1. Table of Contents The Nature of Force Friction and Gravity
Newton’s First and Second Laws
Newton’s Third Law
Rockets and Satellites

2. Book M – Ch 2 – Section 1 (Page 36-39)
The Nature of Force
A force is a push or a pull.
Like velocity and acceleration, a force is described by its strength and by the direction in which it acts.
The strength of a force is measured in the SI unit called newton (N).

3. Book M – Ch 2 – Section 1 (Page 36-39) Combining Forces
- The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39) Combining Forces
The combination of all forces acting on an object is called the net force.

4. Balanced forces acting on an object do not change the object’s motion.
- The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39) Unbalanced Forces vs. Balanced Forces
Unbalanced forces can cause an object to start moving, stop moving, or change directions.
Unbalanced forces acting on an object result in a net force and cause a change in the object’s motion.
Balanced forces are equal forces acting on one object in opposite directions.
Balanced forces acting on an object do not change the object’s motion.

5. - The Nature of Force
Book M – Ch 2 – Section 1 (Page 36-39) Unbalanced Forces vs. Balanced Forces

6. Click the SciLinks button for links on force.
- The Nature of Force
Links on Force
Click the SciLinks button for links on force.

7. Book M – Ch 2 – Section 2 (Page 42-50)
Friction and Gravity
The force that two surfaces exert on each other when they rub against each other is called friction.
The strength of the force of friction depends on two factors:
How hard the surfaces push together.
The types of surfaces involved.

8. Book M – Ch 2 – Section 2 (Page 42-50) Four Types of Friction
Static Friction – the friction that acts on objects that are not moving.
Sliding Friction – occurs when two solid surfaces slide over each other.
Rolling Friction – occurs when an object rolls across a surface.
Fluid Friction – occurs when a solid object moves through a fluid.

9. Book M – Ch 2 – Section 2 (Page 42-50) Gravity
Gravity is a force that pulls objects toward each other.
The law of universal gravitation states that the force of gravity acts between all objects in the universe.
Two factors affect the gravitational attraction between objects:
Mass
Distance

10. Book M – Ch 2 – Section 2 (Page 42-50) Mass vs. Weight
- Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50) Mass vs. Weight
Mass is a measure of the amount of matter in an object.
The force of gravity on a person or object at the surface of a planet is known as weight.

11. Book M – Ch 2 – Section 2 (Page 42-50) Free Fall
- Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50) Free Fall
When the only force acting on an object is gravity, the object is said to be in free fall.
In free fall, the force of gravity is an unbalanced force, which causes the object to accelerate.
Acceleration due to gravity is 9.8 m/s².
This means for every second an object is falling, its velocity increases by meters per second (m/s).

12. Free Fall - Friction and Gravity
Use the graph to answer the following questions.

13. Free Fall - Friction and Gravity Interpreting Graphs:
What variable is on the horizontal axis? The vertical axis?
Time is on the horizontal axis, and speed is on the vertical axis.

14. Free Fall - Friction and Gravity Calculating:
Calculate the slope of the graph. What does the slope tell you about the object’s motion?
The slope is 9.8. The speed increases by 9.8 m/s each second.

15. Free Fall - Friction and Gravity Predicting:
What will the speed of the object be at 6 seconds?
58.8 m/s

16. Free Fall - Friction and Gravity Drawing Conclusions:
Suppose another object of the same size but with a greater mass was dropped instead. How would the speed values change?
The speed values would not change.

17. Book M – Ch 2 – Section 2 (Page 42-50) Air Resistance
- Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50) Air Resistance
Falling objects with a greater surface area experience more air resistance.
This is why a leaf falls more slowly than an acorn.
Air resistance increases
with velocity.
The greatest velocity a
falling object reaches is
called terminal velocity.

18. Book M – Ch 2 – Section 2 (Page 42-50) Projectile Motion
- Friction and Gravity
Book M – Ch 2 – Section 2 (Page 42-50) Projectile Motion
An object that is thrown is called a projectile.

19. Comparing and Contrasting
- Friction and Gravity
Comparing and Contrasting
As you read, compare and contrast friction and gravity by completing a table like the one below.
Friction
Gravity
Pulls objects toward one another
Effect on motion
Opposes motion
Types of surfaces involved, how hard the surfaces push together
Depends on
Mass and distance
Measured in
Newtons
Newtons

20. Click the SciLinks button for links on friction.
- Friction and Gravity
Links on Friction
Click the SciLinks button for links on friction.

21. Click the Video button to watch a movie about free fall.
- Friction and Gravity
Free Fall
Click the Video button to watch a movie about free fall.

22. Book M – Ch 2 Section 3 (pgs. 51-54)
- Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs )
Newton’s First Law of Motion
States that an object at rest will remain at rest, and an object moving at a constant velocity will continue moving at a constant velocity, unless it is acted upon by an unbalanced force.
Inertia - the tendency of an object to resist a change in motion
Dependent on mass - the more mass, the more inertia (resists change more)

23. Book M – Ch 2 Section 3 (pgs. 51-54)
- Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs )
Newton’s Second Law of Motion
Acceleration depends on the object’s mass and on the net force acting on the object
Force – measured in Newtons (N) or (kgm/s2)
Mass – measured in kilograms (kg)
Acceleration – measured in (m/s2)
1 newton as the force required to give a 1-kg mass an acceleration of 1 m/s2

24. Book M – Ch 2 Section 3 (pgs. 51-54)
- Newton’s First and Second Laws
Book M – Ch 2 Section 3 (pgs )
What two ways can you increase acceleration??
Increase force
Decrease mass
Let’s Practice!

25. - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Read and Understand What information have you been given? Mass of the water-skier (m) = 55 kg Acceleration of the water-skier (a) = 2.0 m/s2

26. Calculating Force - Newton’s First and Second Laws
A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Plan and Solve What quantity are you trying to calculate? The net force (Fnet) = __ What formula contains the given quantities and the unknown quantity? a = Fnet/m or Fnet = m X a Perform the calculation. Fnet = m X a = 55 kg X 2.0 m/s2 F = 110 kg • m/s2 F = 110 N

27. - Newton’s First and Second Laws
Calculating Force
A speedboat pulls a 55-kg water-skier. The force causes the skier to accelerate at 2.0 m/s2. Calculate the net force that causes this acceleration. Look Back and Check Does your answer make sense? A net force of 110 N is required to accelerate the water-skier. This may not seem like enough force, but it does not include the force of the speedboat's pull that overcomes friction.

28. Calculating Force Practice Problem - Newton’s First and Second Laws
What is the net force on a 1,000-kg object accelerating at 3 m/s2?
3,000 N (1,000 kg X 3 m/s2)

29. Calculating Force Practice Problem - Newton’s First and Second Laws
What net force is needed to accelerate a 25-kg cart
at 14 m/s2?
350 N (25 kg X 14 m/s2)

30. Click the PHSchool.com button for an activity about Newton’s laws.
- Newton’s First and Second Laws
More on Newton’s Laws
Click the PHSchool.com button for an activity about Newton’s laws.

31. Book M – Ch 2 Section 4 (pgs. 55-61)
- Newton’s Third Law
Book M – Ch 2 Section 4 (pgs )
Newton’s Third Law of Motion
States that if one object exerts a force on another object, then the second object exerts a force of equal strength in the opposite direction on the first object.
For every action there is an equal but opposite reaction

32. Book M – Ch 2 Section 4 (pgs. 55-61)
- Newton’s Third Law
Book M – Ch 2 Section 4 (pgs )
Motion can not always be observed – Ex. Earth’s gravity pulling on a pencil vs. pencil pulling on the earth.
Action-Reaction forces do not cancel (like in Newtons 1st law of motion) because they are acting on different objects

33. Book M – Ch 2 Section 4 (pgs. 55-61)
- Newton’s Third Law
Book M – Ch 2 Section 4 (pgs )
Momentum – a characteristic of a moving object that is related to the mass and the velocity of the object.
Mass – measured in kilograms (kg)
Velocity – measured in meters per second (m/s)
Momentum – measured in kilogram meters per second (kg·m/s)
Described by its direction as well as its quantity
An objects momentum is the same direction as it’s velocity

34. Book M – Ch 2 Section 4 (pgs. 55-61)
- Newton’s Third Law
Book M – Ch 2 Section 4 (pgs )
The more momentum a moving object has, the harder it is to stop.
Increased mass and/or increased velocity will increase momentum
Which has more momentum?
Baseball traveling at 20 m/s or Car traveling at 20 m/s
2 ton car traveling at 10 m/s or
2 ton car traveling at 50 m/s

35. Calculating Momentum - Newton’s Third Law
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Read and Understand What information have you been given? Mass of smaller sledgehammer = 3.0 kg Velocity of smaller sledgehammer = 1.5 m/s Mass of larger sledgehammer = 4.0 kg Velocity of larger sledgehammer = 0.9 m/s

36. Calculating Momentum - Newton’s Third Law
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Plan and Solve What quantities are you trying to calculate? The momentum of each sledgehammer = __ What formula contains the given quantities and the unknown quantity? Momentum = Mass X Velocity Perform the calculation. Smaller sledgehammer = 3.0 kg X 1.5 m/s = 4.5 kg•m/s Larger sledgehammer = 4.0 kg X 0.9 m/s = 3.6 kg•m/s

37. Calculating Momentum - Newton’s Third Law
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or a 4.0-kg sledgehammer swung at 0.9 m/s? Look Back and Check Does your answer make sense? The 3.0-kg hammer has more momentum than the 4.0-kg one. This answer makes sense because the 3.0-kg hammer is swung at a greater velocity.

38. Calculating Momentum Practice Problem - Newton’s Third Law
A golf ball travels at 16 m/s, while a baseball moves at 7 m/s. The mass of the golf ball is kg and the mass of the baseball is 0.14 kg. Which has the greater momentum?
Golf ball: kg X 16 m/s = 0.72 kg•m/s Baseball: 0.14 kg X 7 m/s = 0.98 kg•m/s The baseball has greater momentum.

39. Calculating Momentum Practice Problem - Newton’s Third Law
What is the momentum of a bird with a mass of kg flying at 15 m/s?
0.27 kg•m/s (0.018 kg X 15 m/s = 0.27 kg•m/s)

40. Book M – Ch 2 Section 4 (pgs. 55-61)
- Newton’s Third Law
Book M – Ch 2 Section 4 (pgs )
Law of Conservation of Momentum
states that, in the absence of outside forces, the total momentum of objects that interact does not change.
Example of “outside force” – Friction

41. Conservation of Momentum-
- Newton’s Third Law
Conservation of Momentum
In the absence of friction, momentum is conserved when two train cars collide.

42. Momentum Activity - Newton’s Third Law
Click the Active Art button to open a browser window and access Active Art about momentum.