Chapter 4.1 Notes Resistance (is futile!)

► Newton’s 1st law - Every object in motion stays in motion; Every object at rest stays at rest unless acted on by an outside. ► Newton’s 1st law - Every object in motion stays in motion; Every object at rest stays at rest unless acted on by an outside force. ► Newton’s 2nd law - The acceleration of an object is directly related to the force acting on the object and inversely related to the mass of the object.

► Newton’s 2nd law equation = force = mass x acceleration Newton’s 2nd law Newton’s 2nd law ► F = m x a ► Units of Force = Newtons (N)

► A 1200 kg car accelerates 0 to 100 m/second in 5 seconds. What is the force on the car? ► First find the Acceleration = (v f -v i ) / t ► a = (100 – 0) / 5 = 100 / 5 = 20 m 2 /s ► F = m x a ► F = 1200 x 20 = 24,000 N

► When the only force acting on an object in the downward direction is gravity and it accelerated downward; this acceleration is called gravitational acceleration. ► Force of gravity = F G ► Force of gravity = mass x gravity ► F G = m x g

A 65 kg person stands on a scale, what is the force of gravity on the person? A 65 kg person stands on a scale, what is the force of gravity on the person? ► F G = m x g ► g = 9.8 m/s 2 ► F G = 65 kg x 9.8 m/s 2 ► F G = 637 N

► When you push an object across a surface, the opposing force is called friction. friction ► To accelerate an object, the force must be greater than the frictional force. ► The force of friction resists movement.

► Friction is a result of irregularities in the surfaces of objects. ► The force required to overcome friction is called the static friction force. static friction static friction ► The force needed to keep a constant speed is called the kinetic friction force. kinetic friction kinetic friction

► Imagine your car broke down and you have to push it. Which takes more force, to get it started rolling or to keep it rolling? ► To get it started ► Static friction is greater than kinetic friction.

Friction depends on 3 things: Friction depends on 3 things: The friction force depends on whether or not the surfaces are moving. The friction force depends on whether or not the surfaces are moving. The friction force depends on the materials of which the surfaces are made of. The friction force depends on the materials of which the surfaces are made of. The friction force depends on how hard the surfaces are pressed together. This is called the Normal Force and depends on mass and gravity also known as Weight The friction force depends on how hard the surfaces are pressed together. This is called the Normal Force and depends on mass and gravity also known as Weight

► Coefficients of Friction Equations ► Static force=Coefficient of static x Normal Force (Weight) ► F s = u s x N ► Kinetic Force=Coefficient of kinetic x Normal Force (Weight) ► F k = u k x N

A crate made of wood weighs 85 lbs is on a concrete floor. What force is required to get it moving? A crate made of wood weighs 85 lbs is on a concrete floor. What force is required to get it moving? ► F s = u s x N ► u s ; Page 176 ► u s =.6 ► F s =.6 x 85 = 51

Same crate is now moving, what force is needed to keep it moving at a constant speed? Same crate is now moving, what force is needed to keep it moving at a constant speed? ► F k = u k x N ► u k ; Page 176 ► u k =.4 ► F k =.4 x 85 = 34

► Lubricants reduce friction by keeping the two sliding surfaces apart with a thin layer of fluid. ► Friction is no longer rubbing the surfaces but instead rubbing the lubricant.

► Another way to reduce friction is to roll an object over a surface. This is called rolling frictional force instead of kinetic frictional force.