 meters per second/per second?
Write the formula
F = m x a
Fill in given numbers and units
F = 1400 kg x 2 meters per second/second
Solve for the unknown
2800 kg-meters/second/second or 2800 N
Force is measure in Newtons (N)

42. Are you smarter than Sir Isaac Newton?:

43. Check Your Understanding
Solve the following problems. Show your work:
1. What acceleration will result when a 12 N net force is applied to a 3 kg object? A 6 kg object?
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?

44. F= m/a 9800 kg-m/sec/sec or 9800 N
How did you do?
1. What acceleration will result when a 12 N net force applied to a 3 kg object?
F/m= a 12 N/ 3 kg = 4 m/s/s or m/s2
12 N / 6 kg = 2 m/s2
2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
F/a= m N / 5 m/s/s = 3.2 kg
3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
F= m/a kg-m/sec/sec or 66 N
4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
F= m/a kg-m/sec/sec or 9800 N

45. Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.
We know that objects with different masses accelerate to the ground at the same rate.
However, because of the 2nd Law we know that they don’t hit the ground with the same force.
F = ma
98 N = 10 kg x 9.8 m/s/s
F = ma
9.8 N = 1 kg x 9.8 m/s/s

46. For every action, there is an equal and opposite reaction.
3rd Law
For every action, there is an equal and opposite reaction.

47. 3rd Law
According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body.

48. 3rd Law
There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces.

49. Newton’s 3rd Law in Nature
Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. In turn, the water reacts by pushing the fish forwards, propelling the fish through the water.
The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards).

50. 3rd Law
Flying gracefully through the air, birds depend on Newton’s third law of motion. As the birds push down on the air with their wings, the air pushes their wings up and gives them lift.

51. Third law continued..
Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. In turn, the air reacts by pushing the bird upwards.
The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards).
Action-reaction force pairs make it possible for birds to fly.

52. Other examples of Newton’s Third Law
The baseball forces the bat to the left (an action); the bat forces the ball to the right (the reaction).

53. 3rd Law
The reaction of a rocket is an application of the third law of motion. Various fuels are burned in the engine, producing hot gases.
The hot gases push against the inside tube of the rocket and escape out the bottom of the tube. As the gases move downward, the rocket moves in the opposite direction.

54. Think about it . . .
What happens if you are standing on a skateboard or a slippery floor and push against a wall? You slide in the opposite direction (away from the wall), because you pushed on the wall but the wall pushed back on you with equal and opposite force.
Why does it hurt so much when you stub your toe? When your toe exerts a force on a rock, the rock exerts an equal force back on your toe. The harder you hit your toe against it, the more force the rock exerts back on your toe (and the more your toe hurts).

55. Olympics & Newton’s Laws
Watch the videos and be prepared to explain how Newton’s laws of motion of are being applied:
Science & the Winter Olympics ("Blade Runners")
Science of NHL Hockey: "Newton's Laws of Motion"

56. F C A Item Sampler 2 # 18… F Item Sampler 2 # 19… C
Item Sampler 2 # 23… A A

57. F F D Item Sampler 2 # 26… F Item Sampler 2 # 10… F
Item Sampler 2 # 15… D D

58. B
22.
Item Sampler 2 # 22… F
Item Sampler 2 # 17… B F