Chapter 2 - Forces
Lesson 2 – Friction and Gravity
 The force that two surfaces exert on each other when they
rub against each other is called friction.
 The strength of the force of friction depends on two factors:
 How hard the surfaces push together
 The types of surfaces involved.
 Friction also increases of surfaces push hard against each
other. If you rib your hands together forcefully, there is more
friction than if you rub your hands together lightly.
 There are four types of friction:
 Static Friction
 Sliding Friction
 Rolling Friction
 Fluid Friction
Static Friction
 The friction that acts on objects that are not moving.
 Because of static friction, you must use extra force to start
the motion of stationary objects.
Sliding Friction
 Occurs when two solid surfaces slide over each other.
Rolling Friction
 When an object rolls across a surface. This type of friction is
important to engineers who design certain products. Skates,
skateboards, and bicycles need wheels that move freely.
Fluid Friction
 Fluids, such as water, oil, or air, are materials that flow easily.
Fluid friction occurs when a solid object moves through a
liquid.
Gravity
 Gravity is a force that pulls objects towards each other.
 Two factors affect the gravitational attraction between the objects:
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Mass and Distance.
Mass is a measure of the amount of matter in an object.
The more mass an object has, the greater its gravitational force.
Weight is the force of gravity on a person or object at the surface
of a planet.
Weight varies with the strength of gravitational force but mass
does not.
Gravity and Motion
 When the only force acting on an object is gravity, the object
is said to be in free fall.
 An object in free fall is accelerating.
 In free fall, the force of gravity is an unbalanced force, which
causes an object to accelerate.
Air Resistance
 Objects falling through air experience a type of fluid friction
called air resistance.
 Air resistance is not the same for all falling objects. Objects
with a greater surface area experience more air resistance. If
the leaf and the acorn fell at the same time, the acorn will hit
first.
 The greatest velocity a falling object reaches is called
terminal velocity. Terminal velocity is reached when the force
of air resistance equals the weight of the object.
Projectile Motion
 An object that is thrown is called projectile.
 When you throw an projectile at an upward angle, the force
of gravity reduces its vertical velocity.