Motion and Force Force = anything that changes the state of rest or motion of an object Balanced ForceUnbalanced Force When the forces applied to an object produce a net force of zero Do not cause an object at rest to start moving Do not cause a change in the motion of a moving object When two opposite forces acting on the same object are unequal Net force on object is greater than 0, object accelerates in direction of greater force Acceleration always in direction that combines the directions of the applied forces Net force = combination of all of the forces acting on an object

Force of Friction Friction- force that opposes motion between two surfaces that are in contact Surface of any object is rough When two surfaces in contact, hills and valleys of one surface stick to hills and valleys of the other surface

Friction opposes the applied force Due to friction, a constant force must be applied to a moving object to keep it moving Force pushing object forward must be greater than the force of friction opposing the object’s motion Friction also affects objects that aren’t moving: objects on inclines may not move down due to friction balancing out the force of gravity

Two types of friction Static friction- exists between stationary surfaces Kinetic friction- exists between moving surfaces Static friction is usually greater than kinetic friction When objects slide past each other, friction that occurs is called sliding friction If a round object rolls over a flat surface, friction that occurs is called rolling friction

Friction can be helpful or harmful HelpfulHarmful

Laws of Motion Derived by Isaac Newton First Law Second Law Third Law

Newton’s First Law An object at rest remains at rest and an object in motion will remain in motion unless acted upon by an outside, unbalanced force Reason for most injuries in car crashes: passengers continue to move even after car has stopped Safety equipment like seatbelts and airbags designed to counteract this Inertia- tendency of an object to resist changes in motion Force needed to overcome inertia depends on mass of object

Newton’s first law often called the law of inertia All matter resists changes in motion Mass is a measure of inertia Object with a small mass has less inertia than an object with a large mass Easier to change the motion of an object with a small mass than a large mass

Newton’s Second Law Describes the effect of an unbalanced force on the motion of an object An unbalanced force acting on an object equals the object’s mass times its acceleration Force = mass x acceleration F = ma Consider two cars: a Prius and a F-250: Which will have a greater acceleration with the same force applied?

Force is measured in newtons 1 N = 1 kg x 1 m/s 2 Pound is also used as a unit of force; 1N = lb, 1 lb = N Example- Given: mass, m = 175 kg, acceleration, a = m/s 2 Force = mass x acceleration ( F = ma ) F = 175 kg x m/s 2 F = 115 N

Practice Problems pg. 351 # F = ma = (1.6 x 10 3 kg)(2.0 m/s 2 ) = 3.2 x 10 3 N 2. m = F/a = 1.4 N/9.8 m/s 2 = 0.14 kg 3. a = F/m = 895 N/655 kg = 1.37 m/s 2 Second law can also be written as: a = F/m