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**ToC, Ch 4**|

**Ch 5, Newton"s 3rd Law**|

**Exercises (Discussion Questions)**

**Ex 4.* before the time of Galileo and also Newton, the was believed by many learned scholars the a stone dropped native the peak of a tall mast that a moving ship would loss vertically and hit the deck behind the mast through a street equal to how far the ship had actually moved forward while the stone was falling. In light of your knowledge of Newton"s Laws, what carry out you think about this?**

As we have actually seen ~ above **The mechanically Video**, and also as I have tried to demonstrate in walking ago and forth in front of the great while cram a ball, the round or rock should fall and also hit the deck at the base of the mast.

You are watching: An apple weighs 1 n. when held at rest above your head, the net force on the apple is

**Ex 4.2 have the right to the velocity of an item reverse direction if mintaining a continuous acceleration? If so, give an example; if not, define why.**

Yes, as presented in the sketch here. Take into consideration a block relocating to the best with one acceleration to the **left** as shown here. The block will eventually concerned rest v v = 0. If the acceleration **stays the same,** the block will now relocate to the **left** and continue to rise its speed. Transparent the sketch shown here, the acceleration is constant and **to the left**.

**Ex 4.3 If an object is not accelerating, can you conclude the no force acts on it?**

You have the right to conclude that there is **no net force** on the object. There may be many separate forces acting ~ above it, yet their **sum** is zero!

**Ex 4.5 If it takes 1 N to push horizontally on your book to make it on slide at constant velocity, just how much force of friction acts on the book?**

Horizontally, your 1 N pressure acts come the best while the friction force acts to the left. If the publication slides at **constant** **velocity** that means it moves with **zero acceleration** so the **net force**must be **zero.** Therefore, the friction pressure is additionally **1 N.**

**Ex 4.6 Is it possible to go around a curve in the lack of a force?**

No, going approximately a curve means the **velocity changes** direction. Any readjust in velocity needs an acceleration. And also any acceleration needs a (net) force.

Think the what happens throughout the winter once there may be ice on a curve so the the pressure of friction between tires and road is very small. Then cars have the right to not go about such a curve and also end up in a ditch.

**Ex 4.9 A 400-kg be affected by each other grasping a upright tree slides down an consistent velocity. What is the friction pressure that acts upon the bear?**** The bear whose mass is 400 kg has actually a downward weight that 4,000 N since**

**W = m g = (400 kg)(10 m/s2) = 4,000 N**

**Since the be afflicted with slides down at continuous velocity (or zero acceleration!), the net force ~ above the bear need to be zero. The net force is the sum of the downward weight and the upward friction force. These two must include to zero. For this reason the friction pressure is additionally 4,000N.**

**Ex 4.11 In the orbiting space shuttle you room handed two identical boxes, one filled through sand and the other filled through feathers. How deserve to you phone call which is i beg your pardon without opened the boxes?**

**Shake the boxes back and soon or up and down and also see which is less complicated to accelerate.**

**Ex 4.17 What wake up to your weight once your mass increases?**

**Since weight is proportional to mass, or W = m g, any type of increase in mass synchronizes to a proportional increase in weight.**

**Ex 4.18 What is your own mass in kilograms? her weight in newtons?**

Recall the the a fixed of 1 kg has actually a load of 2.2 lbs. To uncover mass in kilograms, take your weight in lbs and **divide by 2.2** (this method a weight of 2.2 lbs coincides to a massive of 1.0 kg or a load of 22 lbs corresponds to a massive of 10 kg or a weight of 220 lbs coincides to a mass of 100 kg.)

Recall the the load of 1 kg is 9.8 newtons. To uncover your **weight** in newtons, take her **mass** in kilograms and **multiply by 9.8** .

Here is a brief table:

load (in pounds) | massive (in kilograms) | weight (in newtons) |

100 | 45.5 | 446 |

125 | 56.8 | 557 |

150 | 68.2 | 668 |

175 | 79.5 | 779 |

200 | 90.9 | 891 |

225 | 102.3 | 1,003 |

**Ex 4.19 A rocket becomes progressively easier to accelerate together it travels v space. Why is this so?**

The force of the rocket enging -- called the **thrust** -- remains constant. However the **mass** of the rocket decreases together it provides its fuel. As this consistent forces proceeds to action on a smaller and also smaller mass, the **acceleration increases.**

**Ex 4.29 when you jump vertically turn off the ground, what is her acceleration after you with your highest point?**

Once her feet leaving the ground, you"re in **free fall**. When your feet leaving the ground, the **only force** acting on friend is the **force of gravity, her weight**. As soon as your feet leaving the ground, your acceleration is the acceleration the gravity, **downward at 9.8 m/s2.**

**Ex 4.32 A friend claims that as lengthy as a auto is in ~ rest, no forces act top top it. What carry out you say if you"re in the the atmosphere to exactly this statement?**** because that a automobile -- or any object -- at rest, the net force is zero. But having the net pressure to it is in zero is not the very same things as having no forces acting on the automobile at all!**

**Ex 4.35 What is the net pressure on a 1 N apple when you organize it in ~ rest above your head? What is the net force on it after you release it?**

If you"re holding the apple at rest, the **net force** ~ above the apple is **zero.**

Once girlfriend drop the apple, the only force on that is its load of 1 N

**Ex 4.43 how does the terminal speed of a parachutist prior to opening a parachute compare to terminal rate after? Why is there a difference?**

**Ex 4.44 how does the gravitational pressure on a falling body compare with the air resistance that encounters before it will terminal velocity? After?**

**Ex 4. 31 What is the net pressure on an apple the weighs 1 N when you organize it in ~ rest over the floor? What is the net pressure on it when you relax it?**

For **anything** in ~ rest, the **net force** is zero.

When you release the apple, the **only** pressure on the is the pressure of gravity, which we contact the **weight** of the apple, and also we space told that that is **one Newton.**

**Ex 4.44 just how does the weight of a fall body compare to the wait resistance it encounters just before it get terminal velocity? After?**

Before getting to terminal velocity, the falling body is quiet accelerating, for this reason its weight (the pressure of gravity) is still greater than the force due to air resistance.

See more: How Many Toes Does A Rhinoceros Have On Each Foot? ? Perissodactyla

After getting to terminal velocity, the falling body falls with constant speed therefore the net force on it should be zero. This method the pressure of heaviness (its weight) exhilaration **downward** is simply **balanced** by the force of wait resistance exhilaration **upward**.** **

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**ToC, Ch 4**|

**Ch 5, Newton"s third Law**|

**Pb 4.4 If a fixed of 1 kg is increased 1 m/s2 by a pressure of 1 N, what would certainly be the acceleration of 2 kg acted on by a pressure of 2 N?**

Use Newton"s second Law,

F = m a2 = 2 x ?

2 = 2 x a

a = 1

a = 1 m/s2

**Pb 4.5 exactly how much acceleration does a 747 jumbo jet of fixed 30 000 kg experience in takeoff once the thrust of every of four engines is 30 000 N (for a full thrust the 120 000 N)?**

Again, usage Newton"s second Law,

F = m a120 000 = (30 000) a

a = 4

a = 4 m/s2

**Pb 4.7 A firefighter of massive 80 kg slides under a vertical pole through an acceleration of 4 m/s2. What frictional pressure does the pole exert top top her?**

First, us can discover the **net force**, for the is constantly the F that shows up in Newton"s 2nd Law,

Fnet = (80 kg) (4 m/s2)

Fnet = 320 N

What forces comprise this net force Fnet? heaviness pulls **down** through the firefighter"s weight,

w = (80 kg) (10 m/sw)

w = 800 N

Then there is the **force that friction**, Ff, i beg your pardon opposes the firefighter"s motion. For this reason the pressure of friction Ff must suggest **up**. The **net force** Fnet is consisted of of these two forces,

320 N = 800 N - Ff

320 N = 800 N - 480 N

Ff = 480 N

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**ToC, Ch 4**|

**Ch 5, Newton"s 3rd Law**|

**Typical multiple-guess questions:**

1. Fixed is a measure up of

**a) the volume of an item**

b) the dimension of one object

c) how daunting it is to adjust the activity of an object

d) the velocity of one object

2. The weight of things is

a) the very same thing as the fixed of thingsb) the amount of all the pressures on one object

c) the pressure of heaviness on an object

d) constantly less than the mass, even in a vacuum

3. The net pressure on a 1-kg object, in ~ rest, is

a) 9.8 Nb) 4.9 N

c) 1.00 N

d) zero

4. The net force on a 1-kg object, in free fallt, is

a) 9.8 Nb) 4.9 N

c) 1.00 N

d) zero

5. A pressure of 24 N acts on an object who mass is 6 kg. This causes the object to advice at

a) 2 m/s2b) 4 m/s2

c) 6 m/s2

d) 10 m/s2

6. A car, with mass that 1,000 kg, accelerates at 2 m/s2. The net pressure exerted top top the car must be

a) 500 Nb) 1,000 N

c) 2,000 N

d) 10,000 N

7. The weight of a 1,000-kg vehicle is

a) 500 Nb) 1,000 N

c) 2,000 N

d) 10,000 N

8. A fireman, whose weight is 500 N, slides under a pole with an acceleration the 3 m/s2. The forces that plot on him room his load pulling the down and also the pressure of friction pulling up on him to sluggish him down. The force of friction need to be

a) 90 Nb) 150 N

c) 350 N

d) 500 N

** answer to typical multiple-guess questions:**

1. Fixed is a measure up of

**a) the volume of things**

b) the size of one object

**c) how complicated it is to readjust the activity of one object**

d) the velocity of one object

2. The load of things is

**a) the same thing together the fixed of an object**

b) the sum of every the forces on an object

**c) the pressure of gravity on one object**

d) constantly less 보다 the mass, even in a vacuum

3. The net pressure on a 1-kg object, at rest, is

**a) 9.8 N**

b) 4.9 N

c) 1.00 N

**d) zero**

4. The net pressure on a 1-kg object, in free fallt, is

**a) 9.8 N**

b) 4.9 N

c) 1.00 N

d) zero

5. A force of 24 N acts on an object whose mass is 6 kg. This causes the thing to accelerate at

**a) 2 m/s2**

**b) 4 m/s2**

c) 6 m/s2

d) 10 m/s2

F = m a24 N = ( 6 kg ) ( a )

24 N = ( 6 kg ) ( 4 m/s2)

a = 4 m/s2

6. A car, v mass of 1,000 kg, accelerates at 2 m/s2. The net pressure exerted top top the automobile must be

**a) 500 N**

b) 1,000 N

**c) 2,000 N**

d) 10,000 N

7. The weight of a 1,000-kg auto is

**a) 500 N**

b) 1,000 N

c) 2,000 N

**d) 10,000 N**

w = (1,000 kg) (10 m/s2)

w = 10,000 N

8. A fireman, whose load is 500 N, slides down a pole through an acceleration the 3 m/s2. The forces that act on him space his weight pulling him down and the pressure of friction pulling up on him to sluggish him down. The pressure of friction should be

**a) 90 N**

b) 150 N

**c) 350 N**

d) 500 N

F = maFnet = ma

Fnet = Fgravity - Ffrict

Fgravity = w = m g

500 N = m (10 m/s2)

m = 50 kg

Fnet = ma = (50 kg) (3 m/s2)

Fnet = 150 (kg m/s2)

Fnet = 150 N

Fnet = Fgravity - Ffrict

150 N = 500 N - Ffrict

Ffrict = 500 N - 150 N = 350 N

**Ffrict = 350 N**

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**ToC, Ch 4**|

**Ch 5, Newton"s third Law**|

an overview ago to "Table the Contents" ToC, Ch5, Netwon"s 3rd Law (C) 2003, Doug Davis; all rights reserved