Newton's Second Law of Motion

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Newton’s Second Law of Motion : 

1 Newton’s Second Law of Motion I can describe how Newton’s Laws of Motion apply to everyday situations. I can calculate the acceleration, mass or force of an object when using Newton’s 2nd law.

Newton’s Second Law : 

2 Newton’s Second Law Force = Mass x Acceleration Force is measured in Newtons ACCELERATION of GRAVITY(Earth) = 10 m/s2 Weight (force) = mass x gravity (Earth) 2

Formulas to remember : 

Formulas to remember Acceleration = change in velocity time What is the cause of acceleration? FORCE

Force causes Acceleration : 

Force causes Acceleration Example – hockey puck on ice Still until force is placed on it Stays moving in a straight path until another force causes it to accelerate Change direction Speed up Slow down Change in velocity  acceleration

Acceleration : 

5 Acceleration Acceleration is directly related to the size of the force and the direction of the force. It accelerates in the direction you push or pull it.

Net force causes acceleration : 

Net force causes acceleration Combination of force yields acceleration Double the force – double the acceleration

In other words…. : 

7 In other words…. Large Force = Large Acceleration F a

In other words…. : 

8 In other words…. Small Force = Small Acceleration F a So….if you push twice as hard, it accelerates twice as much.

But there is a twist…. : 

9 But there is a twist…. Acceleration is INVERSELY related to the mass of the object.

In other words…..using the same amount of force…. : 

10 In other words…..using the same amount of force…. Large Mass a Small acceleration Small Mass Large acceleration a

Mass resists acceleration : 

Mass resists acceleration Example Full shopping cart vs. empty shopping cart The greater the mass the more force it takes to accelerate the object Acceleration is inversely proportional to mass

Using consistent units : 

Using consistent units a = F m a =acceleration (m/sec2 ) F = force (newtons - N) m = mass (kg)

Friction : 

Friction Is a force Must be in contact Direction opposite to motion Force is needed to overcome friction Caused by irregular surface

Extent of friction : 

Extent of friction Depends on kinds of material How much surface are pressed together Examples Rubber on concrete Steel on steel Guard rails are now concrete instead of steel

Friction in Fluids : 

Friction in Fluids Liquid friction can be quite high Air resistance – notice at high speeds Biking or skiing

Questions : 

Questions Two forces act on a book resting on a table: its weight and the support force from the table. Does a force of friction act as well? No, not unless the book tends to slide. Friction only acts when there is motion

Question : 

Question Suppose a high-flying jet cruises with a constant velocity when the thrust from its engines is a constant 80,000 N. What is the acceleration of the jet? What is the force of air resistance acting on the jet?

Answer : 

Answer The acceleration must be zero because the velocity is constant. Since the acceleration is zero, if follows a = F/m the net force is zero. This means the force of air resistance is 80,000 N and it acts in the direction opposite to the jet’s motion.