Physics Chapter 6 Forces

Newton’s Laws of Motion 1 st Law (Law of inertia) –An object moving at constant velocity keeps moving at that velocity unless a net force acts on it 2 nd Law (F=ma) –The net force acting on an object causes the object to accelerate in the direction of the net force 3 rd Law –For every action there is an equal but opposite reaction

Force –A push or a pull System –The object and what is around it Environment –World around object that exerts forces on it

Contact force –Acts on an object only by touching it Long-range force –A force exerted without contact Force of gravity –An attractive force that exists between all objects Agent –A specific, identifiable, immediate cause of a force

Newton’s 1st Law of Motion Inertia (deals with 1 st law) –The tendency of an object to resist any change in motion Equilibrium –When an object is moving at constant velocity An object at rest is moving at a constant velocity of 0 and is in equilibrium

Free body diagram Uses dots to represent objects Uses lines to represent forces –Lines are always drawn in the direction of the force which acts upon the object –The tail of the line representing the force is always placed on the dot of the object that it is acting on.

Net Force (F net ) –The vector sum of two or more forces on an object Friction (F f ) –The contact force that acts to oppose sliding motion between surfaces –Parallel to the surface and opposite the direction of sliding friction Normal Force (F N ) –contact force exerted by surface on an object –Perpendicular to and away from the surface FORCES

FORCES con’t Spring Force (F sp ) –Restoring force, that is, the push or pull a spring exerts on an object –Opposite the displacement of the object at the end of the spring Tension (F T ) –The pull exerted by a string, rope, or cable when attached to a body and pulled taut –Away from the object and parallel to the string, rope, or cable at the point of attachment

FORCES con’t again Thrust (F thrust ) –A general term for the forces that move objects such as rockets, planes, and cars –In the same direction as the acceleration of the object barring any resistive forces Weight (F g ) –A long-range force due to gravitational attraction between two objects, generally Earth and an object –Straight down toward the center of the earth W = mg

Common Misconceptions When a ball has been thrown, the force of the hand that threw it remains on it A force is needed to keep an object moving Inertia is a force Air does not exert a force The quantity ma is a force

Apparent weight –The force exerted by the scale Weightlessness –Doesn’t mean your weight is zero –It means your APPARENT weight is zero Free fall –When an object is influenced only by the acceleration due to gravity Terminal velocity –When the drag force is equal to the acceleration due to gravity

FRICTION Static friction force –Force exerted on one surface by the other when there is no relative motion between the two surfaces 0 ≤ F f, static ≤ μ s F N Kinetic friction force –Force exerted on one surface by the other when the surfaces are in relative motion F f, kinetic = μ k F N

Periodic Motion Simple harmonic motion –When the force that restores an object to its equilibrium position is directly proportional to the displacement of the object Period (T) –Amount of time needed to repeat one complete cycle of motion Amplitude –The maximum distance the object moves from equilibrium

Mechanical resonance –The increase vibration of the amplitude due to small forces being applied at regular intervals to a vibrating or oscillating object Period of a Pendulum T = 2 l g

If an object is on a flat surface, then the Force due to Weight will equal the Normal Force F W = F N The force of friction (F f ) is always opposing the applied force (F app ) If the net force is zero (means no acceleration or constant velocity) the force due to friction will equal the applied force. F f = F app