Download The concept of Force

WEEK 7 PHYSICS
MUST DO
• Unit 2 Cover Page
• Write three questions
• Write a statement of what you
already know.
Unit 2: Molecules to Cells
My Questions:



Why does science need evidence?
Why don’t we know all the answers?
Will we ever know all the answers?
What I already know: I know that science has a method.
Scientists can work in labs. Science can lead to new
discoveries.
THE CONCEPT OF FORCE
FORCES AND THEIR DIRECTION
• Forces are measured in Newtons (N)
• It is important to identify the direction of a force (vector)
• There are 6 force types we will discuss.
•
•
•
•
•
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Weight
Tension
Normal (contact)
Drag
Upthrust
Frictional
WEIGHT
• Force resulting from gravitational attraction
• W = mg
• g is the gravitational constant of the planet
Units are N/kg or m/s2
• Weight is always directed vertically downward
• If mass is in kg and g in N/kg then Weight has
units of N
TENSION
• A force that arises when any body is
stretched
• A tension force in a string is created when
two opposite forces are applied at both
ends.
• In magnetic forces the “string” is the force
between two molecules.
• Force pulls down where the string attaches
to the top and pulls up on the object
hanging.
• For the sake of idealized physics, the string is
massless so tension is the same at all points
on the string.
NORMAL (CONTACT FORCE)
• When two objects touch there is a
force of reaction or contact
• This force is perpendicular to the
object exerting the force.
DRAG FORCE
• Force that opposes motion of a moving
body through a fluid (gas or liquid)
• A marble falling through honey.
• Drag forces are directed in the opposite
direction of the velocity vector. The
higher the speed the higher the drag
force.
UPTHRUST
• An upward force on an object placed in a fluid
• If upthrust equals the weight of the body, the
body will float in the fluid. If upthrust is less than
the weight the body will sink.
• Upthrust id caused by the upward force the
fluid places on the body.
FRICTIONAL FORCE
• Force opposing the motion of a body.
• Usually occurs when two bodies are in
contact.
• Direction is opposite to the motion.
• Object on an inclined plane
TRY THIS
• A brick of weight 50N rests on a horizontal surface. The maximum frictional
force between the brick and the surface is 30N. When the brick slides on the
surface the frictional force is 10N. A horizontal force is applied to the brick, its
magnitude increasing slowly from zero. Find the frictional force on the brick
for various values of F.
F
0N
12N
28N
29N
30N
30.01N
40N
f
0N
12N
28N
29N
30N
10N
10N
No
motion
No
motion
No
motion
No
motion
No
motion
motion
motion
FREE-BODY DIAGRAMS
• Diagram showing all forces on a
chosen body.
• The body is shown on its own free
from other objects it may be in
contact with.
• In mechanics it is important that
the length of the arrow
representing a force is
proportional to the magnitude of
the force.
HOOKE’S LAW
• When a spring is extended, a force
pulls it back to its original length; if
we try to compress a spring, again
a force tries to pull the string back
to its original position
• Hooks law is the relationship to the amount a spring is extended or compressed. Represented by x
• The tension T is proportional to x so the greater distance you pull/push the spring the more force is
required.
• T = kx (k is a constant of proportionality known as the spring constant which varies from spring to
spring)
• Units are force over extension or N/m
• This law only works in a range of extensions. Outside of that “elastic limit” the math becomes more
complicated.