Types of Forces
• Contact Forces
• Forces at a distance.
Four Main Forces in Universe
•
•
•
•
Gravity Fg.
Electromagnetic EM
Strong Nuclear Strong or Nuclear
Weak Nuclear Weak
•All Forces fall into one of the above
categories.
Common Forces
• Normal Force
(reaction force) –
• perpendicular to the
surfaces in contact
or touching.
What if you’re leaning on a wall.
Where is the normal force?
Where is it now?
Sliding Friction – force acts at contact between
surfaces. Direction of friction opposes velocity
or attempted velocity.
Which way is friction?
Ff = mFn
m is constant
Which way is the book moving?
Right
Left
The box is not moving.
• Tension –
• transmitted through string, rope, or
wire by forces at each end.
Air Resistance – air friction.
Big reason for bad fuel economy in cars.
Spring or Elastic Force –
Force applied by stretched
or compressed spring
described by Hooke’s Law.
Magnetic Force – due to excess
electron spin
Electrical – due to excess
charge buildup.
• Weight Fg = mg
• gravity interaction with mass.
Weight depends on gravity,
Mass does not.
What is mass?
Amount of matter in object.
Question: What will be the mass of a
10-kg object on the moon?
•
•
•
•
1. Zero
2. Less than 10 – kg
3. 10 – kg.
4. Impossible to tell without more
information.
Calculation of Weight Fg and mass.
• g = Fg/m
• Fg = mg.
• Fg = gravity on object =
weight N.
• m = mass, kg.
• g = accl gravity m/s2.
• On Earth g = 9.81 m/s2.
1. A student weighs 850-N on Earth. What
is his mass?
• Fg = mg.
• Fg = m
• g
• 850 N
• 9.81 m/s2.
• = 86.6 kg
2. A 650-N Earth student weighs 12.6 N on
planet Tessaz. What is the acceleration of
gravity on Tessaz?
•
•
•
•
•
•
•
Find mass on Earth.
m = Fg/g
m = 650 N/ 9.81 m/s2.
m = 66 kg.
g = Fg/m
g = 12.6 N/ 66 kg.
g = 0.2 m/s2.
3. Little Jimmy weighs 490-N on Earth. If the
acceleration of gravity on MonsterY is 50 m/s2, what is
the force of gravity on Little Jimmy on MonsterY?
• m = Fg/g
• 490N/9.81 m/s2.
• m = 50 kg.
• Fg = mg
• (50kg)(50m/s2) = 2500N
3. Make a sketch graph of weight (Y) vs. mass (X).
Write the equation for the graph.
What is the slope of the line?
120
Weight vs. Mass
100
Weight N
80
60
40
20
0
0
2
4
6
Mass kg
8
10
12
Hwk Sheet 1 - 15.
Normal Force Fn.
• Contact between surfaces.
Fn
• Always perpendicular to
surface.
• If object rests on horizontal
surface, then:
• Fn = -W = mg.
How does Normal Force Arise?
How does a wall know to push back
harder when I push with increasing
force?
The object will be at rest only if the surface can
exert an equal & opposite force to sustain it,
otherwise the object crashes through the surface.
Friction Between Surfaces
Friction depends on the nature of the
materials in contact and the
smoothness of their surfaces.
. Friction is independent of surface area.
. Ff is directly proportional the normal force.
Ff = mFn. Where m is a constant.
The ratio of Ff, to Fn, is
the coefficient of friction, m.
m=
Ff
Fn
m has
no units.
File Cabinet Demo
m
• m depends on two surfaces in contact
• the type of sliding motion – starting or
moving.
• Does not depend on any other factor.
• It is a constant.
Types of Friction
• Static Friction use mst – Present for objects at
rest. Need to overcome to start objects
moving.
• Kinetic Friction or sliding use mk – Present
for objects in motion. Need to overcome to
keep objects in motion. Always less than
static friction.
Reference Table
1. A 10-kg rubber box is at rest on a dry
horizontal concrete surface.
Calculate:
• a. the weight of the box.
98.1 N
98.1 N
• b. the normal force on the box.
• c. the force needed to start the box moving.
Ff = mstFn. = (0 .9 )98.1 N = 88.9 N
• d. the force to keep the box moving a constant
velocity.
Ff = mstFn. = (0 .68 )98.1 N = 66.7 N