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Newton’s Second Law

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

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Newton’s Second Law

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

  2. 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

  3. 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

  4. 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

  5. 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

  6. 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.

  7. 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.

  8. 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

  9. 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

  10. 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

  11. 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

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

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