Measuring Motion

Chapter 5

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

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?

Velocity: Direction Matters

O Imagine 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

AccelerationO Acceleration 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)

AccelerationO Calculating Average Acceleration

O Avg Accl = final velocity-starting velocity time it takes for velocity to

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

expressed in s.O Therefore, 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

O Centripetal Acceleration- the acceleration that occurs in a circular motion. O Ex: ferris wheel, the moon’s orbit

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 ForceO Forces in the same direction

O Add forces to determine net force

Calculating Net ForceO Forces in opposite directions

O Subtract 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 on force on stationary objectO Cause a nonmoving object to start

moving.O Ex. Stationary soccer ball kicked

5.3 FrictionO Friction - A force that opposes

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

slow down and eventually stop.O What causes friction?

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

FrictionO Rough surfaces have more hills and

valleys than smooth surfaces do.O The rougher the surface, the greater

the friction.O Ex. A soccer ball rolling on grass,

verses a hockey puck on ice.O The 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 Friction

O 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

Is friction helpful or harmful?

O HelpfulO Movement of

tiresO WalkingO Eraser

O HarmfulO Machine wearO Erosion O Burns/blisters

Decreasing FrictionO Lubricants- Substances that are

applied to surfaces to reduce the friction between the surfaces.O Ex: motor oil, wax, greaseO Lubricants are usually liquids, but can

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

Decreasing FrictionO Switching from sliding friction to

rolling friction. O EX. Ball bearings are placed between

the wheels and axles of skates

O Make surfaces that rub against each other smoother.

GravityO Gravity is a force of attraction that

acts between objects with mass.O Objects close to Earth accelerate at

9.8m/s2.O All objects fall at the same rate.

GravityO If gravity affects all objects with

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

you can’t detect this forceO Earth 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.

O All objects in the universe attract each other through gravitational force.

O The size of the force depends on the masses of the objects and the distance between the objects.O Greater the mass the greater the force.O Smaller 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)