Chapter 13 The Nature of Forces

1
Chapter 13 The Nature of Forces

• Forces
• Friction
• Inertia
• Newtons Laws
• Gravity
• Air Resistance

2
Section 1 - Forces

• Force
• a push or pull that one body exerts on another
  that can make an object start or stop moving, OR
  change its speed or direction.
• What forces are being exerted on the football?

Ffriction
Fkick

• Fgrav Force of Gravity
• Fkick Pushing force of the kickers foot
• Ffriction molecules of air rubbing against
  the surface of the football air resistance!

Fgrav
3
Section 1 - Forces

• Balanced Forces

• forces acting on an object that are opposite in
  direction and equal in size
• no change in velocity
• no movement
• the upward force is called the normal force

normal force
weight (force)
4
Section 1 - Forces

• Unbalanced Forces
• Forces that cause a change in motion (these
  forces can be in the same direction OR opposite
  directions.
• velocity changes (object accelerates)

5
Section 1 - Forces

• Balanced Unbalanced Forces

• Draw the resulting force arrow for each of the 3
  force combinations shown to the left!!!

6
Section 2 - Friction

• Friction
• force that opposes (restricts or fights) motion
  between 2 surfaces
• The amount of friction depends on the
• types of surfaces
• force between the surfaces

7
Section 2 - Friction

• Friction is greater...
• between rough surfaces
• when theres a greater force between the surfaces
  (e.g. more weight)

8
Section 2 - Friction

• 3 main types of friction
• Sliding
• Rolling
• Fluid
• What are some Pros and Cons of friction?

Decreasing Frictional Force
9
Section 2 - Friction

• 3 main types of friction
• Sliding when solid objects slide past or over
  each other
• Rolling friction produced by solid objects such
  as wheels or ball bearings.
• Fluid friction produced when an object moves
  through a fluid such as air or water.

10
Section 2 - Friction

• How do people decrease the force of friction?
• We use lubricants.
• Def any substance that changes sliding friction
  to fluid friction.
• Ex motor oil, grease,

11
Section 3 - Newtons First Law

• Newtons First Law of Motion
• An object at rest will remain at rest and an
  object in motion will remain in motion at a
  constant velocity unless acted upon by an outside
  force.

12
Section 3 - Newtons First Law

• Newtons First Law of Motion
• Law of Inertia
• Inertia
• tendency of an object to resist any change in its
  motion (either resting or in motion)
• increases as mass increases

13
Section 3 - Newtons First Law

• Restated
• An object will maintain its speed and direction
  (velocity) or LACK of velocity (remain
  motionless) unless an outside force changes it.

14
Test Your Understanding

• You are a passenger in a car and not wearing your
  seat belt.
• Without increasing or decreasing its speed, the
  car makes a sharp left turn, and you find
  yourself colliding with the right-hand door.
• Which is the correct analysis of the situation?

15
Test Your Understanding

• 1. Before and after the turn, there is a
  rightward force pushing you into the door.
• 2. Starting at the time of collision, the door
  exerts a leftward force on you.
• 3. Both of the above
• 4. Neither of the above

2. Starting at the time of the turn, the door
exerts a leftward force on you.
16
Section 3 - Newtons Second Law

• Newtons Second Law of Motion
• Newtons Second Law shows how force, mass, and
  acceleration are related.

F ma
17
Section 3 - Newtons Second Law
F ma
F force (N) m mass (kg) a accel (m/s2) 1 N
1 kg m/s2
18
Force Calculations

• What force would be required to accelerate a 40
  kg mass by 4 m/s2?

GIVEN F ? m 40 kg a 4 m/s2
WORK F ma F (40 kg)(4 m/s2) F 160 N
19
Force Calculations

• Mr. Miller weighs 557 N. What is his mass?

GIVEN F(W) 557 N m ? a(g) 9.8 m/s2
SHOW YOUR WORK m F a m (557 N) (9.8
m/s2) m 56.8 kg
20
Force Calculations

• A 4.0 kg shotput is thrown with 30 N of force.
  What is its acceleration?

GIVEN m 4.0 kg F 30 N a ?
SHOW YOUR WORK a F m a (30 N) (4.0 kg) a
7.5 m/s2
21
Section 3 - Newtons Third Law

• Newtons Third Law of Motion
• When one object exerts a force on a second
  object, the second object exerts an equal but
  opposite force on the first.

22
Section 3 - Newtons Third Law

• Simply put.
• Every action has an equal and opposite reaction.

23
Section 3 - Newtons Third Law

• How do the arrows indicate how the forces are
  acting in the picture?

24
Section 3 - Newtons Third Law
Explanation The force of the water (blue
arrows) is counterclockwise. The resulting force
(red arrows) against the sprinkler head is in
the opposite direction clockwise. This is
why the sprinkler spins!!
25
Section 3 - Newtons Third Law

• Action-Reaction Pairs

• The hammer exerts a force on the nail to the
  right.
• The nail exerts an equal but opposite force on
  the hammer to the left.
• However, even though the nail exerts as much
  force as it can, it is not as large as the
  hammers force and the net result is the nail
  moving toward the right.

26
Section 4 - Gravity

• Gravity
• force of attraction between any two objects in
  the universe
• increases as...
• mass increases
• distance decreases

G m1 m2 d2
27
Section 4 - Gravity

• Who experiences more gravity - the astronaut or
  the politician?

• Which exerts more gravity - the Earth or the moon?

28
Section 4 - Gravity

• Weight
• the force of gravity on an object

W mg
W weight (N) m mass (kg) g acceleration due to
gravity (m/s2)
WEIGHT depends on gravity (N or gxm/s2)
MASS always the same (kg or g)
29
Section 4 - Gravity

• Would you weigh more on Earth or Jupiter?

• Jupiter because...

Jupiter has a greater mass
Therefore it has greater gravity
And you would have greater weight
30
Test Your Understanding

• Is the following statement true or false?
• An astronaut has less mass on the moon since the
  moon exerts a weaker gravitational force.

• False! Mass does not depend on gravity, weight
  does. The astronaut has less weight on the moon.

31
Section 4 - Gravity

• Acceleration due to gravity (g)

• In the absence of air resistance, all falling
  objects have the same acceleration!
• On Earth g 9.8 m/s2

Animation from Multimedia Physics Studios.
32
Section 4 - Air Resistance

• Air Resistance
• fluid friction or drag
• force that air exerts on a moving object to
  oppose its motion
• depends on

• speed of object
• surface area of object
• shape of object
• density of fluid

33
Section 4 - Air Resistance

• Terminal Velocity
• maximum velocity reached by a falling object
• reached when Fgrav Fair

• no net force
• ? no acceleration
• ? constant velocity

34
Section 4 - Terminal Velocity
35
Section 4 - Air Resistance

• Terminal Velocity

• increasing speed ? increasing air resistance
  until
• Fair Fgrav

Animation from Multimedia Physics Studios.
36
Section 4 - Air Resistance

• Falling with air resistance

• heavier objects fall faster because they
  accelerate to higher speeds before reaching
  terminal velocity

Fgrav Fair

• larger Fgrav
• ? need larger Fair
• ? need higher speed

Animation from Multimedia Physics Studios.
37
JET CAR CHALLENGE

• CHALLENGE
• Construct a car that will travel as far as
  possible (at least 3 meters) using only the
  following materials.

• scissors
• tape
• 4 plastic lids
• 2 skewers

• 2 straws
• 1 balloon
• 1 tray

How do each of Newtons Laws apply?