1. Or Trust in the Force Luke/Ani
Forces
Or Trust in the Force Luke/Ani
Standards:
1b Students know that when forces are balanced no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton’s 1st Law).

2. Forces in General
A Force is a push or pull, an interaction between two bodies
Force is a vector quantity
Adds vectorally (tip to tail)
Can be decomposed into perpendicular components that are independent of one another
Force is measured in Newtons (Kg m/s2)
1 Newton ~ ¼ pound
Question: Define force. How many objects are involved in a force? Is force a vector or scalar quantity? What units are used to measure force?

3. Force Types
Contact forces are forces that require physical contact between the two objects in the interaction.
E.g. Springs, muscular pushes and pulls, friction, normal force, air resistance, ropes, chains
Non-Contact or action at a distance forces do NOT require physical contact between two objects in interaction
E.g. Gravity, Electric and Magnetic Forces, Strong and Weak Nuclear Forces
Forces on object are those things touching it and gravity
Questions: Distinguish between a contact and non-contact force. Give examples of each.

4. Common Forces Force Definition Direction Magnitude Equation Weight
Gravitational force of planet on object
Towards center of planet
Fweight = mg where g is the gravitational field strength (-9.8 N/kg near Earth)
Spring Force
Force of spring on object
In direction opposite spring’s displacement
Fspring = -kx where k measures spring stiffness
Normal Force
Perpendicular force of contact between two surfaces
Perpendicular to surfaces contact
Never larger than net force in opposite direction
Frictional Force
Horizontal force of contact between two surfaces
Parallel to surface contact in direction that opposes motion
Ffriction =mFnormal where mu depends on stickiness of two surfaces
Air Resistance
Force between air molecules and body moving through air
Opposite direction of body’s motion
Fair resistance = Cv2 where C depends on shape and mass of object
Common Forces
Define the following forces, their direction and the magnitude equations they use.

5. Check Question
What is the weight of 2 kg?
2 N
1 N
20 N
0.2 N

6. Net Force
Sum of all forces in a linear direction (eg horizontal and vertical).
If net force is NOT zero then object will accelerate in that direction
If net force is equal to zero then object will move with a constant velocity in that direction (eg stay stationary or retain initial velocity)
Define net force. What type of motion occurs when the net force is zero? What type of motion occurs when the net force is NOT zero? Are all objects with zero net force stationary?

7. Examples of Net Force Stationary box on floor
Mover leaning on stationary box on floor
Mover pushing box on floor at constant velocity
Mover pushing box on floor at constant acceleration
Object falling w/ air resistance

8. Ropes, Chains, Strings and Equilibrium Problems
Ropes, strings, and chains exert a contact force on other objects called a tension.
Equilibrium Tension Problem has an object suspended by strings, chains and ropes.
Since no acceleration occurs, net force = 0.
Forces in horizontal direction balance
Forces in vertical direction balance
As rope becomes more horizontal the tension in the rope must increase
Questions: What are forces due to the pull of a rope, string or chain called? If an object is suspended by some arrangement of chains, strings and ropes what can be said of the horizontal and vertical net force? As any of the chains, ropes or strings are moved so they are more horizontal, what happens to the tension of that connector?