Title: Force
1Force
- Unbalanced forces are the reason objects
accelerate. - If the unbalanced portion of all forces acting on
an object is determined, Newtons second law can
predict the acceleration of that object. - Force is measured in newtons (N)
2Unbalanced Force!!!
SF ma
SF ma means that you must find the vector sum
of the forces. This vector sum is the unbalanced
portion that accelerates the object.
3Types of Forces
- Force due to gravity (Fg) is always pointed down
and is equal to the (mass)(9.8 m/s2) - 9.8 m/s2 9.8 N/Kg
- So, Fg mass(Kg) 9.8 N/Kg
4Types of Forces
- Normal Force (FN) is the force due to the
presence of a surface. - It is always perpendicular to the surface.
- (FN) is what your scale tells you!! Its the
surface pushing back.
5Types of Forces
- Friction Force (Ff) resists motion.
- Always opposite the direction of motion
- mf is coefficient of friction
- mf (Ff) / (FN)
6Types of Forces
- Other forces F?
- Other forces are a result of a push or pull
from an outside source.
7Free Body Diagrams
Free body diagrams are simply diagrams that help
to identify any unbalanced force that will
accelerate an object.
All forces acting on an object act as though
they are acting on the center of gravity of the
object.
8Free Body Diagrams
- Represent the object as a point in space. This
dot is the center of gravity of the object
Fg is always down
Fn always perpendicular to the surface.
9Creating Free Body Diagrams
Imagine that you are pulling straight forward on
a 10 Kg wagon across a horizontal field with a
force of 61 N. The wagon has a coefficient of
friction of 0.23. Find the net force on the
wagon.
10Creating Free Body Diagrams
- Represent the object as a point in space. This
dot is the center of gravity of the object
11Step Two
- Draw and label all of the forces acting on the
object. - The forces should be shown as Pulls on the
center of gravity
Typical Order Fg, Fn, Ff, other forces.
12GSFO!!!!
- Ask yourself four questions IN THIS ORDER!!!!
- Gravity? If yes, Fg straight down.
- Surface? If yes, Fn perpendicular to the
surface. - Friction? If yes, Ff opposite motion.
- Other Forces? F(other)
13Step Three
- Choose a coordinate system.
- Find the magnitude of each force
Fp 61 N
Ff mfFn
14Step Four
Find the magnitude and direction of the net force
SF ma SFy may 0 SFy Fg Fn 0 SFx
max SFx Fp Ff ma SFx 61N -22.5N 38.5N
(10 Kg)a
Fp 61 N
Ff -22.5 N
15Examples
- A ball at rest on a table
- A ball free falling
- A cart being pulled across a frictionless
horizontal surface
16Examples
- A 15 Kg sled that is pulled with a horizontal
force of 17 N across low friction snow (mf
0.04) with a constant velocity. Find the net
force.
17Examples
- A 1200 Kg car is traveling along a level highway
with a force due to the engine of 4500 N
forward yet there is a friction coefficient of
0.12. Find the acceleration of the car.
18Examples
A train with an engine and two cars is
accelerating due to the engines 22000N
force. Find the net force on each car and the
trains acceleration. Assume no friction.
1.2 x 105 Kg 1.4 x 104 Kg 2.2 x 104 Kg
22000 N