Physical Science Ms. Pollock 2008-2009 Newton’s Second Law Physical Science Ms. Pollock 2008-2009

Force, Mass, and Acceleration Throwing and dropping, different forces Greater force = greater acceleration Different masses, different forces Greater mass = greater acceleration Force, mass, and acceleration related mathematically in Newton’s Second Law Acceleration (m/s2) = net force (in newtons) mass (in kg) A = fnet m

Newton's Second Law Example You push a friend on a sled. Your friend and the sled together have a mass of 70 kg. If the net force on the sled is 35 N, what is the sled’s acceleration? A = fnet ÷ m A = 35 N ÷ 70 kg A = 0.5 m/s2

Calculating Net Force with the Second Law Force calculated, if mass and acceleration known Fnet = ma Example: When a tennis player hits a ball with a mass of 0.06 kg, the acceleration is 5,000 m/s2. What is the force exerted on the ball? F = ma F = (0.06 kg)(5,000 m/s2) F = 300 N

Friction Even after force is applied, objects slow down and eventually stop. Deceleration implies that a force is acting on the object. Friction – force that opposes the sliding motion of two surfaces that are touching each other Amount of friction dependent on kinds of surfaces and force pressing surfaces together Caused by microwelds between surfaces in contact

Friction Bumps weld together Larger force = stronger microwelds A force must be applied to break the microwelds and allow one surface to move over the other.

Static Friction No motion = zero acceleration = no net force Static friction – frictional force that prevents two surfaces from sliding past each other Microwelds between objects that cannot be broken, so object does not move.

Sliding Friction With added force, the object moves, but when the force is removed, it stops. Microwelds broken but reformed as objects slide Sliding friction – force that opposes the motion of two surfaces sliding past each other

Rolling Friction Frictional force between rolling object and the surface it rolls on Wheel and surface deformed to allow object to roll Static friction acting over deformed area Also causes rolling object to slow down and stop

Air Resistance Falling object pulled downward by gravity Air resistance – friction- like force that opposes motion of objects moving through air Without air resistance, all objects would fall with the same acceleration. Amount dependent on speed, size, and shape of object

Terminal Velocity Acceleration causes an object’s speed to increase as it falls. Increasing speed increases the air resistance on the object. Air resistance will eventually balance the force of gravity. Acceleration becomes zero, so speed becomes constant. Terminal velocity – highest speed a falling object will reach

Terminal Velocity Depends on size, shape, and mass of falling object Greater surface area = greater air resistance