Force
• A force is a push or a pull on a body.
• Measured in units of Newtons (N).
• The push or pull can result in a change in motion or not at all. • 2 or more forces can act on a body.
Equilibrium?
• Any change in motion requires a force to act.
• A single force will cause motion.
PUSH
• Two or more forces acting on a body can produce a net or overall change in motion.
PUS
H
S
PU
H
• When a body is in equilibrium, its motion does not change. Mechanical Equilibrium
• Mechanical Equilibrium is when forces balance on a body. • There must be at least two forces acting to produce mechanical equilibrium.
Equilibrium
• Many different types of forces can act on a body. They have different names depending on how they act.
• Tension – force due to a rope
• Friction – resistance
• Weight – force of gravity on a body
• Normal force – due to a surface
• Applied force – force applied
• Net Force – sum of all forces acting on a body.
Equilibrium
• All forces acting on a body have a direction. The Net force is found by adding all of these forces together. • The net force has a size and a direction of action.
• If the net force is zero. The motion of a body does not change.
• If the net force is other than zero, the motion of a body will change.
Free Body Diagrams
• Standard representation of the
relative magnitude and direction of all
forces acting upon an object.
• Object represented by a box, forces by
arrows
Free Body Diagrams
• Length of the arrow reflects magnitude of
force
• Orientation of the arrow shows force direction
• Each arrow is labeled to indicate the force
type
• Arrows are always drawn outward from the box
• Must depict all the forces which act on the
object
Free Body Diagrams
Example – A book with a weight of 15 N
is at rest on the top of a table. Draw the
free body diagram.
Fnorm
Fgrav
Net Force
A Net Force is responsible for acceleration of an object.
It is found by using a free body diagram and adding the forces together.
Net Force is the sum of the forces acting on a body.
Net Force
For any object, Net Force is equal to the net mass times the net acceleration.
Fnet = manet
Fnet = Σ F
Σ F = Σma
All objects experiencing a force contribute to the acceleration. We have to look at the system.
Example Problem 1
Billy applies a force of 30 N on a box sitting on the floor with a weight of 20 N and a mass of 2 kg. a. Draw the free body diagram
Fnorm
Fgrav
Fapp
b. Find the net Force on the box.
Example Problem 1
Find the net force in the vertical direction first. • Set up and to the right as positive.
Fnorm
Fgrav
Fapp
Fnet =
• Since there is no vertical motion, the net force vertically is equal to zero.
Example Problem 1
Let us now find the net force in the horizontal direction. Use the same directions as positive as before.
Fnorm
Fgrav
Fapp
Fnet = Fnet = Example Problem 2
An elevator is connected to steel cable from above. The elevator has a weight of 500 N and a mass of 50 kg. Draw the free body diagram.
Ften
Fgrav
Example Problem 2
Write the net force equation.
Ften
Fgrav
Fnet = Example Problem 2
How much is the tension in the cable if there is no net acceleration?
Givens :
Fnet = Ften ­ Fgrav
anet = 0 m/s2
Solve : Fnet = Ften ­ Fgrav
Fnet=manet
manet = Ften ­ Fgrav
0 = Ften ­ Fgrav
So Ften = Fgrav and Ften = 500 N upward.
Example Problem 2
• If the elevator now accelerates upward, will the tension be greater than 500 N or less than 500 N?
Ans ­ Greater than 500 N
• Why?
Ften must be greater than Fgrav to have a Net Force and acceleration upward. The amount depends on the acceleration.
If the elevator accelerated upward at 3 m/s/s, what would be the Tension in the cable?
Fgrav = 500 N
anet = 3 m/s2
m = 50 kg
Ften=?
Fnet = Ften ­ Fgrav