Chapter 2Motion and Force

Motion

How do we know an object is in motion?? Motion: when an object changes position

over time relative to a reference point.

Reference Point: the object that appears to stay in one place as another object moves. Trees and building are reference points that we

use on a daily basis.

Distance vs. Displacement

Distance is how far an object has moved. Displacement is the distance and direction

of an object’s change in position from the starting point.

Distance vs. Displacement

What is the distance you would have to travel to get to the factory?

What would your displacement be?

200m

Displacement is a vector quantity and can be defined by using distance concept. It can be defined as distance between the initial point and final point of an object. It must be the shortest interval connecting the initial and final points, that is a straight line.

Speed

Speed: the distance traveled by an object divided by the time taken to travel that distance.

What are some units of speed you use?m/h, ft/s

What are some SI units of speed?km/h

Average Speed

Generally objects do not travel at a constant speed. We have a formula for calculating average speed.

Average speed = total distance

total time

You Try…

Don’t forget FSAU!!! This weekend I traveled 1,000 kilometers

in 2 hours. What was my average speed?Average speed=total distance/total timeAverage speed=1,000 kilometers/2 hoursAverage speed=500 km/hr

A question… Sarah leaves Northwestern at 3:00 and

travels 10 km in 10 minutes. Jessica leaves at 3:00 and travels 10 km in 5 minutes. Both girls stop moving after 10 km. After 15 minutes are both girls in the same location??No!! Why not?Because they left the school an traveled in

different directions.

Velocity

This is the speed of an object in a particular direction. Velocity MUST include a speed and

DIRECTION. You try…Andrew left northwestern going

south. Andrew traveled 50 km in 25 minutes, what is Andrew’s velocity?2 km/min SOUTH

Is velocity constant?

Velocity changes any time the speed or direction of an object changes. Therefore velocity is only constant in a straight line at a constant speed.

We speak about velocity everyday…It told us when Ida would arrive!

Air traffic control also involves velocity!

Combined Velocity

Pretend you are sitting in an airplane traveling 500 mi/hr east. During the flight you decide to use the restroom. You walk to the restroom at a speed of 0.2 mi/hr. While walking to the restroom you have what is called a resultant velocity.You find this resultant velocity by combining

the two velocities.

How to calculate resultant velocity

First, we must decide whether the velocities are in the same direction. If they are you ADD the two velocities to find

resultant velocity. SO, if in your walk to the restroom you are walking

in the same direction as the plane what is your resultant velocity?

Resultant velocity=500 mi/hr east + 0.2 mi/hr east Resultant velocity=500.2 mi/hr east

Resultant Velocity Continued

What if the restroom is in the back of the plane and causes you to walk west?Then you must subtract your walking velocity

from the planes velocity Resultant velocity=500 mi/hr east – 0.2 mi/hr west Resultant velocity=499.8 mi/hr east

Acceleration This is the rate at which velocity changes.

Something that is accelerating is . . .

. . . . speeding up . . . .

Acceleration is . . . . . . . . . . slowing down . .

. . . or . . .

. . . . changing direction !

Formula for determining Acceleration is . . .

Acceleration = Final velocity – Original velocity Time

Hey . . . . Let’s calculate acceleration !!!

At point A, a runner is jogging at 3 m/s. Forty secondslater, at point B – on a hill – the jogger’s velocity is only 1 m/s. What is the jogger’s acceleration from point A to point B?

Negative acceleration (decrease in speed) is called deceleration.

You’re right! Answer: -0.05 m/s/s

Acceleration = 1 m/s – 3 m/s 40 secs = -2 m/s

40 secs= 0.05 m/s/s

Reason for the m/s/s . . . this means that for every second, the jogger’s decelerates 0.05 m/s.

Deceleration example . . . . .

Another deceleration example . . .

. . . .roller coaster climbing a hill !

Other deceleration examples:

“Pathfinder’s” landing on Mars

parachutes . . . . .

Circular motion . . . . .

. . . . (changing direction) involves the continuous changing of the velocity. This occurs because the direction is continuously changing.

An object in circular motion is accelerating even though its speed may be constant.

Examples include . . . .

Ferris Wheel

. . . and . .

. . . the bicycle !

Which ends our notes covering . . .

“Motion” !

Motion Graphing

Motion can be graphed in a way that allows us to say something about the speed and direction of an object.

In addition, speed and acceleration can be calculated by interpreting these graphs.

Interpret The Graph Below:

Interpret The Graph Below:

The graph shows an object which is not moving (at rest).

The distance stays the same as time goes by because it is not moving.

Interpret The Graph Below:

Interpret The Graph Below:

The graph showsthat the objects distance increases as time passes.The object is moving and so it has velocity.

The straight line shows it is a constant (not changing).

Interpret The Graph Below:

Interpret The Graph Below:

Just like the previous graph, this graph shows an object moving with constant velocity

Interpret The Graph Below:

Interpret The Graph Below:

The curve in the graph shows that the objects velocity is changing as time passes.

This is acceleration.

Interpret The Graph Below:

Interpret The Graph Below:

In the first part of the graph the object is moving with constant velocity.

In the second part of the graph the object is at rest (not moving).

In the third part the object is again moving with constant velocity.

Interpret The Graph Below:

Interpret The Graph Below:

Interpret The Graph Below:

The graph showsthat the objects velocity is increasing as time passes – it is accelerating.

The straight line shows that it is constant acceleration.

Forces all around us

The Nature of Force

• A force gives energy to an object, sometimes causing it to start moving, stop moving, or change direction.

• A force can be a push or pull.

• Newton: the unit used to express force (I pushed the chair with 50 N of force)

• Net Force: the combination of all forces acting on an object

Forces Acting in the Same Direction

When multiple forces act upon an object in the same direction, you ADD the forces to find the net force acting on the object.

Forces Acting in Opposite Directions

When multiple forces act upon an object in the opposite directions, you SUBTRACT the forces to find the net force acting on the object.

Balanced Forces

Balanced forces cause NO CHANGE in motion

Forces that are in opposite directions and equal in size are called balanced forces.

When forces are balanced, there is no change in motion; there would be no overall force (net force) acting on the object.

Balanced Forces

Unbalanced Forces

Unbalanced forces cause a CHANGE in motion

Same direction When two forces are

acting in the same direction, they are added together.

The size of the arrow is relative to the strength of the force.

•This force is unbalanced because there is a change in motion

Unbalanced ForcesSame Direction

They each provide an “applied force” in the same direction. Meanwhile, gravitational force and frictional forces are working against them.

Since Frank and John’s combined forces are greater, then the piano moves!

The net force is “unbalanced”

Frank and John are pushing the piano on its wheels to move it.

Unbalanced Forces-Opposite directions

When two forces act in opposite directions, they combine by subtraction,

If one force is greater

than the other force, the object will move in the direction of the greater force (tug-of-war).

This force is unbalanced

because there is a change in motion

Unbalanced Forces In this game of tug of

war, the red dog is losing ground.

The applied force of the red dog is not as great as the applied force of the gray dog.

Net force is unbalanced, and in the direction of the gray dog.

NEWTON’S FIRST LAW

An object at rest remains at rest, and an object in motion remains in motion at a constant speed and in a straight line unless acted upon by an unbalanced force.

PART 1

An object at rest An object at rest will not move until a push or a

pull is exerted on it. Ex. You must pull on a chair to get it to move out from

under a table.

PART 2

An object in motion In order to slow, speed up or change the

direction of an object an unbalanced force must act upon it.

INERTIA The property of an object that resists

changes in its state of rest or motion.

When stationary difficult to push.

When moving difficult to stop.

INERTIA DEPENDS ON MASS.

Which would have more inertia? A child on a swing or an adult on a swing?

When you are in a car that is accelerating, you feel as if you are being pushed back. Your body (actually the inertia of your body) is resisting the increase in speed. When the car comes to a sudden stop, your body wants to continue in a forward motion.

· Sudden Starts - head rests stop the head from being pushed back. (rapid positive acceleration)

· Sudden Stops - seat belts help your body resist the tendency to keep moving. (rapid negative acceleration)

ENERGIZER

A physics teacher walks 4 meters East, 2 meters South, 4 meters West, and finally 2 meters North. What is the distance and displacement for this teacher?