Motion and Force

Frame of ReferenceMotion of an object in relation to a fixed body

or place.To describe motion accurately and

completelyTHE MOST COMMON FRAME OF

REFERENCE: The Earth

Relative MotionIs the movement in relation to a frame of

referenceExample:

A train going past a platform (see movement)

People riding the train feel no movement

Measuring DisplacementDisplacement—the

direction from the starting point and the length of a straight line from the starting point to the ending pointInvolves distance

and direction

Combining DisplacementVector—is a quantity that has magnitude and

directionAdd displacements using vector addition

If vectors are in the same directions, add them Ex:

10meters 5 meters = 15 meters

If vectors are in the opposite direction, subtract themEx:

20 meters 15 meters = 5 meters

Resultant vector—the vector sum of two or more vectors

SpeedRate of Motion:

Change of position in a given amount of time.

Formula:Speed =

Distance/timeUnits: km/hours, m/s

SpeedAverage Speed

Is computed for the entire duration of a trip

Instantaneous SpeedIs the rate at

which an object is moving at a given moment of time

Speed right now

Speed Practice Problems1. How long does it take a bird to fly 300km if

it travels at a speed of 50 km/hr? 2. Alex rode his bicycle 60 kilometers in 4

hours. How fast was he going?3. Joe can pitch a baseball 32 m/s and the ball

crosses the plate in 1.5 seconds. How far away is the plate?

4. Kayla runs 90 meters with a speed of 15 m/s. How long did it take?

VelocityVelocity= Speed in

a given direction.Velocity is a vectorYou combine

velocities the same way you combine displacementSame direction, add

themOpposite direction,

subtract them

VelocityFormula

V=Δd/ΔtV=velocityd=distancet=time

Units: m/s

Graphs of Speed and VelocityConstant Speed

Not Moving

Constant speed and then not moving

AccelerationRate of change in

velocity. Acceleration is a vector Formula: A = vf – vi or Δv

t tUnit: m/s2

Variables need to solve acceleration problems

2 Velocities Time

DecelerationNegative AccelerationSolve it just like the

acceleration problem but the answer is always

NEGATIVE

Acceleration/ Deceleration Problems1. A car has starts from rest and goes to 20 m/s

in 4 seconds. What is the car’s acceleration?2. A runner starts to run and reaches a velocity

of 100 m/s in 25 seconds? What is the car’s acceleration?

3. A train slows down from 200 m/s to a stop in 50 seconds. What is the car’s deceleration?

4. A train has an acceleration of 40 m/s2 during a period of 15 seconds. What is the train’s velocity?

Graphs of Acceleration(Speed verse Time)

Acceleration

Deceleration

Constant Acceleration

Graphs of Acceleration(distance verse time)

Acceleration Deceleration

Constant Speed

Centripetal ForcesCentripetal Force is

a force that pulls an object towards the center.

Center Seeking

Constant Speed and Circular Motion

An object moving in a circular motion may have constant Speed even though it is accelerating.

Why?

Answer: That object needs to change

direction.

3 ways objects accelerate Change in Speed only Change in Direction only Change in Speed and Direction

Momentum Momentum is the

product of the mass times the velocity.

The heavier an object, the harder it is to stop its movement.

Formula: Momentum = m x v

ForcesForce is a push or

pull on an object.

Can cause a resting object to move or it can accelerate a moving object by changing the object’s speed or directions

Unit for force:N or Newton.

Balanced forces:Where there are two

forces that counteract each other and result in no movement (EQUAL)

Unbalanced forces: One force is greater

than the other force. (UNEQUAL)

FrictionA force that resist

motion and can cause heat

Lubricant help reduce friction

Types of frictionStaticSlidingRollingFluid

Types of FrictionStatic—is the friction force that acts on objects

that are not movingSliding—is the force that opposes the direction

of motion of an object as it slides over a surfaceRolling—the friction force that acts on rolling

objectsFluid—opposed the motion of an object through a

fluidFluids= water and mixture of gasesFluid friction action on an object moving through

the air is known as air resistance

Newton’s Laws of Motion1st Law: Objects at rest remain at rest, or

objects in motion remain in motion unless acted upon by a force.

2nd Law: The acceleration of a body depends on the ratio of the acting force to the mass of the body.

3rd Law: for every action there is an equal and opposite reaction.

1st Law of Motion (Law of Inertia)Inertia: force that is resistant to the direction of the motion

Unbalanced forces

Examples: Inertia belts (seat belts)

2nd Law of MotionConcept:

Acceleration Unbalanced forces

Examples: hitting a golf ball gently vs. hard.

Answer = ___ newtons

2nd Law of Motion Formula:

Force=mass x acceleration F=ma

1. How much force is needed to accelerate a 500.0 kg car at a rate of 4.000 m/s/s?

2. A 100 N causes an object to accelerate at 2 m/s/s. What is the mass of the object?

3. A 1.5 kg ball is kicked with a force of 450 N. What acceleration did the ball receive?

3rd Law of MotionAction/ Reaction

Concept: Action/Reaction of objects

Balanced forcesExamples:

Stationary objects, rockets being launched

Gravity and Free Falling Objects

Gravity: is a force that acts between two masses

Acts downward towards the center of the Earth

All objects fall at a rate of 9.8 m/s/s

Galileo did an experiment at the leaning tower of Pisa in Italy with bowling balls. Both balls fell at the same rate.

Galileo's experiment at the Tower of Pisa

Projectile MotionThe motion of a falling object after it is given an initial forward velocity

Air ResistanceForce that slows

down falling objects due to the atmosphere and surface area of the object.

Newton’s Law of Universal GravitationShows that objects

are attracted to one another by their mass and their distance away from the object.

Feather vs. RockEarth: feather

would float down (air resistance) while the rock would drop at 9.8 m/s/s

Moon: feather and rock would drop at same rate due to no atmosphere and air resistance.

What about a vacuum?Both fall at the same rate due to no air being in the jar.

Gravity and WeightWeight—is the force

of gravity acting on an object

Expressed as Fg

FormulaWeight=mass x

acceleration due to gravity (g)

Fg=mg

g=9.8 m/s/s