Force

1
Force

• Chapter 6

2
Force

• Any push or pull exerted on an object

3
System

• The object with the force applied

4
Environment

• The world surrounding the object

5
Contact Force

• A force that acts on an object by touching it

6
Contact Force

• A baseball bat striking a ball

7
Long-range Force

• A force that acts on an object w/o touching it

8
Long-range Force

• The force of gravity

9
Agent

• Whatever is causing the force

10
Inertia

• The resistance to change
• (in motion)

11
Equilibrium

• When the net forces acting on an object zero

12
Force Vector Diagram

• A Diagram showing the vectors of all forces
  acting on an object.

13
Force Vector Diagram
Force of table on the ball
Weight on table
14
Draw Force Vector Diagrams of

1. A book on a desk
2. A book being pushed across the desk
3. A book falling

15
Newtons Laws of Motion
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Newtons 1st Law
An object will remain at rest or in constant
straight-line motion if the net force acting on
it is zero
17
Newtons 1st Law
The velocity is constant and acceleration is zero
when the net force on an object is zero
18
Newtons 2nd Law
The acceleration of an object is directly
proportioned to the net force applied to it
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Newtons 2nd Law
Fnet m
a
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Newtons 2nd Law
Fnet ma
21
Newtons 3rd Law
For every action, there is an equal opposite
reaction
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Newtons 3rd Law
FA on B -FB on A
23
Two horizontal forces of 23.5 N 16.5 N are
acting in the same direction on a 2.0 kg object.
Calculate 1) net Force on the object2) its
acceleration
24

• Two horizontal forces of 23.5 N 16.5 N are
  acting in opposite directions on a
• 2.0 kg object. Calculate
• net force on the object
• 2) its acceleration

25
Forces of 4.0 N west 3.0 N north are acting on
a 2.0 kg object. Calculate 1) net Force on
the object2) its acceleration
26
Calculate the acceleration of a 1500 g object
falling towards Earth when the Fair friction is
11.7 N.
27
List Newtons Laws of Motion
28
Types of Forces

• Friction Tension
• Normal Thrust
• Spring Weight

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Friction (Ff)

• The contact force that acts to oppose sliding
  motion between surfaces
• Its direction is parallel opposite the
  direction of sliding

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Normal (FN)

• The contact force exerted by a surface on an
  object
• Its direction is perpendicular away from the
  surface

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Spring (Fsp)

• A restoring force, or the push or pull a spring
  exerts on an object
• Its direction is opposite the displacement of an
  object at the end of a spring

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Tension (FT)

• The pull exerted by a string, rope, or cable when
  attached to a body pulled taut
• Its direction away from the object parallel to
  the string at the point of attachment

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Thrust (Fthrust)

• A general term for the force that moves rockets,
  planes, etc
• Its direction is the same direction as the
  acceleration of the object barring any resistive
  forces

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Weight (Fg)

• Force due the gravitational attraction between
  two objects like an object the Earth
• Its direction is straight down towards the center
  of the Earth

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Name describe the 6 types of forces
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Weight (Fg)
Weight Fg mag mg Fg W mg
37
When an object is launched, the only forces
action upon it are the forces gravity air
friction.
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No net force is required to keep an object in
motion. Frictional forces oppose motion.
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Inertia is not a force, but the resistance to the
change in motion or momentum.
40
Air exerts huge balanced frictional forces on
an object. When in motion, the net Ff of air is
large.
41
Terminal Velocity

• The constant velocity that is reached when the
  force of air friction of a falling object equals
  its weight

42
Friction (Ff)
Kinetic frictional force Ff, kinetic Static
frictional force Ff, static
43
Draw Vector Force Diagrams of

• 1) a skydiver gaining downward velocity
• 2) a skydiver at terminal velocity

44
Draw Vector Force Diagrams of
3) a rope pulling a ball up at constant
velocity 4) a rope acceleration a ball upwards
45
An objects weight on Earth is 490 N.
Calculate1) its mass2) its weight in the moon
where gmoon 1.60 m/s2
46
An 500.0 g object on an unknown planet has a
weight of 250 N. Calculate the acceleration
caused by the planets gravity.
47
Static Ff

• The force exerted on one surface by another when
  there is no relative motion

48
Kinetic Ff

• The force exerted on one surface by another when
  in relative motion

49
Forces acting on an object

• FN -W
• FA gt Ff

FN
Fapplied
Ff
Fg or Weight
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Static Ff
Ff, static msFN
51
m
ms is proportionality constant called the
frictional coefficient
52
Kinetic Ff
Ff, kinetic mkFN
53
A 25 N force is required to pull a 50.0 N sled
down the road at a constant speed. Calculate the
sliding frictional coefficient between the sled
the road.
54
A person a sled have a total weight of 490 N.
The sliding frictional coefficient between the
sled the snow is 0.10. Calculate the force
required to pull the sled at constant speed.
55
Calculate the acceleration of the sled if the
applied force pulling on the sled is 299 N. W
490 N m 0.10
56
Calculate the force required to pull a 500.0 g
block with an acceleration of 3.0 m/s2. m 0.50
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Periodic Motion

• Repetitive or vibrational motion like that of a
  spring, swing or pendulum

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Simple Harmonic Motion

• Periodic motion in which the restoring force is
  directly proportional to the displacement

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Period (T)

• The time required to complete one full cycle of
  motion

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Amplitude

• Maximum displacement from the zero point or
  equilibrium

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Pendulum Motion Formula
l

• T 2p ----

ag
62
Calculate the period of a pendulum with a length
of 49 cm
63
Calculate the length of the pendulum of a
grandfather clock whose period is equal 1.0
second
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Fundamental Forces

• Gravitational
• Electromagnetic
• Strong Nuclear
• Weak Nuclear

65
Calculate the force required to pull a 150 g
block at a constant velocity of 180 km/hr. m
0.20
66
A 9.8 kN car went from 0 to 25 m/s in 5.0 s. mK
between car road 0.20. Calculate the force
applied by the engine of the car.
67
Calculate the force required to start a 2.0 kg
block its acceleration when moving. ms 0.20,
mk 0.10
68
Calculate the force required to start a 2.0 kg
block calculate its acceleration when
moving. ms 0.20, mk 0.10
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• A 6.0 kg ball is attached by a rope over a pulley
  to a 4.0 kg ball.
• Draw the problem.
• Calculate each balls acceleration

70

• A 6.0 kg ball is attached by a longrope over a
  pulley to a 4.0 kg ball.
• Calculate air friction at max velocity

71
A 65 kg boy a 35 kg girl are in a tug-of-war.
The girls acceleration is 13 cm/s2. Calculate
the boys acceleration.
72
A 150 g baseball, was hit came to rest in 4.0 s
after going 100.0 m. Calculate vi, a, Ff on
the ball.
73

• A 50.0 kg box falls off a 0.49 km cliff.
• Calculate vi, vf, a, t.
• Calculate Ff if air friction is included

74

• A 10.0 kg box falls off a 0.49 km cliff hits
  the ground in 20.0 s.
• Calculate vf a.
• Calculate Ff if air friction is included

75
Calculate the force required to pull a 250 g
block at a constant velocity of 360 km/hr. m
0.30
76
Calculate the force required to accelerate a 1500
g block along the floor at 3.0 m/s2. m 0.25
77
Calculate the apparent weight of a 50.0 kg person
on a scale on an elevator descending at 2.0 m/s2.
78
Calculate the apparent weight of a 50.0 kg person
on a scale on an elevator ascending at 2.0 m/s2.
79
Calculate the period of the pendulum on Big Ben
which is 4.9 m long.
80
Calculate the force required to accelerate a 10.0
kg block straight up at 25 cm/s2.
81
Calculate the force required to accelerate a 50.0
kg block straight up over a pulley at 5.0 m/s2.
82
Calculate the acceleration of a system of a 55.0
kg block tied to a 45.0 kg block hanging over a
pulley.
83
Calculate the frictional coefficient of a 100.0
kg block if a 150 N force causes it to
accelerate at 50.0 cm/s2.
84
Calculate the frictional coefficient of a 10.0 kg
block if a 98 N force causes it to slide at 30.0
cm/s.
85
A 5.0 N force accelerates a 1000.0 g block at
45.0 cm/s2. Calculate mK.
86
Calculate the acceleration of a system of a 200.0
kg cart on a plane tied to a 50.0 kg block
hanging over a pulley.