Unit 3 - Dynamics How Forces Affect Motion

Unit 3 Part 1 – Newton’s Laws of Motion Physics Book Chapter 4 Conceptual Physics Book Chapters 4-6

Motion Theories  The old way looked at the what  The new way looked and looks at the why  4 th century BC - Aristotle  Did not experiment  Used only logic and observation  Two states of motion natural and violent  Earth at rest everything else moves in complete circles

Motion Theories  1600 Galileo  Used experiments with ramps  Discussed friction came up with the idea of inertia: property of all matter  1670 Newton  Built off of Galileo’s idea

1 st Law of Motion: The Law of Inertia  Newton said, “An object at rest wants to stay at rest unless acted on by a net (unbalanced) force, and an object in motion wants to stay in motion in a straight line at a constant speed unless acted on by an net (unbalanced) force.  Forces do not cause motion they change motion  Inertia is a property of all matter

1 st Law of Motion: The Law of Inertia  Inertia is related to the mass of an object  Mass is the measure of inertia or how easy or hard it is to change an objects position  Mass can also be how much stuff makes up an object.  Units of mass kilograms or slugs  1 slug = kg

1 st Law of Motion: The Law of Inertia Which one has more inertia?

1 st Law of Motion: The Law of Inertia c3d ba1f-c98be5caeb14/science-nfl- football-newtons-first-law-motion

Forces and Free Body Diagrams  A force is defined as any push or pull  To display forces free body diagrams are used.  Include only the forces acting on the object.  Force is a vector! Use an arrow to indicate a force.

Forces and Free Body Diagrams  Fit in two main categories  Contact forces between 2 objects  Shoot a basketball  Push a shopping cart  Friction  Non-contact or at a distance  GRAVITY  Electricity  Magnetism

Forces and Free Body Diagrams  Equilibrium  All forces are equal up = down, lefts = rights  If any one force is not equal then there is a non- zero net force on the object

Forces and Free Body Diagrams  Equilibrium involving gravity

Proportions

Newton’s 2 nd Law: Law of Acceleration

 “When a net external force acts on an object of mass m, there is an acceleration that results.”  The acceleration is directly proportional to the force applied and inversely proportional to the mass.

Newton’s 2 nd Law: Law of Acceleration  Units of force are Newtons and pounds  A Newton is a kg m/s 2  A pound is a slug ft/s 2

Newton’s 2 nd Law: Law of Acceleration WDhvw

Example Problem  Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other person applies a force of 395 N in the same direction. A third force of 560 N also acts on the car, but in the opposite direction. (This force is due to the friction of the tires on the pavement.) Find the acceleration of the car.

Newton’s 3 rd Law of Action/Reaction  Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.  “for every action there is an equal and opposite reaction”  Hammer hits nail and nail stops hammer  Swimming: you push the water backward and the water pushes you forward

Newton’s 3 rd Law of Action/Reaction

Weight and Normal Force

 The normal force is the perpendicular component of the force that a surface exerts on an object with which it is in contact.  Does NOT necessarily point directly upwards!  Equal and opposite of the weight IF no other forces are acting and the object is on a horizontal surface.  Think of “atomic springs” in the surface compressing to produce the normal force.

Newton’s 3 rd Law of Action/Reaction

Friction  Friction is the parallel component of the force that a surface exerts on an object with which it is in contact.  Two types:  If the object IS NOT moving, then the force is called static friction.  If the object IS moving, then the force is called kinetic friction.

Friction

True Weight vs. Apparent Weight  The apparent weight is the force that an object exerts on a scale.  If the scale and the object are either at rest or moving at a constant speed (a=0), then True=Apparent.  If the scale and object are accelerating, then apparent weight will be different!

True Weight vs. Apparent Weight