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Acceleration and Force

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Galileo’s Investigation

Engagement

What are the similarities and differences between a basketball and a tennis ball?

Make a T-Chart

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Galileo’s Investigation

Engagement

What will happen if the balls are released from the same height above the floor?

Try to use Physics vocabulary in your response.

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Galileo’s Investigation

Engagement

What’s going on here, why did they move?

How would you describe the motion of the balls?

– Path

– Speed

Again, try to use Physics vocabulary in your response.

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Force of Gravity: Pull or Push?

Force can be described as a push or pull one object exerts on another.

One specific force is weight. Weight = the force of gravity on an object.

But what causes gravity?

– The presence of mass

Explanation

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Newton’s Universal Law of Gravitation HO 3.2

Masses attract one another about their centers with a force proportional to the product of their masses and inversely proportional to the square of the distance between them (whew!).

Try to write that in an equation form.

Explanation

Force Gm 1 m 2

d2

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Newton’s First Law

Objects require a force to change their velocity.

– Another term for this property of matter is inertia

What is the principle property of an object that determines its inertia?

– mass

Explanation

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Newton’s First Law Example

If we apply the same force (push) to a bus and a bicycle, which object will experience a greater change in velocity?

Explanation

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Newton’s Second Law

Force causes a change in motion

– Change in motion means change in velocity

– Change in velocity means???

A change in velocity occurs in a certain amount of time.

AHA! That’s a rate.

Explanation

F = ma

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Acceleration is a Rate

The speedometer in your car indicates what property of motion?

What happens to the needle if you speed up or slow down?

The speed of the needle is a measure of your acceleration (in a straight line).

Acceleration is the rate of change in velocity.

Explanation

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Newton’s Second Law Example #1

Important note: the acceleration vector is always in the same direction as the force vector.

If we apply a 100N force to a wagon full of rocks initially at rest with a mass of 50 kg, what is its acceleration?

What does this answer tell us?

What is the velocity of the object after 1 second?

– 2 seconds?

– 5 seconds?

Explanation

F = ma

100 N2 m/s2

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Newton’s Second Law Example #2

What happens when the acceleration vector points opposite to the velocity vector?

Explanation

30 m/s

F -5m/s2

How long will it take Mighty Mikey to stop the train?

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Newton’s Third Law

No single isolated force can exist

Equal and Opposite forces exist only between two objects

Explanation

Push on water

WRONG!

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Newton’s Third Law Example

You push on the water The water pushes back on you with an equal force in the

opposite direction So why do you move?

Explanation

Push on water

CORRECT!

Push on swimmer

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Acceleration Due to Gravity

The Tennis Ball and Basketball hit the ground at the same time because they accelerated at the same rate (~10 m/s2).

What force changed their velocities?...Weight.

Weight = mass X gravity (W=mg).

Explanation

Wbb>Wtb

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Acceleration Due to Gravity

Explanation

F = weightF = weight

m = massm= mass

F/m=a F/m = a

a = gregardless of mass!

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Elaboration

Is there a Starbuck’s in the Leaning Tower of Pisa?

Observe the instructor drop a new set of objects.

As a class, discuss the similarities and differences in their motion.

Why is this different?

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Elaboration

Paper Drop Challenge HO 3.3

Materials per group: 2 sheets of paper, stopwatch, 8 paperclips, 30cm of tape, scissors, meter stick.

Have Materials Manager distribute supplies.

You have 10 minutes for this activity.

Make one whole piece of paper drop to the floor as fast as possible from a height of 2 meters and the other drop as slowly as possible from the same height. (Do not attach the scissors or meter stick to your paper.)

GO!

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Elaboration

Paper Drop Challenge

As a group, describe your strategy for making the piece of paper:

– Drop the slowest

– Drop the fastest

Make a table of your drop times for comparison with the class

Explain why your two pieces of paper accelerate at different rates.

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Free Body Diagram HO 3.4

A free body diagram is a pictorial representation of all forces acting on an object.

We consider the object to be isolated from the physical system and choose to examine only the forces directly acting ON the object, not forces applied BY the object.

Explanation

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Free Body Diagram Example

Explanation

Label the forces acting on the block.

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Free Body Diagram Example

Explanation

Force on the block by the ground

Force on the block by the earth

Force on the block by Antonio

Force on the block by the rope.

Force on the block by the ground

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Free Body Diagram Example 2

Forces on coffee filter before release (balanced forces)

Explanation

Weight

Support from my

hand

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Free Body Diagram Example 2

Forces on coffee filter immediately after release (unbalanced force acceleration)

Explanation

Weight

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Free Body Diagram Example 2

Forces on coffee filter while falling

Explanation

Weight

Air resistance

What factors influence the magnitude of

the force of air resistance?

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So Why Different Times?

Force due to gravity is different for the two objects

Their masses are different and therefore they have different inertias

They do not accelerate at the same rate …WHY???

– We must examine ALL the forces acting on the objects in order to determine the Net Force.

Explanation

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Net Force

If you earn $2000/mo but taxes, ins, retirement and other reductions total $350/mo then your:

– Gross Pay = $2000/mo

– Net Pay = $1650/mo

Explanation

Net Force

F

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Net Force

The force of air resistance is greater for the basketball than the coffee filter.

However, the force of air resistance is negligible compared to the weight of the basketball, therefore the Net Force is toward the floor.

Explanation

Air resistance

Weight

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So Why Different Times?

The force of air resistance is also acting on the coffee filter and is comparable in size to the weight.

In fact, at some point, the force of air resistance is equal to the weight of the coffee filter.

When this is the case:

– There is NO net force

– Therefore there is no acceleration

– Therefore the object moves at a constant speed

This equilibrium condition is called terminal velocity

Explanation

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Evaluation

Sky Diver Physics HO 3.5

A skydiver jumped out of a plane 1 sec ago.

– Label the forces on the skydiver

After some time, the skydiver has zero acceleration

– What does this indicate about her speed?

– What is the name for this equilibrium condition of a falling object?

– Label the forces on the skydiver

What factors influence the force of air resistance on the skydiver?

BONUS: What is the terminal velocity of a human?

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Summary

Force

Gravity & Weight

Newton’s Laws

Air resistance

Free Body Diagrams

Net Force

So what?

– This applies to transportation, moving objects, sports, etc.

Summary

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Homework

Reading Assignments HO 3.6

NSES

– p. 123, 126-127;

149, 154-155; 176-181

BSL

– 87-92

Integrated Science

– Forces: p. 26-27, Falling Objects:p. 29-31, Laws of Motion:p. 32-38

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Homework

Concepts and Questions

Integrated Science, Ch. 2

– Applying the Concepts 1-6, 8-9

– Questions for Thought 1, 5, 8