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Ch.2.1 Forces change motion.
 A force is a push or a pull.
 Types of forces
 Contact force – one object pushes or pulls another
 Gravity – force of attraction between two masses
 Friction – force that resists motion between two surfaces that are
pressed together
 Size and direction of forces (like velocity force is a vector)
 Balanced and unbalanced forces
 Balanced forces have same effect as no force
 Unbalanced forces change the motion of an object
 Net force – overall force on an object when all he forces are
combined
 Forces on moving objects
 Object w/forces acting on it can be moving at a constant speed as
long as those forces are balanced.
 Unbalanced force is needed to change an object’s motion
 Newton’s 1st Law relates to force and motion.
 Sir Isaac Newton – mid 1600’s studied effects of forces on objects
 Ancient Greeks vs. Galileo
 Newton restated Galileo’s conclusions in
 Newton’s 1st Law – Objects at rest remain at rest, and object’s in
motion remain in motion unless acted upon by an unbalanced
force.
 Inertia – the resistance of an object to change in speed or the direction of
it’s motion (Newton’s 1st Law also called the law of inertia)
2.2 Force and mass determine acceleration.
 Newton’s 2nd law relates force, mass, and acceleration.
 States that the acceleration of an object increases with increased force and
decreases with increased mass. And the direction of the acceleration is the
same as the direction of the force.
 Force equals mass times acceleration.
F= mass x acceleration
 What force is needed to accelerate a 10 kg shopping cart 3 m/s2?
F = mass x acceleration or F = ma
F = 10 kg x 3 m/s2
F = 30 kg x m/s2
Unit of force is a newton (N).
*kg x m/s2 is the same as a newton
F = 30 N
Calculation of Acceleration
 If a team pulls with a force of 9000N on an airplane with mass of 30,000kg, what
is the acceleration of the airplane?
F=ma, 9000N = 30,000kg x a
9000N = a
30000kg
a= 0.3m/s2
Mass is also a variable in Newton’s 2nd law.
 If the same force acts of 2 objects, the object w/less mass will have the greater
acceleration.
A model rocket is accelerating at 2 m/s2. The force on it is 1N. What is the mass of the
rocket? F = mxa, m =f/a
m=1N
2m/s2
 Forces can change the direction of motion
 Force causes mass to accelerate in the same direction as the force.
 Centripetal Force – any force that keeps an object moving in a circle
 Increasing centripetal force on an object increases it’s acceleration.
2.3 Forces act in pairs.
 Newton noticed that forces always act in pairs.
 Newton’s 3rd law – states that every action has an equal and opposite reaction.
 Action and reaction forces vs. balanced forces
 May be confused
 Balanced forces act on a single object while reaction forces act on
different objects
Newton’s 3 laws describe and predict motion.
 The 3 laws are not independent of each other; they are used together to explain
the motion of objects.
 1st law- in order to start a canoe moving you have to apply force to overcome its
inertia
 2nd law- you choose a canoe that isn’t as massive so that it accelerates easier
 3rd law- how you move your paddle in the water affects how you will move
forward in the water