Chapter 4 -Newton’s 2nd Law
Force Causes Acceleration
• We learned w/ the 1st Law that objects do not
like changes in their motion…
▫ if a net force is applied to an object the motion of
an object will change…
• ‘changing motion’ basically means the same
thing as Acceleration
• So then we can say…. ‘force causes acceleration’
Friction
• Force Caused by irregularities between surfaces
that are in contact with each other
• Always directed opposite motion
• Rolling Friction<Sliding Friction < Static
Friction
• Air Resistance is another kind of friction
Amount of Friction…
• Depends on
▫ Weight of the object
▫ How rough or smooth the surfaces in contact are
• Does not depend on
▫ Surface area of object
▫ Speed of the object
Mass & Weight
• Mass (m)
▫ How much matter is in an object
▫ A measure of an object’s inertia
▫ Mass never changes depending on location
 Same everywhere
• Weight (Fg)
▫
▫
▫
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How much gravity is pulling on an object
Aka… The FORCE of gravity acting on an object
Depends on how strong gravity is
can change from place to place….
How to determine an object’s weight?
• Mass and weight are always directly proportional
• We find weight by using the equation
• Weight = mass x acc. of gravity
• Or..
Fg = mg
• g = 10 m/s2
• an object has a mass of 1 kg has a weight of be 10
Newtons.
• The “Newton” is the standard unit for force, and
since Weight is a force, the Newton is used here.
• 1 N = 1 kg*m/s2
Mass resists acceleration
• All forces do not accelerate all objects equally
• Objects with more mass are more difficult to
accelerate, and thus require more force
Newton’s 2nd Law
• ‘The acceleration of an object is directly
proportional to the net force on it, and
inversely proportional to its mass’
• This means…. The larger the force, the larger
the acceleration
• AND …. The larger the mass, the smaller the
acceleration
• This can be summed up w/ the equation
• ∑F=ma
• a acceleration (m/s2); ∑FNet Force (N);
mmass (kg)
Newton’s Second Law
• Q - When 2 kg stone is thrown it is given an acceleration of 20
m/s2. What is the net force applied to the ball?
• A - Fnet = ma = (2 kg) x (20 m/s2) =
40 N
• Q- What is the weight of an object with a mass
of 70 kg?
• A- Fg = mg = 70kg x 10 m/s2= 700 N
• Q- What is the mass of an object that has a
weight of 1000 N?
• A- Fg=mg so m = Fg/g = 1000N/ 10m/s2= 100 Kg
Free Fall Acceleration –
Why don’t heavier objects fall faster?
• Newton’s 2nd says that the more force an object
has on it the faster it should accelerate….. So
then if heavier objects have a larger weight
(force of gravity), why do they not accelerate
faster?
▫ Ans. True, heavier objects have more force,
but they also have more mass, which means that
it requires more force to accelerate as fast as
lighter objects….
▫ Essentially…. The larger force and larger mass
cancel each other out
▫ All falling objects accelerate downward at 9.8
m/s2
Non FreeFall
 Air Resistance
 Depends on 2 things…
 Frontal area of the falling object
 How fast object is moving
 There is a point where a falling object
is going so fast the air resistance
becomes so strong that the object can
no longer accelerate….. This speed is
called terminal speed
 Object is still falling, but no longer
accelerating
 In most problems we ignore any effects
of air resistance
• Heavier parachutist
must fall faster in order
for air resistance to
cancel out his weight
Skydive / Free Fall Record
• Kittinger- 1960
▫ Video
• Failed attempts
• Failed Skydive Attempt
• Skydiver
• RedBull Stratos Project
• Red Bull Stratus Project
• http://www.space.com/7824-skydiver-plans-supersonic-jump-edgespace.html
• http://spacejump.co.uk/history.htm
• http://www.popularmechanics.com/technology/aviation/safety/space-divethe-race-to-jump-from-120000-feet
• Other project details
• Videos