Forces and Motion Review Macroscopic Forces Determine motion from net forces

What is a Force? A force is simply a push or a pull. Forces are Vectors so they have a magnitude and a direction. A force applied to an object might change its shape (deformation) A force applied to an object might affect the object’s motion

Categories of Forces Contact forces – requires the objects exerting the forces to touch (be in contact with) each other Field Forces – act at a distance or through space. Girl pulls on wagon. Her weight pushes down on the chair. Gravitational field Magnetic field

Force Due to Gravity Fg = m g Gravitational forces occur because the mass of objects causes them pull together. On Earth, the gravitational force is WEIGHT. Weight on Earth is calculated by multiplying mass by 9/8 m/s/s (or about 10 m/s/s) Fg = m g Direction: always toward the center of the Earth.

Friction Force Frictional forces between two objects depend on the type of surfaces that are in contact with each other Direction: in the opposite direction of motion or impending motion. Ff

Elastic Force An elastic force applied to an object deforms the object but once removed allows the object to recover its original form, length, shape. The force present when things stretch or squish FEL

No stretching or squishing Tension Force The force that shows up in a string/rope as the result of its stretching is called tension force. Muscles can produce tension forces. FT No stretching or squishing Direction: Is always along the ‘rope’

Normal or Support Force Support force between and object and a solid surface Direction: it is always perpendicular to the surface at the point where the object touches it FN Direction: 90o to surface

Buoyant Force Force due to a liquid or gas; an object floating in or supported by liquid or air FBUOY Direction: Upward

Applied Forces FApp FPush FPull These are forces applied to an object; they can be pushes or pulls; applied forces are not a ‘type’ of force, but a source of a force in a particular problem. FApp FPush FPull

Magnetic force FEM FMAG Magnetic forces are non-contact (or field) forces; a magnetic force can pull an object toward itself or push it away. FEM FMAG

Common MACROSCOPIC Forces Name: ____________ Symbol Name of Force Contact or Field Force? Direction? Description FG or Fg Gravitational force (or weight force) Gravitational forces occur because the mass of objects causes them to be attracted to each other. On Earth, the gravitational force is WEIGHT. Weight is calculated by multiplying mass by 9/8 m/s/s (or about 10 m/s/s) Fe Elastic force Contact force An elastic force applied to an object deforms the object but once removed allows the object to recover its original form, length, shape. Ff Friction force Frictional forces between two objects depend on the type of surfaces that are in contact with each other; Solid surfaces that are in contact produce friction; Air or liquids can produce frictional forces, too. FT Tension force The force that shows up in a string/rope as the result of its stretching ; Muscles can produce tension forces FN or Fn Normal force Support force between and object and a solid surface; it is always perpendicular to the surface at the point where the object touches it F buoy Buoyant force Support force due to liquid or gas ; an object floating in or supported by air or liquid. F PUSH F PULL F APP Applied force by a person or thing forces applied to an object; they can be pushes or pulls; applied forces are not a ‘type’ of force, but a source of a force in a particular problem. F MAG Magnetic force Non-contact force (Field force) a magnetic force can pull an object toward itself or push it away. A magnetic field is present.

Common MACROSCOPIC Forces Name: ____________ Symbol Name of Force Contact or Field Force? Direction Description FG or Fg Gravitational force (or weight force) Non-contact force (Field force) Toward the center of the Earth Gravitational forces occur because the mass of objects causes them to be attracted to each other. On Earth, the gravitational force is WEIGHT. Weight is calculated by multiplying mass by 9/8 m/s/s (or about 10 m/s/s) FEL Elastic force Contact force Varies An elastic force applied to an object deforms the object but once removed allows the object to recover its original form, length, shape. Ff Friction force Opposite direction of motion or impending motion Frictional forces between two objects depend on the type of surfaces that are in contact with each other; Solid surfaces that are in contact produce friction; Air or liquids can produce frictional forces, too. FT Tension force Along the rope, string, etc. The force that shows up in a string/rope as the result of its stretching ; Muscles can produce tension forces FN or Fn Normal force Always perpendicular (90o) from surface. Support force between and object and a solid surface; it is always perpendicular to the surface at the point where the object touches it F buoy Buoyant force Upward Support force due to a liquid or gas ; an object floating in or supported by air or liquid. F PUSH F PULL F APP Applied force by a person or thing forces applied to an object; they can be pushes or pulls; applied forces are not a ‘type’ of force, but a source of a force in a particular problem. F MAG Magnetic force a magnetic force can pull an object toward itself or push it away. A magnetic field is present.

Vector Addition of Forces  

+4 N +3 N = +7 N -3 N = +1 N +4 N 5 N 3 N = 4 N Use the Pythagorean Theorem

= 0 N 3 N 1 N To the right is positive Up is positive When the net force is 0 , the forces are BALANCED - velocity stays the same.

= -1 N 3 N 2 N 1 N To the right is positive Up is positive When the net force is NOT 0, the forces are UNBALANCED - velocity changes SO ACCELERATION OCCURS!!

Motion Implications based on Force Analysis Balanced Forces Unbalanced Forces Equal Forces ΣF = 0 Unequal Forces ΣF ≠ 0 Static Equilibrium Dynamic Object is at rest. (still) v = 0 & a = 0 Object is moving at constant velocity. v ≠ 0 & a = 0 Disequilibrium Object accelerates in the direction of the net force. a ≠ 0 Motion Implications

A. 60 N ΣF = 6oN + 60 N Net Forces? ΣF = 0 N Equilibrium? Yes, in the x and y directions. ‘x’ Motion: At rest or constant vel. ‘y’ Motion: Net Forces? Equilibrium? Motion?

B. 12 N 5 N ΣF = 12N + 5 N Net Forces? ΣF = 7 N Equilibrium? No, in the y direction. ‘y’ Motion: Accelerates upward Yes, in the x direction. ‘x’ Motion: At rest or constant vel. Net Forces? Equilibrium? Motion?

C. 6.2 N 2.1 N ΣFy = 6.2N + 6.2 N Net Forces? ΣFy = 0 N (y direction) ΣFx = 2.1N Equilibrium? Yes, in the y direction ‘y’ Motion: At rest or constant vel. No, in the x direction ‘x’ Motion: Accelerates left 2.1 N Net Forces? Equilibrium? Motion?

C. B. A. 12 N 5 N 6.2 N ΣF = 6.2N + 6.2 N ΣF = 0 N (y direction) Equilibrium? Yes, in the y direction ‘y’ Motion: At rest or constant vel. No, in the x direction ‘x’ Motion: Accelerates left A. 60 N ΣF = 6oN + 60 N ΣF = 0 N Equilibrium? Yes, in the x and y directions. ‘x’ Motion: At rest or constant vel. ‘y’ Motion: 2.1 N ΣF = 12N + 5 N ΣF = 7 N Equilibrium? No, in the y direction. ‘y’ Motion: Accelerates upward Yes, in the x direction. ‘x’ Motion: At rest or constant vel.

1. Complete the rectangle with the vectors 3. To create a balanced force diagram, you need to place a new vector in the opposite direction (but the same magnitude). 2. Draw the diagonal of the rectangle to create the ‘Resultant’ vector..