Forces are usually divided into two types.
1. Contact forces occur because of physical contact between
objects.
Examples: pushing open a door
pulling on a rope
2. Field forces – Act 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 magnet
CONTACT FORCES
Normal force 𝐅𝐧
 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!)
Fn
mg
Called the ‘normal’ force
because it is always
normal (perpendicular) to
the surface.
CONTACT FORCES
Normal force 𝐅𝐧
 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!)
Fn
Fn
Ffr
Fn
F
mg
mg
CONTACT FORCES
Friction force Ffr
 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.
motion
friction
CONTACT FORCES
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!)
B/c drag increases with speed,
object moving through the air reach
a terminal velocity – a maximum
speed at which Fg = Fdrag so there is
no more acceleration.
CONTACT FORCES
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!)
Without drag, raindrops would fall
340 m/h.
With drag, they only fall 17 m/h.
CONTACT FORCES
Tension
 the force that the end of the rope exerts on whatever is
attached to it.
 Direction of the force is along the rope.
T2
T1
physics
T2
What is the
relative force
along the two
yellow arrows?
Why?
CONTACT FORCES
Spring Force
 Force due to the elasticity of a material
 Depends on the elasticity of the spring
 Direction is opposite displacement
CONTACT FORCES: THINK PAIR SHARE
Type of Force
Normal
Friction
Drag
Tension
Spring
Direction
CONTACT FORCES: THINK PAIR SHARE
Type of Force
Direction
Normal
Perpendicular to surface, opposite applied / gravitational forces
Friction
Opposite motion
Drag
Opposite motion
Tension
Along the rope & opposite motion
Spring
Opposite displacement
FIELD FORCES
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 FORCES
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
HOW TO SOLVE FORCE PROBLEMS
1.
2.
3.
Draw a free body diagram – label all the forces acting
on one object.
Add up the forces
Apply Newton’s second law: F = ma.
3. Identify forces that act on the system
How to draw a force diagram
Label them on diagram
1. Choose
NONE body to be isolated
decision: cart
dog or the cart? F
dog
Make a simple sketch of the system – point system
F2.
fr
Fnet
mg
4. Find out the net force by
adding the force vectors
5. Apply Newton’s second law
Fnet = ma
WE DO PROBLEMS

Superman lifts a 35,000 kg truck off Jimmy Olsen at a constant
velocity of 10 m/s. How much force did Superman apply to the
truck?
Fapp
F = 0 because there is no
net
acceleration.
Fgravity

Fapplied = Fgravity = mg = 35000*9.8 =
340000 N
Now Lois Lane is the one trapped under a semi. Superman
applies a 450,000 N force to fling the truck off of Lois. How
fast does the truck accelerate?
Fapp
Fapplied - Fgravity = ma
450000 - (35000*9.8) = 3.1 m/s2
35000
Fgravity
YOU DO PROBLEMS

A bookbag is hanging from your shoulder by one strap. Draw a
free body diagram, making sure that your forces are
proportional.
F
tension
Fgravity

A fish accelerates as it swims horizontally through the water. It
experiences an applied force (forward from its fins), drag,
buoyancy (upward force from the water), and gravity. Draw the
free body diagram, making sure that your forces are
proportional.
Fbouyancy
Fdrag
Fapplied
Fgravity
YOU DO PROBLEMS
Does a net force exit? Yes or no.
Yes … it is accelerating
No. It has constant velocity
No. It has constant velocity.
No, it has constant velocity.
Yes, it is accelerating.
Yes, it is accelerating.
YOU DO PROBLEMS
For each problem, determine the value of the unknown forces.
YOU DO PROBLEMS
For each problem, determine the acceleration of each block.
Each block has a mass of 10 kg.
A: No acceleration; forces
are balanced.
B: Fnet= 5N left
a = F/m = 0.5 m/s2 left
C: No acceleration;
forces are balanced.
D: Fnet = 15 N up
a = F/m = 1.5 m/s2 up