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# Do Now: PowerPoint PPT Presentation

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:

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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.

### 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

11/4/10

Reminder: test 12/8/11!

### Newton’s Third Law:

For every action, there is an equal and opposite reaction

### Examples:

Gravity pulls down on an object

The floor reacts by pushing up on the object.

### 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:

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?

### 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:

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.

### 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:

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):

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

### Equilibrium

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

### 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)

Newton's First Law is the Law of what?

### Review: (10 s)

Describe Newton's Second Law in 3 letters.

### Review: (30 s)

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

### Review: (15 s)

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

### Review: (20 s)

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

### Review: (10 s)

What is the sum of 2 or more vectors?

### 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)

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

### 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 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 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 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 s)

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

### Review: (15 s)

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

• 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

One Newton is equivalent to what other units?

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

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?

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

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?

The SI unit for acceleration is what?

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

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?

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

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

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"

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

### 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