Conceptual PhysicsUnit 3 Newton’s Second Law of motion:

Force and Acceleration

Force causes acceleration:To increase the acceleration of an object, you

must increase the net force acting on the object.

Net force: the combination of all forces that act on an object. (fig 4-3, pg38)

Acceleration depends on the amount of the net force. It is directly proportional to the net force.

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Mass resists acceleration: acceleration depends on the mass being pushed or pulled.

Acceleration is inversely proportional to mass; that means that the two values change in opposition direction:

If you double a mass, the acceleration is half for the same force.

Newton’s second law:Net force= mass x acceleration

F= ma

If a car can accelerate at 2m/s2, 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?

Newton’s second law:Net force= mass x acceleration

F= maIf a car can accelerate at 2m/s2, what

acceleration can it attain if it is towing another car of equal mass?

The force from the engine is the same, but now the mass is doubled, therefore the acceleration will be half.

What kind of motion does a constant force produce on an object of fixed mass?

Constant acceleration.

Friction: is a force that affects motion. Acts upon materials that are in contact to each

other and acts in a direction to oppose motion.It occurs due to irregularities on the surface of

objects.Occurs in solids, liquids and gases ( called

fluids: because they flow)Air resistance: friction acting on something

moving through air.

Pressure: amount of force per unit of area.Pressure= force P= F

area of application AUnits: N/m2 or pascals (Pa)Ex. You exert more pressure on the floor when you are standing on one foot than when standing on both feet.

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Free fall explained:Galileo showed that falling objects accelerate

equally, regardless of their masses. This is true if air resistance is negligible

P47For a free falling object, the only force acting

upon it is its weight. But its weight is proportional to its mass.

F = F = g (acceleration due to gravity) m mThe accelerations are equal!!!

http://videos.howstuffworks.com/hsw/10695-elements-of-physics-falling-objects-video.htmEffect of Mass On Free Fallhttp://www.stmary.ws/highschool/physics/home/videos/apolloGravity.html

Falling and Air ResistanceWhen air resistance of a falling object equal its

weight, the net force is zero and no further acceleration occurs.

Terminal speed or velocity: acceleration terminates.(Is zero)

Examples

1) a)Calculate the acceleration if you push with a 20N horizontal force on a 2 kg block on a horizontal friction free air table. b) What acceleration occurs if the friction force is 4N?

Examples

2)Calculate the acceleration of a 300 000 kg jumbo jet just before takeoff when the thrust of each of its four engines is 30 000 N.

ClassworkTextbook answer question p 51 #1-20 except

#10,11 problems p 666 #1-4, 7,8, 11-13

Conceptual PhysicsUnit 3 Newton’s Second Law of motion:

Force and Acceleration