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**ToC, Ch 4**|

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

**Exercises (Discussion Questions)**

**Ex 4.* prior to the time that Galileo and also Newton, that was assumed by many learned scholars the a rock dropped from the height of a high mast the a moving ship would autumn vertically and also hit the deck behind the mast through a street equal come how much the ship had moved forward while the rock was falling. In light of your knowledge of Newton"s Laws, what do you think about this?**

As we have actually seen ~ above **The mechanical Video**, and as I have tried to show in walking ago and soon in former of the classroom while cram a ball, the sphere or stone should fall and also hit the deck in ~ 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 can the velocity of an item reverse direction if mintaining a constant acceleration? If so, offer an example; if not, define why.**

Yes, as shown in the sketch here. Consider a block relocating to the appropriate with one acceleration come the **left** as displayed here. The block will certainly eventually come to rest through v = 0. If the acceleration **stays the same,** the block will now move to the **left** and also continue to rise its speed. Throughout the sketch shown here, the acceleration is constant and **to the left**.

**Ex 4.3 If things is not accelerating, have the right to you conclude that no force acts on it?**

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

**Ex 4.5 If that takes 1 N to press horizontally on your publication to do it on slide at constant velocity, how much force of friction acts on the book?**

Horizontally, her 1 N pressure acts to the appropriate while the friction pressure acts to the left. If the book slides at **constant** **velocity** that way 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 absence of a force?**

No, going around a curve way the **velocity changes** direction. Any adjust in velocity needs an acceleration. And any acceleration requires a (net) force.

Think of what happens during the winter once there may be ice on a curve so the the force of friction between tires and also road is an extremely small. Climate cars have the right to not go roughly such a curve and also end up in a ditch.

**Ex 4.9 A 400-kg be afflicted with grasping a vertical tree slides under an constant velocity. What is the friction pressure that acts on 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 bear slides under at constant velocity (or zero acceleration!), the net force top top the bear should be zero. The net force is the amount of the downward weight and also the upward friction force. These 2 must include to zero. So the friction pressure is additionally 4,000N.**

**Ex 4.11 In the orbiting an are shuttle you are handed two identical boxes, one filled v sand and also the other filled with feathers. How can you phone call which is i m sorry without opening the boxes?**

**Shake the boxes earlier and soon or up and also down and see i beg your pardon is much easier to accelerate.**

**Ex 4.17 What wake up to her weight as soon as your massive increases?**

**Since load is proportional to mass, or W = m g, any increase in mass corresponds to a proportional increase in weight.**

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

Recall that the a massive of 1 kg has a weight of 2.2 lbs. To find mass in kilograms, take her weight in lbs and **divide by 2.2** (this way a weight of 2.2 lbs corresponds to a mass of 1.0 kg or a weight of 22 lbs coincides to a mass of 10 kg or a load of 220 lbs synchronizes to a mass of 100 kg.)

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

Here is a quick 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 i do not care progressively easier to accelerate together it travels v space. Why is this so?**

The force of the rocket enging -- called the **thrust** -- remains constant. But the **mass** of the rocket decreases as it provides its fuel. As this consistent forces continues to act on a smaller and smaller mass, the **acceleration increases.**

**Ex 4.29 as soon as you run vertically turn off the ground, what is your acceleration after ~ you reach your highest possible point?**

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

**Ex 4.32 A friend states that as long as a auto is in ~ rest, no pressures act on it. What carry out you speak if you"re in the mood to exactly this statement?**** for a vehicle -- or any object -- in ~ rest, the net force is zero. Yet having the net force to be zero is not the exact same things as having actually no forces acting on the car at all!**

**Ex 4.35 What is the net pressure on a 1 N apple once you organize it at rest above your head? What is the net force on the after you relax it?**

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

Once you drop the apple, the only force on that is its weight of 1 N

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

**Ex 4.44 just how does the gravitational force on a falling body compare through the air resistance the encounters prior to it will terminal velocity? After?**

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

For **anything** at rest, the **net force** is zero.

When you relax the apple, the **only** force on that is the force of gravity, i beg your pardon we contact the **weight** that the apple, and we space told that that is **one Newton.**

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

Before getting to terminal velocity, the falling human body is quiet accelerating, so its weight (the force of gravity) is still greater than the force as result of air resistance.

See more: How Many Square Yards Is An Acre ? Acres To Square Yards Conversion (Ac To Sq Yd)

After reaching terminal velocity, the falling body falls with constant speed for this reason the net pressure on it have to be zero. This way the pressure of heaviness (its weight) acting **downward** is simply **balanced** by the force of air resistance acting **upward**.** **

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**ToC, Ch 4**|

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

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

Use Newton"s 2nd Law,

F = m a2 = 2 x ?

2 = 2 x a

a = 1

a = 1 m/s2

**Pb 4.5 how much acceleration does a 747 jumbo jet of mass 30 000 kg endure in takeoff when the thrust of each of four engines is 30 000 N (for a full thrust that 120 000 N)?**

Again, use 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 upright 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 that is always the F that shows up in Newton"s second Law,

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

Fnet = 320 N

What forces comprise this net pressure Fnet? gravity pulls **down** with the firefighter"s weight,

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

w = 800 N

Then over there is the **force of friction**, Ff, which opposes the firefighter"s motion. For this reason the force of friction Ff must allude **up**. The **net force** Fnet is comprised of these 2 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. Mass is a measure of

**a) the volume of an item**

b) the size of one object

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

d) the velocity of an object

2. The load of things is

a) the very same thing as the fixed of an itemb) the sum of all the pressures on one object

c) the pressure of gravity on an object

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

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

a) 9.8 Nb) 4.9 N

c) 1.00 N

d) zero

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

a) 9.8 Nb) 4.9 N

c) 1.00 N

d) zero

5. A force 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, through mass of 1,000 kg, increases at 2 m/s2. The net pressure exerted ~ above the automobile must be

a) 500 Nb) 1,000 N

c) 2,000 N

d) 10,000 N

7. The load 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 load is 500 N, slides under a pole v an acceleration of 3 m/s2. The pressures that action on him space his load pulling him down and also the pressure of friction pulling increase on that to slow-moving him down. The pressure of friction have to be

a) 90 Nb) 150 N

c) 350 N

d) 500 N

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

1. Mass is a measure up of

**a) the volume of things**

b) the dimension of an object

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

d) the velocity of one object

2. The load of an item is

**a) the very same thing together the mass of an object**

b) the sum of all the pressures on one object

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

d) constantly less 보다 the mass, also 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 totally free fallt, is

**a) 9.8 N**

b) 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 reasons 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, through mass the 1,000 kg, speeds up at 2 m/s2. The net force exerted on the vehicle 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 automobile 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 weight is 500 N, slides down a pole with an acceleration that 3 m/s2. The forces that action on him space his load pulling that down and also the pressure of friction pulling up on him to sluggish him down. The force of friction have to 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**|

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