Free Body Diagrams, Speed, Velocity, Acceleration

AKS 8A: Force and Motion

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Speed

Vocabulary

Motion = change in position with respect to time

• Must have a reference point

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Reference Point = object that is used for comparison & it’s assumed to be stationary

Speed

Speed = measure of the distance traveled per unit time…aka how fast something is moving

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Multiple Choice

A train travels 25 km in .5 hrs. How fast is the train going?

• 25 miles/hr
• 5 miles/hr
• 125 km/hr
• 50 km/hr

Multiple Choice

A car travels 30 miles/hr for 3 hours, calculate the distance it will travel

• 90 miles
• 10 miles
• 600 miles
• 90 km

Multiple Choice

What is the speed of a man who runs 10 miles in 1 hr?

• 20 miles
• 10 m/min
• 10 miles/hr
• 20 hrs

•Graphs can be used to represent motion

•Graphs have an X and a Y axis

•The formula tells you what goes on the axis (Speed Formula is S=D/T)

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•Distance: Tells us where the object is.  How NEAR or FAR from 0.

•Time: Tells how long the object has been traveling.

Graphing Motion:

•What is the objects distance at 4 seconds of travel?

• How long has the object been traveling when it is at 6m?

• How many seconds is the object at 5m?

• How many seconds does it take the object to go from 0 to 5 m?

Practice Reading a Graph:

• Distance/Time graphs have four shapes.

​• Each graph shape means something different about the motion.​

How to read a Distance/ Time Graph:

Memorize the graph shapes and then you can read distance/time graph you have on a quiz or test.

Multiple Select

Describe the object's motion from A to B (TWO Answers):

• Speeding Up

• Slowing Down

• Constant Speed

• Not Moving

Multiple Choice

Describe the object's motion from B to C:

• Speeding Up

• Slowing Down

• Constant Speed

• Not Moving

Multiple Choice

Describe the object's motion from C to D:

• Speeding Up

• Slowing Down

• Constant Speed

• Not Moving

Multiple Choice

What is happening at D?

• Stationary

• Accelerating

• Fast steady speed; moving away from the starting position

• Slower steady speed; moving away from the starting position

Multiple Choice

What is happening at C?

• Stationary

• Accelerating

• Fast steady speed; moving away from the starting position

• Slower steady speed; moving away from the starting position

Multiple Choice

How long did it take this object to travel 10 m?

• 0 s

• 10 s

• 15 s

• 20 s

Multiple Choice

Segment A-B The bus is _________________.

• at rest

• returning (to original position)

• moving forward

Multiple Choice

Find the average speed from 30 to 60 minutes on this graph. (Hint: distance traveled during time range /divided by time)

• 0.03 km/min

• 0.05 km/min

• 0.67 km/min

• 0.07 km/min

Multiple Choice

What is the speed of segment A?

• 0.5 m/s

• 2 m/s

• 3 m/s

• 0 m/s

Velocity is different from speed. Speed measures how fast something is going. It does not consider direction. Velocity, however, is a measure of how FAST something is going IN A PARTICULAR DIRECTION.

Our focus for today is VELOCITY!

Velocity

If I say Sha'Carri ran 24 mph, I'm only telling you her speed...

But if I say she ran 24 mph Northwest, I'm expressing her VELOCITY.​

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Which is expressing velocity?

Multiple Choice

Which one of the following expresses velocity?

• Kevin is moving North.

• Kevin drove Northeast at 45 mph.

• Kevin rode his bicycle 20 mph.

• Kevin will ride his bike to Northside Playground and catch a ride for the 45 mile trip.

Multiple Choice

Speed is different from velocity because...

• Velocity has a direction.

• Velocity does not include time.

• Velocity does not include distance.

• Speed does not include time.

What is Acceleration

● Acceleration is the change in velocity per unit time.
● Acceleration can result from

A change in speed (increase or decrease)

A change in direction (up, down, back, forth...)

Changes in both

Mr. Posada’s
Bugati

Formula

Acceleration =

Change in velocity

Change in time

a = Δv

Δt

vf - vi

=

Δt

Ariel dropped a golf ball from her second story window.

The ball starts from rest and hits the sidewalk 1.5 s later
with a velocity of 14.7 m/s. Find the average acceleration
of the golf ball.
vf - vi

Δt

14.7 m/s - 0 m/s

1.5 s

a =

= 9.8 m/s2

Multiple Choice

What are the units for acceleration?

• m/s²

• m

• sec

• s/m

Acceleration Graph

Match

Match the following graphs to the right description.

Speeding up

No acceleration (constant speed)

Slowing down

• Speeding up

• No acceleration (constant speed)

• Slowing down

What part of the graph shows no
acceleration?

What part of the graph shows the highest
acceleration?

Multiple Choice

What is the acceleration in the first ten seconds?

(15m/s - 0m/s) / 10s =

• 4 m/s2

• 1.5 m/s²

• 10 m/s2

• 0 m/s2

Multiple Choice

In which part of the graph is the object slowing down?

• A

• B

• C

• D

Multiple Choice

What is the average acceleration at 30 seconds, according to the graph?

• 0.833 $$\frac{m}{s^2}$$

• 25 $$\frac{m}{s^2}$$

• 1.2 $$\frac{m}{s^2}$$

• 15 $$\frac{m}{s^2}$$

Multiple Choice

Karina is slowing down on her skateboard. She starts at a speed of 5.5m/s and slows to 1.0 m/s over a time of 3.0 seconds. What is Karina's acceleration?

• -1.5 $$\frac{m}{s^2}$$

• 8.5 m/s

• -2.5 m/s

• 4.5 km/s

Multiple Choice

A roller coaster's velocity at the top of a hill is 10m/s. 2 seconds later, it reaches the bottom of the hill with a velocity of 26 m/s. What was the acceleration of the coaster?

• 20 m/s

• 10 m/s2

• 8 $$\frac{m}{s^2}$$

• 16 m/s3

• Free-body diagrams help us add up all the forces in order to determine the NET Force.

• This helps us to predict how the object will move.

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Free-Body Diagrams

Let's look at the forces acting on an box sitting on a table.

• ​This box is sitting at rest.

• The force of gravity is equal to the normal force.​

• Normal Force - the force that a surface exerts on an object that is pressing on it.​

• The normal force is equal and opposite to the force pressing the object into the surface.

The book is not moving, so the forces must be equal.

• When the forces being applied to an object are equal, the object isn't moving.

• This means that these are Balanced Forces.

• Balanced Forces - forces that result in a net force (total force) of zero.​

When forces are balanced, their NET force is zero. This means the object is in equilibrium.

• Equilibrium - the state in which the net force on an object is zero.

• If the object is not in equilibrium. The object will move!

Multiple Choice

Is this a balanced force?

• Yes

• No

Draw

Circle the guy who successfully pushes the box.

Multiple Choice

What is the NET Force?

• 10 N

• 5 N

• 10 N to the right.

• 5 N to the right.