Measuring Motion

Chapter 5

Section 1: Measuring MotionO Objectives:

O Describe the motion of an object by the position of

the object in relation to a reference point

O Identify the two factors that determine speed

O Explain the difference between speed and velocity

O Analyze the relationship between velocity and

acceleration

O Explain how changes in motion can be measured

and represented in a graph

Observing Motion by Using a Reference

PointO How do you know an object is

moving?

O When you watch the motion of an object, you are actually watching the object in relation to another object that appears to stay in place.

O The object that appears to stay in place is a reference point.

Observing Motion by Using a Reference

PointO When an object changes position over

time relative to a reference point, the object is in motion.

O Common Reference Points:O treeO buildingO doorO objects in motion – a bird flying while in a

hot air balloon.

Speed Depends on Distance and Time

O Speed-Distance traveled by an object divided by the time taken to travel that distance.O Ex. A balloon traveled 50 m in 10 s.

What is the balloons speed?

 O The SI unit for speed is meters per

second (m/s).

Determining Average Speed

O Most of the time, objects do not travel at a constant speed – you probably do not walk at a constant speed from one class to the next.

O Average speed= total distance Total time

Section 1: Measuring Motion

SPEED = distance time

Section 1: Measuring Motion

1. Write the formula

2. Identify variables

3. Plug in numbers

4. Solve and circle

A snail moves 1mm in 5 seconds. What is the speed in METERS/SECOND?

Section 1: Measuring Motion

1. Write the formula

2. Identify variables

3. Plug in numbers

4. Solve and circle

A car is driving along at 25km/hr. How long will it take me to drive 400km?

Section 1: Measuring Motion

1. Write the formula

2. Identify variables

3. Plug in numbers

4. Solve and circle

A bee is flying by at a speed of 1.5 m/s. How far will the bee travel in one minute?

Average SpeedO Examples:

O An athlete swims a distance from one end of a 50m pool to the other end in a time of 25s. What is the athletes average speed?

O Jake jogs to a store 72 m away in a time of 36s. What is Jake’s average speed?

Recognizing Speed on a Graph

Distance (m)

Time (s)

Distance vs. Time

What is the average speed?

Section 1: Measuring Motion

Speed ≠ Velocity

O _____________ is the speed of an object in a given direction

OVelocity MUST include a ___________!

Speed

direction

Section 1: Measuring Motion

OWhich is it? Speed or Velocity?

O 7 miles per hour

O 2.345 inches/minute east

O 18.7 ft/s down

O 7.77 km/day

Velocity: Direction Matters

OImagine that 2 students leave the same classroom at the same time. They both walk at 10km/hr for 5 min, 12 km/hr for 8 min, and 5 km/hr for 10 min. Why don’t they end up at the same place?

VelocityO Velocity- Speed of an object in a particular direction

O You must always include a direction in your answer

O Constant velocity only occurs if neither speed nor direction changes.

O Velocities can be added or subtracted together to give a resultant velocity. O Add velocities that are in the same direction.

O Ex: Walking on a bus as the bus is moving forward.

O Subtract velocities that are in opposite directions.O Ex. Walking to the back of the bus while it is moving

forward

Finding Resultant Velocities

O Same direction

O Resultant Velocity =

15 km/min east

2 km/min east

Finding Resultant Velocities

O Opposite directions

O Resultant Velocity=

15 km/min east

2 km/min west

AccelerationOAcceleration is the rate at which

velocity changes.O Velocity changes if:

O speed changesO direction changesO both change

O Increase in velocity = positive acceleration

O Decrease in velocity = negative acceleration (deceleration)

AccelerationOCalculating Average Acceleration

OAvg. Acceleration = final velocity-starting velocity

time it takes for velocity to change

OA = vf – vi = A = Δv t tO Velocity is expressed in m/s and time is

expressed in s.OTherefore, acceleration is expressed in

meters per second per second (m/s/s) or m/s2

Calculating Acceleration

1 m/s 2 m/s 3 m/s 4 m/s 5 m/s

0:01 0:02 0:03 0:04 0:05

Equation:

Calculation:

Acceleration ProblemsO A skater goes from a standstill to a speed of 6.7 m/s

in 12 seconds.  What is the acceleration of the skater?

O A plane passes point A at a velocity of 240 m/s north. Forty seconds later, it passes point B at a velocity of 260 m/s north. What is the planes average acceleration?

Centripetal Acceleration

OCentripetal Acceleration- the acceleration that occurs in a circular motion. OEx: Ferris wheel, the moon’s orbit

Section 2: What is a Force?OObjectivesObjectives

O Describe forces, and explain how forces

act on objects.

O How do you determine the net force when

more than one force is acting on an

object?

O Compare balanced and unbalanced forces.

O Describe how unbalanced forces cause

changes in motion.

5.2 ForcesO A force is a push or pull

that causes a resting object to move, or it can accelerate a moving object by changing its speed or direction.

O Unit = Newton (N)O 1 kg m/s2

O Combination of all forces acting on an object is called the net force.

Types of ForcesO Balanced Force

O When the forces on an object are balanced, the net force on an object is zero and there is no change in the object’s motion.

O Unbalanced forceO When the forces

on an object are unbalanced, there is a net force and the object accelerates.

Calculating Net ForceOForces in the same

directionOAdd forces to determine net

force

Calculating Net ForceOForces in opposite

directionsOSubtract forces to determine

net force

ForcesO Affect of force on moving object

O Change in speed or directionO Ex. When soccer ball is passed to

another player and is kicked

O Affect of force on stationary objectO Cause a nonmoving object to start

moving.O Ex. Stationary soccer ball kicked

Section 3: A Force that Opposes Motion

Objectives:ODescribe friction

OExplain why friction occurs

OList the two types of friction, and give examples of each

FrictionOFriction - A force that opposes

motion between two surfaces that are in contact.OFriction causes moving objects to

slow down and eventually stop.OWhat causes friction?

OWhen the hills and valleys of one surface come in contact with the hills and valleys of the other surface

FrictionORough surfaces have more hills

and valleys than smooth surfaces do.OThe rougher the surface, the greater

the friction.OEx. A soccer ball rolling on grass,

verses a hockey puck on ice.OThe greater the weight

of the object, the greater the friction will be

Types of FrictionO Static Friction is the

friction force that acts on a stationary object. It opposes the applied force.

O Kinetic Friction is the force that opposes the direction of motion of an object as it slides over a surface. O Solid over solid

Types of Kinetic FrictionO Rolling Friction is

the force of friction felt on rolling objects O Example: wheels

and balls.O Fluid friction

opposes the motion of a moving object in a fluid (gas or liquid)O Example: Air

resistance

Section 3: A Force that Opposes Motion

Is friction helpful or harmful?

OHelpfulOMovement of

tiresOWalkingOEraser

OHarmfulOMachine

wearOErosion OBurns/

blisters

Decreasing FrictionOLubricants- Substances that

are applied to surfaces to reduce the friction between the surfaces.OEx: motor oil, wax, greaseOLubricants are usually liquids,

but can be solids or gases. (Air in air-hockey)

Decreasing FrictionOSwitching from sliding friction to

rolling friction. OEX. Ball bearings are placed between

the wheels and axles of skates

OMake surfaces that rub against each other smoother.

GravityOGravity is a force of attraction

that acts between objects with mass.OObjects close to Earth accelerate

at 9.8m/s2.OAll objects fall at the same rate.

GravityOIf gravity affects all objects with

mass – why aren’t all objects stuck together?OMasses of most objects are too small,

you can’t detect this forceOEarth has a huge mass, so the

gravitational force of Earth is large. It pulls everything toward the center of the Earth. – Dropped objects fall to the floor.

OAll objects in the universe attract each other through gravitational force.

OThe size of the force depends on the masses of the objects and the distance between the objects.OGreater the mass the greater the force.OSmaller the distance, the greater the

force.

Law of Universal Gravitation

Mass vs. WeightO Mass is the amount

of matter in an object.

O Unit = kilogramO Measuring device is

a balance.

O Weight is a measure of gravitational force on an object.

O Unit – NewtonO Measuring device is a

spring scale. O Weight = mass x

gravity N = (kg) x

(m/s2)