Physics Unit
Review Summary
Speed, Velocity, Acceleration
Speed is a measure of how fast an object is moving. Velocity is a measure of how fast an object is
traveling in a certain direction. Both speed and velocity include the distance traveled compared to the
amount of time taken to cover this distance. Units are always the distance unit divided by the time unit
(ex: km/sec, mile/hr, km/hr, etc.)
Speed = Distance
Velocity = Distance in a specific direction
Time
Time
Acceleration is a change in velocity over time. Acceleration refers to an increase in speed, a decrease in
speed (deceleration or negative acceleration) or a change in direction. Acceleration of an object moving
in a straight line can be calculated using the following formula: Units are always the distance unit divided
by the time unit squared (ex: km/sec2, miles/hr2, km/hr2, etc.)
Acceleration =
Final velocity – Initial velocity
or
Change in velocity
Time
Time
Speed is graphed by showing distance vs. time. Acceleration is graphed by showing speed vs. time.
Forces
Forces are pushes and pulls acting on an object. Forces are shown using vectors (arrows) which show the
direction and strength of the force. Unbalanced forces cause motion in the direction of the largest force.
There are two forces that are acting on every object, gravity (which pulls downward toward the Earth) and
the normal force (that pushes upward against gravity). Gravity depends on two factors (1) the mass of
the objects, and (2) the distance between the objects. Acceleration due to gravity is 9.8 m/s2 regardless of
the mass of the object (which is why all things of similar shapes drop at the same time). Moving objects
are also affected by the force of friction (a force caused by objects rubbing against each other and acts in
the direction opposing motion), which slows motion. The strength of friction depends on the type of
surfaces and how hard they push together. There are 4 types of friction:
(1) Rolling friction;
(3) Static Friction; and
(2) Sliding friction;
(4) Fluid Friction (which includes air resistance).
Newton’s Three Laws of Motion
Newton’s First Law – The Law of Inertia: An object at rest will stay at rest and an object in motion will
stay in motion, unless acted on by an outside force. Remember that inertia is the resistance to change in
motion (like the way you continue to move forward when mom hits the brakes really hard and the seatbelt
has to stop your motion).
Newton’s Second Law – The Law of Acceleration: An object that has an unbalanced force acting on it
will accelerate in the direction of that force. Acceleration depends upon the object’s mass and the net
force acting on it.
Newton’s Third Law: For every action, there is an opposite and equal reaction. Picture the Newton’s
Cradle or a rocket blasting off to recall this law – or those balloon racers that we did.
Physics Unit
Review Summary
Work, Power and Simple Machines
For work to be done, an object must move some distance in the direction of a force put on it (like books
being lifted upward onto a shelf). Power is the amount of work that is done per unit of time. They can be
calculated using the following formulas:
Work = Force x Distance
Power = Work
Time
The unit for work is the joule (J) which is equal to a Newton x meter. The unit for power is the watt (W)
which is equal to a joule divided by a second.
Simple machines make work easier by changing the amount of force, the direction of the force or the
distance over which the force is exerted. Simple machines can be combined into a compound machine.
There are 6 types of simple machines:
1. Inclined plane (a ramp which increases the distance over which force is exerted);
2. Wedge (ex: a knife which changes the direction of the force);
3. Screw (which increases the distance over which force is exerted);
4. Pulley (which increases the distance and changes the direction of the force);
5. Lever (which changes the direction of the force); and
6. Wheel & Axle (which increases the distance of the force).
Energy and Heat
Energy produces work. Some energy is kinetic (related to motion) and some is potential (stored energy
caused by position or shape of the object). An object has more potential energy when it is higher up off
of the ground. An object that is moving faster has more kinetic energy than a slow moving object. These
can be calculated using the following formulas:
Kinetic Energy = weight x height
Potential Energy = ½ mass x velocity2
Energy comes in 6 forms and can change (or transform) from one form to another.
1.
Mechanical energy – total potential and kinetic energy of a moving object;
2.
Thermal energy – heat energy;
3.
Electrical energy – energy from the flow of electric charges;
4.
Electromagnetic energy – energy that moves in waves;
5.
Nuclear energy – the energy that is contained in the atom;
6.
Chemical energy – energy stored in the bonds between atoms.
Heat is the transfer of thermal energy. Heat is always transferred from a warmer area/object to a cooler
area/object. Heat can be transferred in three ways:
 Conduction – heat moving from a warmer object in direct contact with (touching) another object (like
the way a spoon gets hot when you leave it in a pot of soup on the stove);
 Convection – heat moves through liquids or gases in an upward motion when it is warmer and falling
back down when it cools forming a convection current (like the one that moves Earth’s plates);
 Radiation – heat moves in waves (like heat from the sun which is radiated to Earth).