Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and...
-
Upload
rachel-west -
Category
Documents
-
view
233 -
download
8
Transcript of Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and...
![Page 1: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/1.jpg)
Chapter 4
Forces and the Laws of Motion
![Page 2: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/2.jpg)
Newton’s First Law
An object at rest remains at rest, and an object in motion continues in motion with constant velocity (that
is, constant speed in a straight line) unless the object experiences a net external force.
“Equilibrium”
∑ �⃗�=0
![Page 3: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/3.jpg)
• Is the object in equilibrium?
• Describe the motion of the object.
![Page 4: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/4.jpg)
Newton’s Second Law
The acceleration of an object is directly proportional to the net force acting on the object and inversely
proportional to the object’s mass.
1
net force = mass x acceleration
![Page 5: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/5.jpg)
ExampleThe net force on a golf cart is 390 N north. If the cart has a total mass of 270 kg, what are the magnitude and direction of the cart’s acceleration?
![Page 6: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/6.jpg)
PracticeTwo students reach for a jar of mustard at the same time. One student pulls to the left with a force of 13.2 N, while the other students pulls to the right with a force of 12.9 N. If the jar has a net acceleration of 0.44 m/s2 to the left, what is the mass of the jar?
13.2 N 12.9 N
![Page 7: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/7.jpg)
PracticeA 2.0-kg fish pulled upward by a fisherman rises 1.9 m in 2.4 s, starting from rest. Assuming the acceleration is constant, find the magnitude and direction of the net force acting on the fish during this interval.
upwards
upwards
![Page 8: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/8.jpg)
Practice
FT,1 = 23.2 NFT,2 = 23.2 N
![Page 9: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/9.jpg)
Practice
* Note: force of gravity = 25,000 N
(a) Fnet = 1830 N at 41.0° below the horizontal(b) anet = 0.718 m/s2 (c) ∆x = 51.5 m
![Page 10: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/10.jpg)
ExampleA Goliath beetle with a mass of 0.080 kg is placed on a slope that makes an angle of 37.0° with the horizontal. If the force of gravity is 0.784 N, find the acceleration of the beetle along the slope (a) if there is no friction, (b) if there is a force of friction applied upward along the slope equal to 1.4 N.
37.0°
![Page 11: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/11.jpg)
y
x
(a) no friction
down the incline down the incline
(b) with friction?
up the incline37.0°
0.784 N
![Page 12: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/12.jpg)
Practice
* Note: force of gravity = 4008 N
(a) Fnet = 76 N at 30.0° above the horizontal(b) anet = 0.186 m/s2 at 30.0° above the horizontal(c) ∆t = 8.03 seconds
![Page 13: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/13.jpg)
Newton’s Third Law
If two objects interact, the magnitude of the force exerted on object 1 by object 2 is equal to the magnitude of the
force simultaneously exerted on object 2 by object 1, and these two forces are opposite in direction.
“For every action, there is an equal and opposite reaction.”
These two forces are called an “action-reaction pair”
�⃗� 1on2=− �⃗� 2on1
![Page 14: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/14.jpg)
Action-reaction pair
![Page 15: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/15.jpg)
Action-reaction pair
![Page 16: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/16.jpg)
Action-reaction pair: Force of Gravity
𝐹 𝑔=𝐺𝑚1𝑚2
𝑟2𝐺=6.673×10−11 N m
2
k g2
�⃗� 1on2�⃗� 2on1
Which force is bigger?
![Page 17: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/17.jpg)
Weight
The weight of an object is the force of gravity acting on the object.
On earth,
𝑊=𝑚�⃗�
![Page 18: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/18.jpg)
Mass vs. Weight
• Which is greater: your mass on Earth or your mass on the moon?
• Which is greater: your weight on Earth or your weight on the moon?
![Page 19: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/19.jpg)
Normal Force
The normal force acts on a surface in the direction perpendicular to the surface
�⃗� 𝑔
�⃗� 𝑛
�⃗� 𝑔
�⃗� 𝑛
𝐹 𝑔=𝐹𝑛?
![Page 20: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/20.jpg)
ExampleA student attaches a rope to a 20.0 kg box of books. He pulls with a force of 90.0 N at an angle of 30.0° from the horizontal. Find the magnitude of the normal force and the net acceleration of the box.
90.0 N
forward30.0°
�⃗� 𝑔
�⃗� 𝑛
![Page 21: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/21.jpg)
Force of Friction
![Page 22: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/22.jpg)
Force of Friction
• Friction opposes the applied force
Static friction: resists the initiation of sliding motion between two surfaces that are in contact and at rest
Kinetic friction: opposes the movement of two surfaces that are in contact and sliding over each other
![Page 23: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/23.jpg)
Force of Friction
• The force of friction depends on – the normal force– the materials in contact
• Coefficient of friction (µ): the ratio of the force of friction to the normal force
𝐹 𝑓=𝜇𝐹 𝑛
![Page 24: Chapter 4 Forces and the Laws of Motion. Newton’s First Law An object at rest remains at rest, and an object in motion continues in motion with constant.](https://reader033.fdocuments.us/reader033/viewer/2022042608/56649eca5503460f94bd86ff/html5/thumbnails/24.jpg)
Force of Friction