Forces

Physics of Motion Kinematics – study of HOW objects move. Galileo described motion but did not try to explain what caused the motion to happen. Dynamics – study of WHY objects move. Newton developed three important laws of motion. Each relate force and acceleration.

Vector quantity – magnitude and direction. Force Push or Pull Vector quantity – magnitude and direction. Free Body Diagram – shows all the forces acting on an object. I pull left with 10 N Friction pulls right with 5 N (Draw)

Ff – Friction Fg – Weight FT – Tension on a rope or string FN – normal force – the sum of the forces acting perpendicular to the motion. Fapp – applied force that is applied to an object by another object or by a person.

Balanced Force – the sum of all forces acting on an object cancel (equal zero). Net Force – the sum of all the forces acting on an object can be represented by a resulting force. The resulting force has the same effect as all the forces acting on the object. Force has units of Newton, N

Four Fundamental Forces in Nature Strong Nuclear Force Strongest force known Operates only over short distances - less than 10-15m. Holds protons and neutrons together in the nucleus of an atom Responsible for nuclear fusion and fission.

Electromagnetic Force 2nd strongest Force Responsible for all electrical and magnetic phenomena Charged particles create an electric field when not moving. Charged particles create a magnetic field when they are moving. Electrons in orbit in the electron clouds when in chemical bonds

Weak Nuclear Force Responsible for radioactive decay Gravitational Force Weakest of all the forces Force of attraction between two objects Causes weight, keeps planets orbiting around the sun, tides from the moon Depends on the mass of the two objects and the distance between the two objects.

Newton’s 1st Law of Motion Inertia – property of an object that makes it resist changes in motion. Depends on the object’s mass Mass  Inertia  and Mass  Inertia  Measure inertia by the amount of force required to cause a change in the objects motion.

Newton’s 1st Law of Motion “Law of Inertia” An object at rest (not moving) will stay at rest until a net force acts on it. An object moving at a constant velocity (same speed and same direction) will keep moving at that velocity until a net force acts on it.

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Balanced forces do not change an objects motion Balanced forces do not change an objects motion. Net force = 0 and acc = 0 A rock in space will keep moving at a constant velocity because nothing is stopping it. A car moves at a constant velocity because the force of the engine is balanced with the force of friction from the road. You could go through the windshield during an accident because of your inertia if nothing is holding you back (like your seatbelt)!

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Newton’s 2nd Law FNet = ma A net force causes an object to accelerate. Acceleration is directly proportional to the net force and inversely proportional to the mass. FNet = ma FNet = Forces acting on an object parallel to the motion of the object. Acceleration is in the same direction as the net force.

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FNet  acc  or FNet  acc  for constant mass Direct Relationship mass  acc  or mass  acc  for constant force Inverse Relationship

Newton’s 3rd Law For every action force there is an equal but opposite reaction force. Forces come in pairs – same magnitude but opposite in direction.

-Faction = Freaction -(maaa) = mrar Accelerations are not necessarily equal – depends on the masses of the objects!

Newton’s Laws and Seatbelts If you do not wear your seatbelt, you keep going when the car stops in an accident. Please, wear your seatbelt Please do not drink alcohol – it is against the law to drink until you are 21 years old. If you are 21 – do not drink and drive. car_accident.mpeg

Example Problems Homework – Forces and Newton’s Laws