The Nature of Force

1
Table of Contents

• The Nature of Force
• Friction and Gravity
• Newtons First and Second Laws
• Newtons Third Law
• Rockets and Satellites

2
Combining Forces
- The Nature of Force

• The combination of all forces acting on an object
  is called the net force.

3
Unbalanced Forces
- The Nature of Force

• Unbalanced forces acting on an object result in a
  net force and cause a change in the objects
  motion.

4
Balanced Forces
- The Nature of Force

• Balanced forces acting on an object do not change
  the objects motion.

5
Asking Questions
- The Nature of Force

• Before you read, preview the red headings. In a
  graphic organizer like the one below, ask a what
  or how question for each heading. As you read,
  write answers to your questions.

Question
Answer
What is a force?
A force is a push or pull.
What happens when forces combine?
Forces combine to produce a net force.
6
Links on Force
- The Nature of Force

• Click the SciLinks button for links on force.

7
End of SectionThe Nature of Force
8
Gravity
- Friction and Gravity

• Two factors affect the gravitational attraction
  between objects mass and distance.

9
Gravity
- Friction and Gravity

• The force of gravity on a person or object at the
  surface of a planet is known as weight.

10
Free Fall
- Friction and Gravity

• Use the graph to answer the following questions.

11
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.

12
Free Fall
- Friction and Gravity

• Calculating
• Calculate the slope of the graph. What does the
  slope tell you about the objects motion?

• The slope is 9.8. The speed increases by 9.8 m/s
  each second.

13
Free Fall
- Friction and Gravity

• Predicting
• What will the speed of the object be at 6 seconds?

• 58.8 m/s

14
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.

15
Air Resistance
- Friction and Gravity

• Falling objects with a greater surface area
  experience more air resistance.

16
Comparing and Contrasting
- Friction and Gravity

• 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
17
Links on Friction
- Friction and Gravity

• Click the SciLinks button for links on friction.

18
Free Fall
- Friction and Gravity

• Click the Video button to watch a movie about
  free fall.

19
End of SectionFriction and Gravity
20
Calculating Force
- Newtons 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.
• 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

21
Calculating Force
- Newtons 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

22
Calculating Force
- Newtons 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.
• 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.

23
Calculating Force
- Newtons First and Second Laws

• Practice Problem
• 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)

24
Calculating Force
- Newtons First and Second Laws

• Practice Problem
• What net force is needed to accelerate a 25-kg
  cart
• at 14 m/s2?

• 350 N (25 kg X 14 m/s2)

25
Outlining
- Newtons First and Second Laws

• As you read, make an outline about Newtons first
  and second laws. Use the red headings for the
  main topics and the blue headings for the
  subtopics.

Newtons First and Second Laws

• Newtons First Law of Motion
• Inertia
• Inertia Depends on Mass
• The Second Law of Motion
• Changes in Force and Mass

26
More on Newtons Laws
- Newtons First and Second Laws

• Click the PHSchool.com button for an activity
• about Newtons laws.

27
End of SectionNewtons First and Second Laws
28
Calculating Momentum
- Newtons 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

29
Calculating Momentum
- Newtons 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 km X 1.5 m/s 4.5
  kgm/s
• Smaller sledgehammer 4.0 km X 0.9 m/s 3.6
  kgm/s

30
Calculating Momentum
- Newtons 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.

31
Calculating Momentum
- Newtons Third Law

• Practice Problem
• A golf ball travels at 16 m/s, while a baseball
  moves at 7 m/s. The mass of the golf ball is
  0.045 kg and the mass of the baseball is 0.14 kg.
  Which has the greater momentum?

• Golf ball 0.045 kg X 16 m/s 0.72 kgm/s
• Baseball 0.14 kg X 7 m/s 0.98 kgm/s
• The baseball has greater momentum.

32
Calculating Momentum
- Newtons Third Law

• Practice Problem
• What is the momentum of a bird with a mass of
  0.018 kg flying at 15 m/s?

• 0.27 kgm/s (0.018 kg X 15 m/s 0.27 kgm/s)

33
Conservation of Momentum
-
- Newtons Third Law

• In the absence of friction, momentum is conserved
  when two train cars collide.

34
Momentum Activity
- Newtons Third Law

• Click the Active Art button to open a browser
  window and access Active Art about momentum.

35
Previewing Visuals
- Newtons Third Law

• Before you read, preview Figure 18. Then write
  two questions that you have about the diagram in
  a graphic organizer like the one below. As you
  read, answer your questions.

Conservation of Momentum
Q. What happens when two moving objects collide?
A. In the absence of friction, the total momentum
is the same before and after the collision.
Q. What is the momentum of an object?
A. Its mass multiplied by its velocity
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End of SectionNewtons Third Law
37
What Is a Satellite?
- Rockets and Satellites

• A projectile follows a curved path because the
  horizontal and vertical motions combine.

38
What Is a Satellite?
- Rockets and Satellites

• The faster a projectile is thrown, the father it
  travels before it hits the ground. A projectile
  with enough velocity moves in a circular orbit.

39
What Is a Satellite?
- Rockets and Satellites

• Depending on their uses, artificial satellites
  orbit at different heights.

40
Identifying Main Ideas
- Rockets and Satellites

• As you read the section What Is a Satellite?
  write the main idea in a graphic organizer like
  the one below. Then write three supporting
  details that further explain the main idea.

Main Idea
A satellite stays in orbit due to
Detail
Detail
Detail
its inertia
Earths gravity
Earths shape
41
End of SectionRockets and Satellites
42
Graphic Organizer
Type of Friction
Occurs When
Example
Friction between an unmoving book and desk
An object is not moving
Static
Two solid surfaces slide over each other
Rubber pads on a bicycles brakes
Sliding
An object rolls across a surface
Rolling
Ball bearings in skateboard wheels
A solid object moves through a fluid
Fluid
Air resistance
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End of SectionGraphic Organizer