Unit 2 – Motion, Force, Gravity, Projectile
Motion and Friction
NYS Standards:
5.1a The motion of an object is always judged with respect to some other object or point. The
idea of absolute motion or rest is misleading.
5.1b The motion of an object can be described by its position, direction of motion, and speed.
5.1c An objects motion is the result of the combined effect of all forces acting on the object. A
moving object that is not subjected to a force will continue to move at a constant speed in a
straight line. An object at rest will remain at rest.
5.1d Force is directly related to an objects mass and acceleration. The greater the force, the
greater the change in motion.
5.1e For every action there is an equal and opposite reaction.
5.2a Every object exerts gravitational force on every other object. Gravitational force depends on
how much mass the objects have on how far apart they are. Gravity is one of the forces acting on
orbiting objects and projectiles.
Motion
-When an object changes position over time when
compared with a reference point.
Object in motion 
 Reference Point
 There are three ways to categorize motion.
o Position
o Direction of Motion
o Speed
 The direction of the objects motion is dependent on the
reference point.
 If the reference point changes, the direction of the object
can also be looked at differently.
Force
-A push or pull on an object.
 There are three characteristics of forces.
o Have size
o Direction
o Units = Newtons (N)
 Ex: 40.0 N West
 If an object is moving or even if it is sitting still, there are
still forces acting upon it.
o If the forces acting on the object are balanced, or equal
to one another but in opposite directions…then the
object doesn’t move.
o If one of the forces is larger, it will move the object in
the direction of the force.
o We use force diagrams to determine the motion of an
object.
 Force Diagrams
 1. If the forces are moving in the same
direction, you add the forces together.
 2. If the forces are moving in the opposite
direction, you subtract them, leaving the
object to move in the direction of the greater
force.
o Example 1
 Are the forces acting on this
object balanced or unbalanced?
What is the net force acting on
the object? What direction is the
object going to move?
Net Force = 10.0N East + 50.0N East
Net Force = 60.0 N East, the object is unbalanced
o Example 2
 Are the forces acting on this
object balanced or unbalanced?
What is the net force acting on
the object? What direction is the
object going to move?
Net Force = 75.0N West - 35.0N East
Net Force = 40.0 N West, the object is unbalanced
o Example 3
 Are the forces acting on this
object balanced or unbalanced?
What is the net force acting on
the object? What direction is the
object going to move?
Net Force = 20.0N West - 20.0N East
Net Force = 0.0 N the object does not move, the object is balanced
Newton’s Three Laws of Motion
 1. An object in motion tends to stay in motion (or an object
at rest tends to stay at rest) unless acted upon by an outside
fore. Also called The Law of Inertia.
o Inertia – The tendency of all objects to resist any
change in motion.
 2. Force = mass x acceleration
o Example
An object has a mass of 50.0kg and moves with an
acceleration of 5.0m/s2. What is the amount of force
the object has?
F = mass x acceleration
F = 50.0kg x 5.0m/s2
F = 250.0 N
 3. For every action, there is an equal and opposite reaction.
Gravity
-Gravity is a force of attraction that exists between all objects in
the universe.
 The pull of gravity is directly related to mass. As the mass of
an object increases, the gravitational force increases.
 The pull of gravity is also related to distance between objects.
As the distance between two objects decreases, the
gradational force increases.
 The direction of Earth’s gravitational pull is toward the
center of Earth.
Projectile Motion
-The curved path an object follows when thrown or propelled near
the surface of the Earth.
-Examples: Throwing a ball, arrows shot from a bow, a leaping
grasshopper, a catapult, and shooting a basket.
 Two Components
o Horizontal Motion – Motion that is parallel to the
ground.
o Vertical Motion - Motion that is perpendicular to the
ground
o Both motions combine to form a curved path.
Friction
-A force that opposes motion between two surfaces that are
touching.
-There is NO SUCH THING as a completely smooth
surface.
-Heat is the most common form of energy that results from
friction. The greater the friction, the greater the heat
generated.
 The Source of Friction
o There are microscopic hills and valleys on the surface
of every object. As the surfaces move back and forth,
the hills and valleys stick to one another.
 What Controls the Amount of Friction
o 1. Roughness of surfaces
 Rougher the surface = more friction
o 2. Force pushing the objects together
 Greater the force = more friction
 Two Types of Friction
o 1. Sliding Friction
 Requires a great amount of force to move the
object. There is a large amount of opposing force.
o 2. Rolling Friction
 Requires much less force to move the object.
There is a small amount of opposing force.
 How Can We Increase or Decrease Friction?
o Decrease Friction
 Lubricants – Substances that are applied to
surfaces to reduce friction between them.
 Examples – Oils, Waxes, Greases, and
Water.
o Increase Friction
 1. Increase the force acting on the object.
 2. Increase the roughness of the surface.
 Examples – Paperweights, Sand on roads,
and Sanding surfaces.