1. MOTION AND FORCES Newton's laws predict the motion of most objects. As a basis for understanding this concept: a. Students know how to solve problems that involve constant speed and average speed.
Motion Graphs: b. Students know that when forces are balanced, no acceleration occurs; thus an object continues to move at a constant speed or stays at rest (Newton's first law).
Distance Formula Use this formula to relate distance, speed, and time for an object that is moving at constant speed or at an average speed.
π₯ = π£ π‘ distance velocity time [ m ] [ m/s ] [ s ]
A really easy way to visual velocity is by creating a position vs. time graph. The motion will be represented by a line on the graph, with the slope of the line being equal to the velocity.
π£ =βπ₯βπ‘
=πππ πππ’π
= π₯ β π₯!π‘ β π‘!
steady
speeding up
fast, steady
rest
backwards
Distan
ce (m
)
Time (s)
acceleration (π) unit: meter per second per second [m/sΒ²]
a measure of the rate of change of velocity.
position (π₯) unit: meter [m] a measure of the location of an object.
velocity (π£) unit: meter per second [m/s] a measure of the rate of change of position. velocity is an objectβs speed and its direction. time (π‘) unit: second [s] a measure of how much time has passed.
Change of Speed Formula Use this formula to find the change of speed
due to an acceleration.
βπ£ = π π‘ velocity change acceleration time
[ m/s ] [ m/sΒ² ] [ s ]
The Law of Inertia: An object at rest will tend to stay at rest. An object in motion will tend to stay in motion. In other wordsβ¦
Force causes acceleration!
The velocity of an object cannot change unless there is an unbalanced force acting on it. If the net force on an object is zero (balanced) then the object is either at rest or moving at constant speed in a straight line path.
Physics A Final Exam β Study Guide
Net force is the sum of all the forces. β4N
net πΉ = (β4) + (6)
6N 4N β6N
net πΉ = 2N
net πΉ = (4) + (β6)
net πΉ = β2N
c. Students know how to apply the law π =ππ to solve one-Ββdimensional motion problems that involve constant forces (Newton's second law).
EXAMPLE: How much force is required to accelerate a 2-Ββkg mass at 6 m/sΒ²?
EXAMPLE: A 20 newton force pushes a 5-Ββkg mass to the right. If the force of friction is 5 N, what is the acceleration of the mass?
EXAMPLE: How much force is required to accelerate a 4-Ββkg mass at 6 m/sΒ²?
d. Students know that when one object exerts a force on a second object, the second object always exerts a force of equal magnitude and in the opposite direction (Newton's third law). Ex 1. A baseball player hits a baseball. ACTION: the bat pushes the baseball forward. REACTION: the baseball pushes the bat backward. Ex 2. A cannonball is fired out of a cannon. ACTION: the cannon pushes the ball forward. REACTION: the ball pushes the cannon backward.
πΉ = π π net force mass acceleration
[N] [kg] [m/sΒ²]
Newtonβs 2nd Law Acceleration is directly proportional to net force and inversely proportional to mass.
βThe force on object B from object A is equal to the force on object A from object B, but in the opposite direction.β
πΉ!" = βπΉ!" Since a force is part of an interaction between two objects, forces always come in pairs!
ACTION and REACTION
Since the two forces are equal, the object with less mass will
experience a greater acceleration.
This is the reason why the baseball goes flying, but the bat
appears to be unaffected.
action reaction
action
reaction
Since the two forces are equal, the object with less mass will
experience a greater acceleration.
This is the reason why the
cannonball goes flying, but the cannon barely moves at all.
mass (π) unit: kilogram [kg] a measure of the amount of material in an object.
force (πΉ) unit: newton [N] a push or a pull. part of an interaction between two objects.
πΉ = ππ πΉ = (2)(6) πΉ = 12 N 5 kg
20 N 5 N
πΉ = ππ net πΉ = (20) + (β5)
net πΉ = 15 N (15) = (5)π
π = 3 m/sΒ²
πΉ = ππ πΉ = (4)(6) πΉ = 24 N
DIRECT: double the mass requires double
the force!
e. Students know the relationship between the universal law of gravitation and the effect of gravity on an object at the surface of Earth. f. Students know applying a force to an object perpendicular to the direction of its motion causes the object to change direction but not speed (e.g., Earthβs gravitational force causes a satellite in a circular orbit to change direction but not speed).
Suppose an object is moving to the right and a force πΉ is applied on the object at some angle. g. Students know circular motion requires the application of a constant force directed toward the center of the circle.
πΉ =πΊπππ!
The Law of Universal Gravitation: There exists an attractive force between any two masses. The strength of this force is directly proportional to mass and inversely proportional to the square of the distance between the two masses.
where π is the first mass, π is the second mass, π is the distance between them, and πΊ = 6.67Γ10!!! NmΒ²/kgΒ²
What is the force of gravity on an object at the surface of Earth?
Weight (π€) unit: newton [N] A measure of the gravitational force acting between two objects.
πΉ =πΊπππ!
π
π
π Plug-Ββin for mass
and radius of Earth
πΉ =(6.67Γ10β11)(5.98Γ10β24)π
(6.38Γ106)!
πΉ = 9.8 π
π€ = ππ
at surface of Earth: π = 9.8 m/sΒ²
βacceleration of gravityβ
Mass vs. Weight: Weight is not equal to mass! Mass is a measure of the amount of material in an object, whereas weight is a measure of the gravitational force on an object. In places with less gravity, the weight of an object will decrease, but its mass will still be the same.
velocity
πΉ πΉοΏ½
πΉ|| The component parallel to the direction of motion
The force πΉ can be broken down into two components (parts):
The component perpendicular to the direction of motion
πΉοΏ½ πΉ||
changes the speed of the object
changes the direction of the object
πΉ
π£ According to Newton's 1st law, if an object is moving in a circle, it must have a force acting on it (since it is not moving along a straight line.) This force changes the direction of the object without changing its speed. Therefore, the force must be perpendicular to the velocity. In other words, it points to the center of the circle. This is called aβ¦ Centripetal Force!
circular motion stops when the force is removed
a centripetal force points to the center
of the circle.
2. CONSERVATION OF ENERGY AND MOMENTUM The laws of conservation of energy and momentum provide a way to predict and describe the movement of objects. As a basis for understanding this concept: a. Students know how to calculate kinetic energy by using the formula π¬ = π
πππΒ².
Kinetic energy is the energy of motion. An object at rest has no kinetic energy. The kinetic energy of an object equals the work that was needed to create the observed motion of the object. b. Students know how to calculate changes in gravitational potential energy near Earth by using the formula βπ¬ =ππβπ. When a force is applied to accelerate an object, we say the work done is transformed into kinetic energy. But what if a force is applied to lift up an object? Instead of changing speed, this force changes height. The work done to lift an object is transformed into gravitational potential energy.
EXAMPLE: A 50kg downhill skier is moving at 20 m/s when she is 40m from the bottom of the hill.
Work (π) unit: joule [J]
The transfer or transformation of energy by means of a force πΉ across a distance π.
Kinetic Energy (πΎπΈ) unit: joule [J]
The mechanical energy of an object associated with its movement (SPEED).
πΎπΈ =12ππ£!
Work Formula Use this formula to calculate the
work done on an object by a force πΉ acting through a displacement π.
π = πΉ π work force displacement [ J ] [ N ] [ m ]
(gains energy)
Parallel
π
πΉ
positive work
Anti-ΒβParallel
π
πΉ
negative work
Perpendicular
π
πΉ
zero work (loses energy)
(constant energy)
Kinetic Energy Formula The work needed to accelerate a
mass π to a speed π£ is
ππΈ = ππβ
Potential Energy Formula the work done in lifting an object of
weight ππ through a vertical distance β
Potential Energy (ππΈ) unit: joule [J]
The mechanical energy of an object associated with its position (HEIGHT).
(a) What is her kinetic energy? (b) What is her potential energy?
πΎπΈ =12ππ£
!
πΎπΈ =12 (50)(20)(20)
πΎπΈ = (25)(400)
πΎπΈ = 10,000 J
ππΈ = ππβ
ππΈ = (50)(10)(40)
ππΈ = (500)(40)
ππΈ = 20,000 J
20 m/s
40 m
c. Students know how to solve problems involving conservation of energy in simple systems, such as falling objects. As an object falls, it loses potential energy (height) and gains kinetic energy (speed). The potential energy that is lost is transformed into an equal amount of kinetic energy. Therefore, the amount of mechanical energy (πΎπΈ + ππΈ) is unchanged. For a free-Ββfalling body, mechanical energy is conserved. EXAMPLE A 2.5-Ββkg brick falls to the ground from a 3-Ββm-Ββhigh roof. What is the approximate kinetic energy of the brick just before it touches the ground? d. Students know how to calculate momentum as the product ππ. EXAMPLE What is the momentum of a 2,000kg car moving at 30m/s? e. Students know momentum is a separately conserved quantity different from energy. The Law of Conservation of Momentum: βIn the absence of an external force, the momentum of a system remains unchanged.β
rising
falling
πΎπΈ (speed)
ππΈ (height)
πΎπΈ + ππΈ = πΎπΈ! + ππΈ! 0 0
ππβ = πΎπΈβ² (2.5)(10)(3) = πΎπΈβ²
(25)(3) = πΎπΈβ²
πΎπΈ! = 75 J
3 m
at rest
on ground
Conservation of Energy The kinetic energy of the brick at the ground will be equal to the potential energy it had
when it was above the ground.
πΎπΈ = 0
ππΈ = 0
Momentum (π) unit: [kg m/s]
A measure of how difficult it is to stop a body.
π = π π£ momentum mass velocity [kg m/s] [kg] [m/s]
Momentum Formula Use this formula to calculate the
momentum of an object
π = ππ£
π = (2000)(30)
π = 60,000 kg m/s
Remember: Momentum is mass AND speed. An object can have a lot of momentum if it has a large mass, a large speed, or BOTH!
no external force momentum is constant
wall provides an external force momentum changes!
πΉ
π = π! π! + π! = π!! + π!!
π!π£! + π!π£! = π!π£!! + π!π£!!
f. Students know an unbalanced force on an object produces a change in its momentum.
According to Newton's 1st Law, if there is a net force on an object, its velocity will change. Since momentum depends on velocity, this means a net force causes a change in momentum. g. Students know how to solve problems involving elastic and inelastic collisions in one dimension by using the principles of conservation of momentum and energy.
A collision results when two object collide (crash) with each other. Momentum is always conserved in collisions. Kinetic energy is only conserved in a special type of collision called an elastic collision. All other collisions are said to be inelastic because some kinetic energy is lost and transformed into heat.
Elastic Collision Inelastic Collision EXAMPLE: An 8kg block crashes into a 2kg block moving to the left at 10m/s. They stick together and move off to the right at 6m/s. What was the velocity of the 8kg block before the collision?
Motion resulting from a constant force πΉ acting on an object for a time βπ‘ causes a change in momentum of πΉβπ‘. This change in momentum is called an impulse.
βπ = πΉβπ‘ πΉ =βπβπ‘
The relationship between force and time is inverse.
πΉ β1βπ‘
Changing momentum in MORE time requires LESS force.
Safety Application The airbag in a car can save your
lift during a car crash. The "cushion" of the airbag increases the time it takes for you to stop. This decreases the force needed to bring your body to a stop.
Changing momentum in LESS time requires MORE force.
Objects are moving at the same speed. Kinetic energy was conserved!
Objects are moving slower than before. Kinetic energy was lost!
π! + π! = π! π! + (2)(10) = (8+ 2)(6)
π! + 20 = 60
π! = 40 kgm/s
β20 β20
π = ππ£ (40) = (8)π£
π£ = 5 m/s
8
8
First we use conservation to find the unknown
momentum.
Now apply π = ππ£ to the object in question.