Chapter
2
Force
and Newton’s
Laws
Isaac Newton
l
4 January 1643 – 31 March 1727
l
An English physicist,
mathematician, astronomer, natural philosopher, and alchemist.
l
An apple fell on his head?
Forces
l A force is a push or pull on an object
¡ Force is
measured in units called newtons, abbreviated “N”
¡ One newton = 1 (kg × m) ÷ s2
l The combination of all the forces acting on an object
is the net force.
¡ Ex: Gravity
pulls you towards the Earth, the Earth pushes you back so you don’t sink, and
your friend pushes you forward. What happens?
Unbalanced Forces
l When forces balanced forces, they cancel each
other out and do not change an object’s motion
l When forces are unbalanced forces,
the motion of an object changes.
¡ Ex: your friend pushes you
Newton’s
First Law of Motion
l An object will remain at rest or move with constant
speed unless a force is applied
l Also known as the Law of Inertia
Friction
l Friction is
a force that resists sliding between two touching surfaces or through air or
water
¡ Ex: Low
friction when you walk on ice so there is little resistance between your shoe
and the ice
¡ Ex: High
friction when walking on rubber so there is high resistance between your shoe
and the rubber
l Friction slows down an object’s motion
Types of Friction
l Static friction
– the type of friction that prevents an object from moving when a force is
applied
¡ Increases with heavier objects
¡ Ex: when you try to push a refrigerator
Types of Friction
l Sliding friction – slows down a sliding object. Due to the microscopic roughness of two
surfaces
¡ Ex: roll a ball
on a field, it eventually stops
l Rolling friction – friction between the ground and a wheel that allows the wheel to
roll
¡ Ex: wheels roll on cement, but not well on ice
Newton’s
Second Law of Motion
l Newton’s 2nd Law of motion explains that an object acted upon by a force will
accelerate in the direction of the force.
¡ Force = mass × acceleration OR
¡ Acceleration = force ÷ mass
Gravity
l Gravity is
the attractive force between objects
¡ The amount of gravity depends on the mass of the
objects and the distance between them
¡ Ex: you all
attracted to your pencil, you’re just not big enough to notice it
l The measure of gravitational pull is called weight
¡ Weight changes on different planets
¡ Different than your mass, which is how much matter you
possess (measured in kilograms)
Using the 2nd Law
l
Unknown force?
¡
Mass = 50 kg, acceleration (gravity) = 9.8
m/s2
¡F=M×A so F =
(50 kg) ×(9.8 m/s2). F= 490 N
¡This object
pushes on the earth (or crashes into the earth) with 490 N of force
l
Unknown acceleration?
¡You push a 10
kg sled with 5 N of force
¡
A = F÷M so A = (5 N) ÷ (10 kg). A = 0.5
m/s2
Centripetal Force
l In circular motion, the centripetal force is
always perpendicular to the motion.
¡ Ex: satellites
are always moving “straight,” but the earth is pulling them down. The overall movement is a circle
Terminal Velocity
l The terminal velocity is reached when the force
of gravity is balanced by air resistance
l The size of the air resistance depends on the shape of
an object and its speed
l A skydiver
¡ Terminal velocity about 200 km/hr when arms spread out
¡ Terminal velocity about 300 km/hr when arms in and head
down
Speeding Up or Slowing Down
l An object can speed up, slow down, or turn in the
direction of the net force when unbalanced forces act on it
¡ Ex: You kick a slow rolling soccer ball
¡ Ex: You apply
the brakes on a bicycle (force of friction)
Newton’s
Third Law of Motion
l Newton’s 3rd Law states that forces always act in equal opposite pairs
¡ For every action, there is an equal and opposite
reaction
¡ Ex: Newton’s
cradle
Action-Reaction Forces
l Action-reaction forces are always the same size but are in opposite
directions and act on different objectives
l When one object is much bigger than the other, the
action-reaction force is not noticeable
¡ Ex: Try pushing
the Earth
Types of Action-Reaction Forces
l Air and water exert action-reaction forces with
objects such as hands (swimming) or canoe paddles
l A rocket launches due to the equal but opposite forces
of the burning fuel
Weightlessness
l Remember weight is a measure of the pull of gravity
l Weightlessness is the condition that occurs in free
fall when the weight of an object seems to be zero
¡ Ex: weight
decreases when elevator drops