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Do Now:. A horizontal force of 20.0 N is needed to keep a 10.0 kg box at constant velocity over a horizontal surface. Calculate the frictional coefficient, µ. Please take out your labs!!! . Do Now (12/5):.

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Do now

Do Now:

  • A horizontal force of 20.0 N is needed to keep a 10.0 kg box at constant velocity over a horizontal surface. Calculate the frictional coefficient, µ.

  • Please take out your labs!!! 


Do now 12 5

Do Now (12/5):

  • A horizontal force of 20.0 N is needed to keep a 10.0 kg box at constant velocity over a horizontal surface. Calculate the frictional coefficient, µ. 


Newton s third law review

Newton’s Third Law/Review

11/4/10

Reminder: test 12/8/11!


Newton s third law

Newton’s Third Law:

For every action, there is an equal and opposite reaction


Examples

Examples:

Gravity pulls down on an object

The floor reacts by pushing up on the object.


Remember

Remember:

Newton's Third Law is NOT the same as balanced forces acting on the same object (i.e. Fg and Fn). It applies to pairs of forces BETWEEN objects.


Example

Example:

While driving down the road, a firefly strikes the windshield of a bus and makes a quite obvious mess in front of the face of the driver. This is a clear case of Newton's third law of motion. The firefly hit the bus and the bus hits the firefly. Which of the two forces is greater: the force on the firefly or the force on the bus?


Example1

Example:

For years, space travel was thought to be impossible because there was nothing that rockets could push off of in space in order to provide the propulsion necessary to accelerate. This inability of a rocket to provide propulsion is because ...

a. ... space is void of air so the rockets have nothing to push off of.

b. ... gravity is absent in space.

c. ... space is void of air and so there is no air resistance in space.

d. ... nonsense! Rockets do accelerate in space and have been able to do so for a long time.


Review

Review:

You may be familiar with the fact that a rifle recoils when fired. This is the result of action-reaction force pairs. A gunpowder explosion creates hot gases that expand outward allowing the rifle to push forward on the bullet. Consistent with Newton's third law of motion, the bullet pushes backwards upon the rifle. The acceleration of the recoiling rifle is ...

a. greater than the acceleration of the bullet.

b. smaller than the acceleration of the bullet.

c. the same size as the acceleration of the bullet.


Review1

Review:

In the top picture (below), Kent Budgett is pulling upon a rope that is attached to a wall. In the bottom picture, the Kent is pulling upon a rope that is attached to an elephant. In each case, the force scale reads 500 Newton. Kent is pulling ...


Practice

Practice:

Please use the rest of class to work on the paper “Newton’s Third Law Problems!” It will be due 12/7. Thanks!


Do now 12 6

Do Now (12/6):

A gun recoils after you shoot a bullet. Why? Use Newton’s Laws to explain.


Equilibrium

Equilibrium

When balanced forces act on an object (the net force is zero)


Classwork

Classwork

Please work on the paper Chapter 4 Assessment. We will check and grade it 15 minutes before the end of class. Use your notes, talk with your peers, and raise your hand if you need help.


Review 5 s

Review: (5 s)

Newton's First Law is the Law of what?


Review 10 s

Review: (10 s)

Describe Newton's Second Law in 3 letters.


Review 30 s

Review: (30 s)

Use Newton's Third Law to explain why a gun kicks back.


Review 15 s

Review: (15 s)

What would happen if you threw a ball in outer space?


Review 20 s

Review: (20 s)

The single force that will bring the net forces acting upon an object back to zero is called what?


Review 10 s1

Review: (10 s)

What is the sum of 2 or more vectors?


Review 30 s1

Review: (30 s)

An 18 N force east & a 12 N force west act on an object simultaneously. The resultant force is


Review 45 s

Review: (45 s)

If a dog weighs 4.9 x 102 N, its mass is _____ kg


Review 10 s2

Review: (10 s)

If a dime & a ball of cotton were dropped in a vacuum (no air resistance), which one reaches the bottom first?


Review 20 s1

Review: (20 s)

  • When the net force acting on an object is zero, the object will stay at rest or at constant velocity. This statement corresponds to which of Newton's laws?


Review 20 s2

Review: (20 s)

  • If you throw a bowling ball while standing on a skateboard, you will roll backwards. This statement corresponds to which of Newton's laws.


Review 20 s3

Review: (20 s)

  • When unbalanced forces act upon a object, the object will accelerate in the direction of the force. This statement corresponds to which of Newton's laws?


Review 20 s4

Review: (20 s)

If your standing (facing forward) on a moving flat wagon that stops suddenly, what will happen to you?


Review 15 s1

Review: (15 s)

How is the acceleration of an object related to its mass?


Do now

  • The acceleration due to gravity:

  • a)is constant for all planets

  • b)varies slightly over the Earth's surface

  • c)is constant over the Earth's surface


Do now

One Newton is equivalent to what other units?


Do now

Three balls (1 kg, 5 kg, & 10 kg) are dropped from a cliff. Which one lands first?


Do now

What is the coefficient of friction between a 50 N sled and the ice below if the force of friction on the sled as it is pulled at constant speed is 5 N?


Do now

A little boy pushes a wagon with his dog in it. The mass of the dog and wagon together is 40 kg. The wagon accelerates at 0.8 m/s2. What force is the boy pulling with


Do now

A crate is dragged across an ice covered lake. The box accelerates at 0.08 m/s2 and is pulled by a 47 N force. What is the mass of the box?


Do now

The SI unit for acceleration is what?


Do now

The net force that is required to give a mass of 60-kg an acceleration of 8 m/sec2 is ____ .


Do now

Three men stand on the same rooftop. At the same instant, man A drops a 1 kg ball, man B drops a 5 kg ball, and man C throws a 10 kg ball horizontally. Which ball lands first?


Do now

A man whose mass is 50 kg has a weight of _____ on the Earth.


Do now

If Fnet , the vector sum of all forces acting on an object, is not equal to zero then the object _____ .

A) has no inertia

B) has uniform velocity

C) is in equilibrium

D) will accelerate


Do now

Which expression most closely relates to Newton's third law of motion?

A) F = ma

B) T2/r3

C) inertia

D) equal areas

E) "action = reaction"


Do now

The ______ of an object depends on the strength of the gravitational field it is in.


Challenge

Challenge!

A bathroom scale on an unmoving elevator shows your weight to be 530 N. If you stand on the scale while riding up at a constant speed in an elevator, the reading will be: A) less than 530 N B) equal to 530 N C) greater than 530 N

You stand on the scale in the previous question while riding up in an elevator. While the elevator is slowing as it approaches the proper floor, the reading on the scale will be _____ .

A) less than 530 N B) equal to 530 N C) greater than 530 N


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