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Mass Vs. Weight
• Mass
•
The quantity of matter in
an object
•
•
Unit = kg
Mass stays the same no
matter where you are
• Weight
• The force of gravity on
an object
• Unit = N
• Weight = mass x
gravity
(W = mg)
Problem #1
• 1. What is the mass of a boy
whose weight is 35N?
W = mg
m = W/g
m = 35N/10m/s2
m = 3.5 kg
Problem #2
• Does a 2 kg rock have twice the
mass of a 1 kg rock?
• Twice the inertia?
• Twice the weight?
• Answer: Yes, Yes, Yes
Gravity
• Anything that has mass is attracted by the
force of gravity.
• The Law of Gravitation states:
Any two masses exert an attractive force
on each other.
• This attractive force depends on:
The mass of the two forces
The distance between the objects
• As mass increases, the
force increases
• As distance decreases, the
force increases
Example:
Earth is close enough to us and has a large enough
mass that you can feel its gravitational attraction.
But…
Your textbook is also close, but does not have enough
mass to exert an attraction that you can feel.
Force
Remember:
• Force is a push or pull on an object.
• Net Force is the total (combination) of all
forces acting on an object.
• Balanced forces (equilibrium) are forces
that are equal in size and opposite in
direction
• No change in velocity
• net force = zero
• Unbalanced force – changes the velocity of
an object because the net force is NOT
equal to zero
Force:
• Unbalanced
forces
net force = 10N→
net force = 2N →
• Balanced forces
net force = 0
Friction
• The force that opposes motion
between two surfaces that are
touching each other.
(an opposing force)
• The amount of friction depends on:
• the kind of surfaces touching (what they
are made of)
• the force pressing the surfaces together.
• Some types of friction:
• Static friction -the force exerted on one
surface by another when there is no motion
between the two surfaces.
• Kinetic friction –the force exerted on one
surface by another when the two surfaces rub
against each other because one or both of them
are moving. Ex. sliding friction & rolling friction
Air Resistance
• An opposing force that acts on
objects falling through the air.
• The amount of air resistance
depends on:
• speed of the object
• size of the object
• shape of the object