Motion and Forces Notes
1. A force is a push or pull
 Anytime you change the motion of an object you use force.
 You use forces every day.
 Examples: writing, running, shooting basketball, jumping rope, riding
a bicycle
2. Types of force
 3 types of forces are:
1. contact forces- when one object pushes or pulls another object by
touching; ex. writing with a pencil
2. gravity- the force of attraction between 2 masses
3. friction- the force that resists motion between 2 surfaces that are
pressed together
 A variety of forces always change and effect an objects motion
everyday.
3. Size and direction of forces
 Force is a vector.
 In visuals red arrows represent forces & blue boxes represent mass.
 Direction of an arrow shows the direction of the force.
 Length of the arrow shows the size of the force.
 Force has both size and direction.
4. Balanced and unbalanced forces
 Net forces: overall force acting on an object when all the forces are
combined
 If the net force is 0 then the object is balanced
 Unbalanced force is needed to change an object’s motion.
5. Forces on moving objects
 Force is used on a moving object every time it’s moving in the same
direction.
 An object with forces acting on it can be moving at a constant velocity
as long as those forces are balanced.
 Different forces have different effects on objects.
 Ex. to turn your bike, you apply an unbalanced force by leaning to one
side and turning the handle bars
6. Newton’s 1st law relates force and motion
 mid 1600s English scientist Sir Isaac Newton studied the effects of the
forces on objects
 Newton formulated 3 laws of motion.
 Newton’s idea built on other scientist such as Galileo.
7. Galileo’s Thought Experiment
 In the absence of friction the world will continue moving with out
force acting upon it.
 Force is needed to stop an object in motion----friction!
 Galileo had a different view than the Greeks.
 An object at rest has zero velocity.
8. Newton’s First Law
 You will find Newton’s law around you.
 Newton used Galileo’s ideas.
9. Inertia
 It is closely related to mass.
 It is the resistance of an object to a change in the speed or direction of its
motion.
 When you measure the mass of an object, you are also measuring its
inertia.
 Inertia is the reason seat belts are needed in a car!

10. Conflicting Ideas about Motion/Forces
Greeks
Galileo
Newton
Force keeps an object in
Force is needed to change An object at rest stays at
motion.
motion.
rest; an object in motion
stays in motion.
Force causes continuing
Force is not required to
It requires an unbalanced
motion
keep an object moving.
force to change motion.
Friction slows down
motion.
11. Forces change motion.
Newton’s First Law: Objects at rest remain at rest, and objects in motion
remain in motion with the same velocity, unless acted upon by an unbalanced force.
12. Force and mass determine acceleration.
Newton’s Second Law: The acceleration of an object increases with
increased force and decreases with increased mass, and is in the same direction as
the force.
a. same mass, larger force = increased acceleration
b. larger mass, same force = decreased acceleration
13. Forces act in pairs.
Newton’s Third Law: When one object exerts a force on another object, the
second object exerts an equal and opposite force on the first object.
14. Friction is the force that happens when 2 things rub together.
 The effect of friction is to slow things down.
 Friction can be useful (ex. snow tires, walking running track & playing
basketball.)
 Friction can get in the way of movement (ex. sliding across the floor, ice
skating & playing hockey.)
15. Ways to reduce friction:
 Wheels reduce a vehicle’s contact with the ground.
 A smooth surface produces less friction than a rough one (ex. smooth race
car tires.)
 Oil is used to reduce friction in an engine and on a bike’s chain.
16. Fluid friction is contact with water.
 Boats ride higher in the water to avoid this type of friction.
 Jet and water skiers have to overcome friction.
17. Gravity is the force objects exert on each other because of their mass.
 It is a universal force.
 Greater mass results in greater force.
 Greater distance results in smaller force.
 Mass is the amount of matter something contains. It remains the
same no matter where the object is.
 Weight is the effect of gravity on the object. Weight depends on the
force of gravity exerted upon an object.
 Isaac Newton discovered in the 1600s that the force of gravity
depends upon the amount of matter (mass) in bodies and the
distances between the bodies.
 Gravity keeps the planets in orbit.
 Earth’s gravity accelerates objects when they fall.
18. A magnet is a rock, a piece of metal, or other solid that has the property of attracting
iron or steel.






Magnets work because of an invisible force, called magnetism. This force
surrounds a magnet.
The movement of electric charges creates the magnetic force. These are atoms
with a positively charged core and surrounding electrons.
Each atom in an object creates a small magnetic force. The forces combine and
the object behaves as a magnet. These objects need to be made out of iron,
nickel, or cobalt. These materials are called ferromagnetic materials.
A magnet has 2 poles, just like the Earth. It has a north and a south pole. The
magnetic force pushes out from the north pole of the magnet and travels to the
south pole of another magnet. The magnetic force can be felt all around the
magnet. The closer you get to the magnet, the stronger the force will be.
Opposite poles attract (pulls towards) and same poles repel (pushes away).
Magnets are used: to hold notes on a refrigerator, in computers and electric
motors, in medical devices & x-rays, in CD & DVD players.
19. 3 Types of friction:

sliding friction: examples include pencil on paper and walking


rolling friction: examples include pulling a wagon and roller blading
fluid friction (liquids & gases): examples include swimming, skydiving, leaf
falling & rain drops ; also known as air resistance
**Friction affects temperature by causing it to increase. ex. rubbing hand together.
Work and Energy Notes
1. Force is necessary to do work.
2. Work is the use of force to move an object some distance.
3. Reading a page in a book is not doing work but turning the page in a book is
doing work.
4. Moving objects can do work.
5. The gravitational force of Earth does work on water and other natural materials.
6. Work transfers energy.
7. Energy is the ability of a person or an object to do work or to cause a change.
8. Work changes potential and kinetic energy.
9. Potential energy is energy stored in an object because of its position or shape.
10. Kinetic energy is energy that an object has due to its motion.
11. Gravitational potential energy is potential energy that is caused by gravity.
12. Mechanical energy is the energy possessed by an object due to its motion or
position. (potential + kinetic energy)
13. The Law of Conservation of Energy states
 Energy cannot be created or destroyed.
 Energy is transferred or transformed.
 The total amount of energy never changes.
14. In most energy transformations, some of the energy is transformed into heat.
15. Forms of Energy: thermal, chemical, nuclear, electromagnetic, radiant,
mechanical, sound, gravitational, motion
16. Energy in the atmosphere
 Radiation: energy that travels across distances in the form of certain types of
waves
 Conduction: the transfer of heat energy from one substance to another by
direct contact
 Convection: transfer of energy from place to place by the motion of gas or
liquid
 Solar energy is absorbed on Earth’s surface by oceans, landforms, and living
things.
 Solar energy is absorbed in the atmosphere by gas molecules, clouds, and dust
particles.
 It is the energy of the downward movement of cool air and the upward
movement of warm air that causes weather such as clouds, rain, snow,
and storms.