Forces Chapter 4

Force Symbol: F Metric Unit = Newton (N) 1 N = 1 kg X m/s2 (derived unit) Standard Unit = Pound (lbs) 1 lb = 4.44 N Definition: push or pull in any direction Causes changes in velocity Vector Quantity

2 Categories of Forces Contact Forces - Physical touching of the object Applied Tension Normal Friction Field Forces – no touching required Gravitational Electrical Magnetic Use bowling ball and tennis ball to show the difference between gravitational force and acceleration.

Newton’s Laws Law of Inertia 2. ΣF = ma or Fnet = ma 3. Every action has an equal and opposite reaction (Action-Reaction)

#1: Law of Inertia States that: Objects in motion stay in motion, and objects at rest tend to stay at rest unless acted on by an outside force. Ex. Table cloth trick https://www.youtube.com/watch?v=o94Pm-Cty3M

Therefore ΣF or Fnet = ma #2: ΣF = ma Force is directly proportional to mass and acceleration ΣF or net force α acceleration As acceleration ↑, force ↑ ΣF or net force α mass As mass ↑, force ↑ Mass α 1/acceleration As mass ↑, acceleration ↓ Therefore ΣF or Fnet = ma

#2:continued If Fnet = ma, then zero acceleration means there is no net force. Does this mean there are no external forces? NO Object is in a state of Equilibrium in which all external forces are balanced

#3: Every action has an equal and opposite reaction force If I push an object, the object will push back on me. Example: I push against a wall with a force the wall pushes back on me with the same force. Example: I stand applying a force on the floor, and the floor pushes back on me with the same force.

How Objects Move If you believe (which you should) every force has an equal and opposite force, then why would anything ever move? To understand why things move you must look at the individual forces acting on an object.

The Individual Forces (we will work with this unit) 1. Gravitational Force (Fg) 2. Normal Force (FN) 3. Applied Force (FA) 4. Tensional Force (FT) 5. Frictional Force (Ff, also Fs)

Gravitational Force, Fg Downward force from the acceleration of gravity on a mass Also known as weight. This force is always down Explains why astronauts are considered “weightless” in space.

VERY IMPORTANT You must understand there is a difference between mass and weight. Mass = measure of inertia measured in kg Weight = measure of gravitational force measured in Newton’s (N) or pounds (lbs) Weight = mass x gravity W = mg lab Fg

Normal Force, FN Force perpendicular to the surface. The definition of normal is perpendicular. The support force of the resting surface. Explain why things aren’t always falling Bathroom scale

Applied Force, FA Force any object uses on the object in question. Can be in any direction. Example: I push a box along the floor. Example: I use a rope to pull a wagon.

Tensional Force, FT A force that is provided by a material suspended between two objects Can be either: Normal or Support Force if the objects are static Applied Force is objects that are in motion

Frictional Force, Ff Force caused by two surfaces rubbing together. Parallel to the surface. Perpendicular to the normal. Opposite the attempted motion.

Free Body Diagrams Free Body Diagrams (FBD) are used to represent ONLY the forces acting on the object in question. isolates the individual forces on the object. used to solve problems involving forces

Only draw the forces that act on the BOX. Free Body Diagram Example: I push a box along a floor to the right FN Ff FA Fg Only draw the forces that act on the BOX.

How Objects Move: Not a FBD Action-Reaction pair: must be equal in size and opposite in direction Is the box moving? FN Ff FA If the applied force pair is larger than the friction pair, the object will move right Fg

Action Reaction Pairs Remember: Each forces has an equal but opposite reaction force Examples: Fg is the attraction of the box to the earth the reaction pair is the earth attracted to the object. FN is the force of the floor on the box the reaction force is the box pushing on the floor.

Action-Reaction Pairs, Cont… FA is the force applied by my hand the reaction force is the force the box applies to my hand. Ff is the force of the floor rubbing on the box  the reaction force is the force of the box rubbing on the floor.

Action Reaction Pairs, example Fperson on hammer Fhammer on person Fnail on hammer Fhammer on nail Fwood on nail Fnail on wood FA

Free Body Diagrams The free body diagram of an object is not concerned with the reaction pair. only draw the forces acting on the object, but ignore any forces the object supplies. Explain with people pulling each others arms Problems 1-12 13-25 Adding Forces Lab

Rules For Adding Forces Draw the free body diagram Rotate the axis if necessary (only rotate if a majority of the forces will align on the axis if rotated) Write sum of the forces equations and substitute in the individual forces. ΣF = max ΣF = may Do you have acceleration? ax = ? ay = ?

Finding Fnet or ΣF ΣF is the result of all forces acting on an object. EX: FN = 100 N Ff = - 50 N FA = 100 N Find the mass of the box. Find the net force acting on the box. Find the acceleration of the box. Fg = -100 N

Finding Fnet or ΣF ΣF is the result of all forces acting on an object. EX: FN = 100 N Ff = - 50 N FA = 100 N a) Fg = mg , 10.19kg b) Σ Fy = FA + Ff = 0 N Σ Fx = FN + Fg = +50 N, right c) Σ Fx = ma, ax = 4.91 m/s2 Σ Fy = 0, ay = 0 Fg = -100 N

Equilibrium Equilibrium exists when the net forces on an object equal zero. ΣF = 0 Equilibrium can occur when an object is at rest or moving with a constant velocity.

Forces at an Angle FN FA FAY FAX Ff Fg

What about forces at an angle on a ramp? Why does this box stay on the ramp? Didn’t we say that an object at rest is in equilibrium? How can three forces be balanced? FNX FN FNY FA FAY FAX Fg

Lets look at some examples of finding net force.

More on Friction There are two types of friction Static Friction – the resistive force that keeps an object from moving. Kinetic Friction – the resistive force that acts while an object is in motion. Static Friction is always larger than kinetic friction. This is why it is harder to get objects moving than it is to keep them moving.

Friction When an object is pushed static friction pushes back. Eventually Fs max is reached. At that point the object begins to accelerate or move. Friction is still acting, but now it is kinetic friction. Friction depends on the surfaces in contact and the normal force. The contact surfaces each have a static and kinetic relationship. Friction Lab Problems 26-36 37-41 42-44(44 honors), 45-54

The Coefficient of Friction