Kragte 11 e
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Transcript of Kragte 11 e
Introduction to forces
A force is a pull or a push that an object experiences due to its interaction with other objects.
Definition
Types of ForcesContact Forces
Non-contact Forces
Forces that are exerted by
objects that make contact
with each other
Forces exerted by objects that do NOT make
contact with each other
Introduction to forces
Tension ():- A force exerted by a rope or cable when it is pulled.
- The same everywhere in the rope/cable.
Contact ForcesIntroduction to forces
Friction ():- A force between and object and the
surface on which it rests parallel to the surface.
- Always opposes motion
𝒗ሬሬԦ �⃗�
Contact ForcesIntroduction to forces
Normal force ():- The force that a surface exerts on the
object that rests on it.- Always perpendicular from the surface
on the object.
Contact ForcesIntroduction to forces
Weight ():- The force with which the earth
attracts an object- Always downwards-
where
Contact ForcesIntroduction to forces
Magnetic Force ():- The force that magnets exert on
other ferromagnetic objects.- Repulsive or Attractive
Non-contact ForcesIntroduction to forces
Electrostatic Force ():- The force that charged objects
exert on other objects- Repulsive or Attractive
Non-contact ForcesIntroduction to forces
Example 1Identify all the forces present in the following pictures:
On the trolley
a) On the balloon
b) On the basket
Introduction to forces
Representation of ForcesForce Diagrams
The object itself is represented diagramatically
Forces are shown with arrows where it really works on the object.
Lengths of the arrows shows the relative magnitudes of the forces
Representation of ForcesExample 2
Draw a force diagram that shows all the forces that are exerted on the trolley
𝑭𝒈ሬሬሬሬሬԦ
𝑭𝑵ሬሬሬሬሬԦ 𝑭𝑻ሬሬሬሬԦ
𝒇ሬԦ
𝑭𝑷𝒍𝒂𝒏𝒕𝒆ሬሬሬሬሬሬሬሬሬሬሬሬሬሬԦ
𝑭𝑻𝒙ሬሬሬሬሬሬԦ 𝑭𝑻𝒚ሬሬሬሬሬሬሬԦ
Freebody diagrams FREE OF A BODY Object represented by only a dot All arrows must point from the dot
outwards Lengths of the arrows shows the
relative magnitudes of the forces
Representation of Forces
Example 3Draw a free body diagram that shows all the forces exerted on the trolley𝑭𝑵ሬሬሬሬሬԦ
𝑭𝑻ሬሬሬሬԦ
𝒇ሬԦ 𝑭𝑷𝒍𝒂𝒏𝒕𝒆ሬሬሬሬሬሬሬሬሬሬሬሬሬሬԦ
𝑭𝑻𝒙ሬሬሬሬሬሬԦ
𝑭𝑻𝒚ሬሬሬሬሬሬሬԦ 𝑭𝒈ሬሬሬሬሬԦ
Representation of Forces
Triangle of ForcesForces in equilibrium
All the forces exerted on a certain point cancel each other
The resultant of the forces are zero The object remains in rest or moves
with a constant velocity When all the forces are drawn head
to tail it forms a CLOSED vector diagram.
When three forces that are exerted on the same point
are in equilibrium, their magnitudes and direction can be shown by the three
sides of a triangle.
Triangle of Forces
FrictionTypes of Friction
Static Friction Kinetic Friction
Exerted on
Symbol
Properties
Formula
Objects in rest Moving Objects
Changes as the applied force
changesAlways constant
𝒇𝒌ሬሬሬሬԦ 𝒇𝒔ሬሬሬሬԦ
𝒇𝒌ሬሬሬሬԦ= 𝝁𝒌𝑭𝑵ሬሬሬሬሬԦ 𝒇𝒔 𝒎𝒂𝒙ሬሬሬሬሬሬሬሬሬሬሬሬԦ= 𝝁𝒔𝑭𝑵ሬሬሬሬሬԦ
WrywingTipes wrywing𝒇𝒔ሬሬሬሬԦ 𝑭𝑻ሬሬሬሬԦ
𝒇ሬԦ
𝑭𝑻ሬሬሬሬԦ
𝒇𝒌ሬሬሬሬԦ
𝒗ሬሬԦ= 𝟎 𝒗ሬሬԦ> 𝟎
𝒇𝒔ሬሬሬሬԦ 𝒇𝒌ሬሬሬሬԦ 𝒇𝒔 𝒎𝒂𝒙ሬሬሬሬሬሬሬሬሬሬሬሬԦ
FrictionMaximum Static Friction
The static friction that an object experiences just before it moves. Static friction increases as the force it opposes increases, until it reaches a maximum. If the applied force increases further, the object starts to move. The object now experiences kinetic friction.
FrictionProperties
Strongly dependent on surface roughness. Directly proportional to the normal force. Indipendent on the surface area of the surfaces in contact. Kinetic friction is independent of the speed at which the object moves. Only the maximum static friction can be calculated with a formula 𝒇𝒌ሬሬሬሬԦ< 𝒇𝒔 𝒎𝒂𝒙ሬሬሬሬሬሬሬሬሬሬሬሬԦ
FrictionCoefisient of Friction
μ No unit Property of the surfaces in contact Mostly smaller than one The bigger μ, the bigger the friction 𝜇𝑘 < 𝜇𝑠
Net force of two or more forces
The net force of all the forces that are exerted on an object, is the vector sum of all the
forces that are exerted on the object.
Also known as the resultant force The net forces in the x-axis an
y-axis are calculated separately
Inclined forces are separated into perpendicular components.
𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ
𝑭𝑵𝑬𝑻 𝒙ሬሬሬሬሬሬሬሬሬሬሬሬሬԦ= 𝑭𝒙ሬሬሬሬԦ 𝑭𝑵𝑬𝑻 𝒚ሬሬሬሬሬሬሬሬሬሬሬሬሬԦ= 𝑭𝒚ሬሬሬሬԦ
Net force of two or more forces
Net ForceExample 1
A book is pulled over a rough surface with a constant force. The kinetic friction coefficient between the surface and the book is 0,5. All the forces are in equilibrium.a) Draw a free body diagramb) Determine the normal forcec) Calculate the kinetic frictiond) Calculate the applied force
Net forceExample 2
The same book as in the previous example is now being pulled across the table by an 5 N force, that makes an angle of 30° with the horizontal. a) Calculate the kinetic friction.b) How does the value of the friction
compare to the value in the previous exmaple? Explain.
c) Calculate the net force in the x-axis
Forces on an inclined plane
θθ𝑭𝒈ሬሬሬሬሬԦ 𝑭𝒈⊥ሬሬሬሬሬሬሬԦ
𝑭𝒈∥ሬሬሬሬሬሬԦ
𝒔𝒊𝒏𝜽= 𝑭𝒈∥ሬሬሬሬሬሬԦ𝑭𝒈ሬሬሬሬሬԦ
𝑭𝒈∥ሬሬሬሬሬሬԦ= 𝑭𝒈ሬሬሬሬሬԦ𝒔𝒊𝒏𝜽
𝑭𝒈⊥ሬሬሬሬሬሬሬԦ= 𝑭𝒈ሬሬሬሬሬԦ𝒄𝒐𝒔𝜽
𝒄𝒐𝒔𝜽= 𝑭𝒈⊥ሬሬሬሬሬሬሬԦ𝑭𝒈ሬሬሬሬሬԦ
90°
∥⊥
Net ForceExample 3
All the forces exerted on the crate in the diagram are in equilibrium. Answer the following questions:a) Draw a free body diagram.b) Calculate and . c) Calculate the normal force.d) Calculate the kinetic friction.e) Calculate
𝑭𝒈∥ሬሬሬሬሬሬԦ 𝑭𝒈⊥ሬሬሬሬሬሬሬԦ
𝝁𝒌
Newton IIIAction-Reaction
If object A exerts a force on object B, object B will exert a force on object A that has the same magnitude, but opposite
direction.
Newton IIIAction-Reaction
A B
A:
𝑭𝒈ሬሬሬሬሬԦ
𝑭𝑵ሬሬሬሬሬԦ
𝒇ሬԦ
𝑭𝑩𝑨ሬሬሬሬሬሬሬԦ 𝑭𝑻ሬሬሬሬԦ
𝑭𝑵ሬሬሬሬሬԦ 𝒇ሬԦ
𝑭𝒈ሬሬሬሬሬԦ
𝑭𝑨𝑩ሬሬሬሬሬሬሬԦ
B:
𝑭𝑻ሬሬሬሬԦ
Newton IIIForce pairs
but opposite in direction Is NOT exerted on the same object Do NOT cancel eachother Works SIMULTANEOUSLY
- THEREFORE: For each action there is a reaction
𝑭𝑨𝑩ሬሬሬሬሬሬሬԦ= 𝑭𝑩𝑨ሬሬሬሬሬሬሬԦ
A:
𝑭𝒈ሬሬሬሬሬԦ
𝑭𝑵ሬሬሬሬሬԦ
𝒇ሬԦ
𝑭𝑩𝑨ሬሬሬሬሬሬሬԦ 𝑭𝑻ሬሬሬሬԦ
𝑭𝑵ሬሬሬሬሬԦ 𝒇ሬԦ
𝑭𝒈ሬሬሬሬሬԦ
𝑭𝑨𝑩ሬሬሬሬሬሬሬԦ
B:
Newton IIIForce pairs
Newton III Force Pairs
Newton ILaw of Inertia
An object will remain in a state of rest, or move with a constant
velocity in a straight line, unless a net force is exerted on the object.
Newton ILaw of Innertia
In Symbols:𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬԦ= 𝟎 𝑵
if
𝒗ሬሬԦ= 𝟎 𝒎∙𝒔−𝟏 ∴ 𝒂ሬሬԦ= 𝟎 𝒎∙𝒔−𝟐
�⃗�=𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕
Newton IInnertia
A property of the object All objects with mass have innertia Bigger mass = More innertia
Newton IIForce and AccelerationIf a net force is exerted on an
object, the object will accelerate in the direction of the force. The
acceleration is directly proportional to the force and inversely
proportional to the mass of the object.
Newton IIForce and Acceleration
In Symbols:𝑭𝑵𝒆𝒕ሬሬሬሬሬሬሬሬԦ= 𝒎𝒂ሬሬԦ
where �⃗� 𝑵𝑬𝑻=𝑵𝒆𝒕𝑭𝒐𝒓𝒄𝒆 (𝑵)𝒎=𝒎𝒂𝒔𝒔 (𝒌𝒈)𝒂=𝒂𝒄𝒄𝒆𝒍𝒆𝒓𝒂𝒕𝒊𝒐𝒏 (𝒎∙ 𝒔−𝟐)
Newton IIForce and Acceleration𝒂ሬሬԦ∝ 𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ
Direct proportionality =
Straight line through origin
𝒂ሬሬԦ
𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ
Newton’s LawsSolving of Problems
1) Separate inclined forces into perpendicular components
2) Draw free body diagrams3) Identify the applicable axis.
Handle the x-axis and y-axis separately.
4) Obtain an equation for𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ
𝒗ሬሬԦ= 𝟎 𝒎∙𝒔−𝟏
Newton I Newton II
𝒂ሬሬԦ≠ 𝟎 𝒎∙𝒔−𝟐
5) Decide which law you will use:
𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ= 𝟎 𝐍 𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ= 𝒎𝒂ሬሬԦ
Newton’s LawsSolving of Problems
�⃗�=𝒄𝒐𝒏𝒔𝒕𝒂𝒏𝒕
6) Check for friction:μ is given
YES NO
Calculate with:𝑭𝑵𝑬𝑻ሬሬሬሬሬሬሬሬሬሬԦ= 𝑭ሬሬԦ
Calculate 𝑭𝑵ሬሬሬሬሬԦ
Calculate with:𝒇ሬԦ 𝒇ሬԦ= 𝝁𝑭𝑵ሬሬሬሬሬԦ
𝒇ሬԦ
Newton’s LawsSolving of Problems
Example 1A Block, mass 5 kg, is being pulled across a horizontal table with a constant force of 50 N. The magnitude of the friction is, 20 N. Calculate:a) The acceleration of the block.b) The friction coefficient between the
block and the table.
Newton’s Laws
A boy pushes a crate of 50 kg over the floor with the aid of a rod that makes an angle of 40° with the horizontal. He exerts a force of 300 N on the rod. The friction coefficient between the crate and the floor is 0,2.
Newton’s Laws
Example 2
a) Calculate the friction between the floor and the crate.
b) Calculate the acceleration of the crate.
300 N
40°
Example 2Newton’s Laws
A person ski’s down a slope that makes an angle of 40° with the horizontal. The total mass of the skier and his ski’s is 50 kg. The friction coefficient between the snow and the ski’s are 0,1.a) Calculate the net force the person
experience parallel to the surface.b) Calculate the acceleration of the skier.
Example 3Newton’s Laws
The diagram shows a 3 kg block (B) and a 2 kg block (A) that is being pushed forward by a force of 30 N so that the system accelerates to the right. The applied force makes an angle of 15° with the horizontal. Each block experiences a friction force of 5 N.
Example 4Newton’s Laws
15°
a) Calculate the acceleration of the system.
b) Calculate the force that A exerts on B.c) Calculate the force that B exerts on A.
A B
Example 4Newton’s Laws
The friction coefficient between Block B and the table top is 0,034. Assume that the ropes have negligible mass and that the pulleys are frictionless. Calculate:a) The acceleration of the system.b) The tension in the ropes..
Example 5Newton’s Laws
A man with a mass of 70 kg stands on a scale in a lift. Calculate the reading on the scale if the lift:a) Is in rest.b) Moves upward with a constant velocity
of 3,2 m·s-1.c) Accelerates upward at 3,2 m·s-2.d) Accelerates downward at 3,2 m·s-2.e) Freefall
Example 6Newton’s Laws
Universal Gravitation
Between any two objects with mass there exist a gravitation force that is directly proportional to the product of
their masses and inversely proportional to the square of the
distance between their centre points.
Newton’s Laws
Universal GravitationSymbols
= Gravitation force (N) = Mass of objects (kg) = Distance between objects (m) = Universal Gravitation
Constant= 6,67 x 10-11 N·m2·kg-2
�⃗�𝑮=𝑮𝒎𝟏𝒎𝟐
𝒓𝟐
Universal GravitationSymbols: On a Planet
�⃗�𝑮=𝑮𝒎𝑴𝑹𝟐
= Massa van planeet (kg) = Massa van vorrwerp
(kg) = Radius van planeet (m)
Universal GravitationRelationship between G and g
�⃗�𝑮=𝑮𝒎𝑴𝑹𝟐
Attraction force by earth on object:
�⃗�𝒈=𝒎𝒈and
�⃗�𝑮= �⃗�𝒈but
Universele GravitasieGewig
Force with which planets attract an object
Symbol: Unit: N Vector Function of the mass and
radius of a planet
Universal GravitationExample 1
Two spherical objects m1 and m2, with their centre points a distance r metre apart, exerts a gravitation force of 6 N on each other. Determine the magnitude of the force if:a) The mass m1 doubles.b) The distance between them halves.
Universal GravitationExample 2
Two metal spheres with masses 8 x 104 kg and 2 x 103 kg respectively is placed 340 cm apart. Calculate the gravitation force between them.
Universal GravitationExample 3
An astronaut with a mass of 80 kg on the earth lands on planet X with his spaceship. Planet X has a radius that is half that of the earth ant a mass that is double that of the earth.
a) Calculate the value of g on planet X.b) Calculate the garvitation force that the
man experiences on planet X.