Do It Now

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# Do It Now - PowerPoint PPT Presentation

Do It Now . Newton’s Laws Quiz. 10-11 or 10-12: Physics. Notes on Free Body diagrams Practice with Free Body diagrams Force calculations Homework : Finish free body diagrams and Freaky Forces; QUIZ next class. Proficiencies.

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Presentation Transcript
Do It Now
• Newton’s Laws Quiz
10-11 or 10-12: Physics
• Notes on Free Body diagrams
• Practice with Free Body diagrams
• Force calculations
• Homework: Finish free body diagrams and Freaky Forces; QUIZ next class
Proficiencies
• 4D Calculate the effect of forces on objects, including the law of inertia, the relationship between force and acceleration, and the nature of force pairs between objects.
• P4E Develop and interpret free-body diagrams.

Shows ALL the F

Freebody

Speeding up

+

Sum of all the F

∑F = Fnet

-

Slowing down

0

At rest

Constant speed

Not a force

kg

Same anywhere in the universe

Measure of inertia

Caused by supporting surface

Reaction to weight

perpendicular

Usually = weight

weight

Unless freefall

Pounds/2.2 lb/kg = ?

downward

Or an inclined plane

Depends on location

X 26.8 m/s/s

In space, weight = 0

Mass is NOT = 0

X 9.81 m/s/s

X 2.7 m/s/s

Wt

Only overcome Ffriction

acceleration

Gravity does not slow objects moving horizontally. ONLY friction does

To move an object

If Fnet not = 0

Let’s do some freebody diagrams
• You will need some colored pencils, please.
Draw a 25.0 kg box at rest on a table.
• Label the forces.

Fnormal = Wt

Fnet = ?

F= ma=( 9.81 m/s/s)(25 kg)

Draw a 5.0 kg box moving at a constant speed to the right. The force of friction is 12.5N

Since the box is moving at a CONSTANT speed, the Fapplied = Ffriction.

Fnormal = Wt

Fnet vertical = ?

FFriction = 12.5N

F= ma =( 9.81 m/s/s)(5 kg)

Fnet horizontal = ?

Draw a 7.2 kg box moving at a 6.5 m/s/s to the right. The force of friction is 23.5N. What is the applied force?

Since the box is accelerating, the

Fnet = Fapplied - Ffriction.

Fnormal = Wt

Fnet vertical = ?

FFriction = 23.5N

F = ma =( 9.81 m/s/s)(7.2 kg)

Fnet horizontal = ma = (7.2 kg)(6.5 m/s/s) = ?

46.8N = Fapplied – 23.5N. What is Fapplied?

Problem Solving
• The formulas we had for kinematics continue here, and we’ll remove all the gravity/projectile motion formulas and focus on accelerated motion:
• Fnet = ma
• ∆d = vf – vi/∆t
• vf2 – vi2= 2a∆d
• ∆d = vi∆t+ .5 a∆t2
Problem Solving
• What net force does it take to accelerate a 12.5 kg fish from rest to 6.58 m/s in 3.22s?
• If the force of friction between the fish and the table is 15.8 N, what is the applied force?
Stopping Forces
• Stopping forces can be extremely high if the stopping time is very short.
• Many things are designed to increase the impact time – seat belts, air bags, bungee cords, soft soled shoes.
• A driver hits a steering wheel with 45.8 kg of his mass, moving at 35 m/s and stops hard, in .59s. What is his impact force?
• The air bag increases the stopping time to 3.88s. What is his impact force now?
Stopping Forces
• A 65.8 kg athlete jumps down from a bleacher that is 1.87 m above the ground.
• What is the weight of the athlete?
• What speed does she strike the ground?
• If she hits with a force of 750N, how long does it take for her shoe to bring her to a stop?
Stopping Forces
• Why bend your knees on impact?
• How do squishy things increase impact time?
• Let’s make a list of things that do this on purpose:
Vectors and Forces
• In physics, we often use a force table for force experiments.
Vectors and Forces
• Concurrent forces are forces acting at the same time on an object.
• Equilibrium is when the object experiencing the forces is either AT REST or moving at a CONSTANT speed.
• The force that puts concurrent forces into equilibrium is called a RESULTANT.
Vectors and Forces

The force that produces equilibrium is equal in magnitude and opposite in direction.

• If this is the net force, how does the object move?
• Accelerates
• East of north
• What force would put this force in equilbrium?
Concurrent Forces

Why is the red arrow longer than the blue ones this time?

Why isn’t the red arrow directly opposite in direction?

• Now there are two concurrent forces. What does the object do?
• Accelerates
• East of north
• More magnitude than last time and the resultant direction is at a different angle.
• What force would put this force in equilbrium?
Vectors and Forces
• Mathematically, this could get messy. Luckily, you don’t have to use trig or solve for multiple vectors.
• What you do need to do is understand what happens as the angle between the vectors changes.
Concurrent Forces
• Let’s look at two forces – one is 8 N due north, the other is 6 N due north.

What is the resultant force?

8N

8N + 6N = 14 N north

6N

What force would put this in equilibrium?

14 N due south

Concurrent Forces
• What happens if the angle between the two goes from zero to 30 degrees?

7N + 5N = 12 N north

Plus some components outward

What is the resultant force?

The N part drops to say 5N

The N part drops to say 7N because some of it is outward now

What force would put this in equilibrium?

12 N due south and some components inward

Concurrent Forces
• What happens if the angle between the two goes from zero to 90 degrees?

5N + 3N = 8 N north

Plus more components outward

What is the resultant force?

The N part drops to say 3N

The N part drops to say 5N because some of it is outward now

What force would put this in equilibrium?

8 N due south and morecomponents inward

Concurrent Forces
• What happens if the angle between the two goes from zero to 180 degrees?

8N - 6N = 2 N blue direction

What is the resultant force?

What force would put this in equilibrium?

2 N due red direction

Vectors and Forces
• Mathematically, this could get messy. Luckily, you don’t have to use trig or solve for multiple vectors.
• What you do need to do is understand what happens as the angle between the vectors changes.
Vectors and Forces
• force table allows you to put a collection of forces into equilibrium.
• Spring scales pull on a ring in the center of the table.
• The system is in equilibrium when the forces are balanced out.
Vectors and Forces
• Each spring scale has vertical and horizontal components of motion. Draw this example:

This vector has one component because we choose to set it at the 0 deg position.

Could it possibly be in equilibrium now?

What direction would the spring scales move?

25 N

19 N

The vector or force that would put it in equilibrium would also have a left and a down vector component.

? N

This vector has a to the right component and a down component (as we look at it)

Vectors and Forces
• You would use trig to solve for each component, add them, then use Pythagorean theorem to solve for the missing vector.

25 N

19 N

? N

Forces at angles
• Here’s an example of weight and tension:

If the mass of the elevator is 435 kg, what is the tension in the rope?

Would this increase or decrease if the elevator accelerated upward?

If it accelerated downward?

Tension = weight

Weight = Fup

Homework:
• Freaky Forces – combines horizontal kinematics and dynamics formulas.
• Free Body diagram practice