Newton’s Second Law of Motion-Force and Acceleration Chapter 5

Change in Motion-Acceleration Newton’s First Law of Motion pertains to objects either at rest or undergoing constant velocity Objects in equilibrium and moving at a constant velocity have no net force acting on them Most of the motion we experience in real life undergoes change- accelerated motion

Acceleration revisited Acceleration describes how quickly motion changes Acceleration is the change in velocity per certain time interval a =  v /  t Acceleration is caused by force

Force Causes Acceleration Apply a force to a hockey puck at rest on the ice and it moves The puck has accelerated, or changed it state of motion When the hockey stick no longer pushes it, it moves at a constant velocity (minimal friction) Apply another force by striking the puck again and its acceleration changes again Force causes acceleration

Questions Is an object at rest accelerating? How much acceleration does an object moving at a constant speed have? Both examples are of objects in equilibrium No net force is acting on the objects Acceleration is zero

Acceleration depends on net force The combination of forces acting on an object is the net force Net force must be increased to increase acceleration If net force is doubled, acceleration will double If a hockey puck is hit, it will accelerate, or change velocity If the hockey puck is hit twice as hard, it will move twice the speed If it’s hit three times as hard, the puck will travel three times the speed Acceleration ~ Net Force

Mass resists acceleration Push on an empty shopping cart, then push equally hard on the same shopping cart loaded with groceries Which one will accelerate more? Acceleration depends on the mass being pushed The same force applied to twice the mass results in half the acceleration If the loaded cart is twice as heavy as the empty cart, what is the loaded cart’s relative acceleration? Acceleration ~ 1 / Mass

Newton’s Second Law of Motion Newton came up with one of the most important rules of nature ever proposed: The acceleration produced by a net force on an object is directly proportional to the magnitude of the net force, is in the same direction as the net force, and is inversely proportional to the mass of the object Acceleration ~ Net Force / Mass What units should be used for each variable in this equation?

Questions If a car can accelerate at 2m/s 2, what acceleration can it attain if it is towing another car of equal mass? What kind of motion does a constant force produce on an object of fixed mass? What acceleration is produced by a force of 2000N applied to a 1000-kg car? What if the force is 4000N? What if the car had a mass of 500-kg?

Friction Friction is a force that also affects motion Friction acts on materials that are in contact with each other and always acts in a direction to oppose motion Friction is due to irregularities in the surfaces of the two objects in contact Friction depends on the materials an object is made of Fluid friction occurs when an object moves through a liquid or gas (materials that flow) Water resistance Air resistance

Friction When the frictional force equals the applied force, the object moves at a constant velocity The friction force balances the applied force and the acceleration is zero