What types of forces do you already know Different types of forces Forces are usually divided into two types C ontact forces occur because of physical contact between objects ID: 724276
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Slide1
T
ypes
of
forces
And
Free Body Diagrams Slide2
What types of forces
do you already know?Slide3
Different types of forces
Forces are usually divided into
two
types
.
C
ontact
forces
occur
because of physical contact between objects.
Examples: pushing open a door
pulling on a rope
Field
forces
–
A
ct at a
distance through
space. The
presence
of an object effects the space around
it, creating a
region of space around the object called a
field.
Example: gravitational field
magnetic field around a magnetSlide4
Contact ForcesNormal force
Force that prevents
an object from falling through the surface of another
body
Always acts perpendicular to the surface
Always equals the forces applied to the surface
(or surface will break!)
mg
F
n
Called the ‘normal’ force because it is always normal (perpendicular) to the surface.Slide5
Contact ForcesNormal force
Force that prevents
an object from falling through the surface of another
body
Always acts perpendicular to the surface
Always equals the forces applied to the surface
(or surface will break!)
mg
F
n
mg
F
F
n
F
fr
F
n
mg sin
q
q
q
mg cos
q
mg Slide6
Friction force F
fr
motion
friction
Friction is
a force that is created whenever two surfaces move or try to move across each other.
Friction
always opposes the motion or attempted
motion
of one surface across another surface.
Friction
is
dependent
on the
texture/roughness
of both surfaces
.
Friction
is also
dependent
on the
force which presses the surfaces together
.
Contact ForcesSlide7
Air resistance / Drag
When an object moves through air or any other fluid, the fluid exerts a
friction-like
force on the moving object. The force is called
drag
.
Drag depends
upon the speed of the object, becoming larger as the speed increases.
(UNLIKE FRICTION!)Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)
Contact Forces
B/c drag increases with speed, object moving through the air reach a terminal velocity – a maximum speed at which
F
g
= F
drag so there is no more acceleration.Slide8
Air resistance / Drag
When an object moves through air or any other fluid, the fluid exerts a
frictionlike
force on the moving object. The force is called
drag
.
Drag depends
upon the speed of the object, becoming larger as the speed increases.
(UNLIKE FRICTION!)Drag also depends upon the size and the shape of the object and the density and kind of fluid. (UNLIKE FRICTION!)
Contact Forces
Without drag, raindrops would fall 340 m/h.
With drag, they only fall 17 m/h.Slide9
physics
Tension
the
force that the end of the rope exerts on whatever is attached to it.
Direction
of the force is along the rope.
T
1
T
2
T
2
Contact Forces
What is the relative force along the two yellow arrows?
Why?Slide10
Spring Force
Force due to the elasticity of a material
Depends on the elasticity of the spring
Direction is opposite displacement
Contact ForcesSlide11
Contact Forces: Think PaIR SHARE
Type of Force
Direction
Normal
Friction
Drag
Tension
SpringSlide12
Contact Forces: Think PaIR SHARE
Type of Force
Direction
Normal
Perpendicular to surface, opposite applied
/ gravitational forces
Friction
Opposite motion
DragOpposite motionTensionAlong the rope & opposite motionSpringOpposite displacementSlide13
Field Forces
Relative Strength
Action Distance
Gravitational Force
attraction between objects due to their masses
10
-45
Infinite – but decreases with square of distance
Electromagnetic Force
between charges
10
-2
Infinite – but decreases with square of distance
Strong Nuclear Force
keeps nucleus together
1
Very short!
Weak Nuclear Force
arise in certain radioactive processes
10
-8
Very
very
short!
Field ForcesSlide14
Field Forces
Relative Strength
Action Distance
Gravitational Force
attraction between objects due to their masses
10
-45
Infinite – but decreases with square of distance
Electromagnetic Force
between charges
10
-2
Infinite – but decreases with square of distance
Strong Nuclear Force
keeps nucleus together
1
Very short!
Weak Nuclear Force
arise in certain radioactive processes
10
-8
Very
very
short!
At the atomic level – all contact forces are result of repulsive electromagnetic
forces – the repulsion of atoms’ electric fields
Field ForcesSlide15
How to Solve Force Problems
Draw a
free body diagram
– label all the forces acting on
one
object.
Add up the forces
Apply Newton’s second law: F = ma.Slide16
How to draw a force diagram
2. Make a simple sketch of the system – point system
1. Choose ONE body to be isolated
dog or the cart?
fr
dog
net
3. Identify forces that act on the system
Label them on diagram
4. Find out the net force by adding the force vectors
decision: cart
5. Apply Newton’s second law