Download 1 Chapter 4: Forces and the Laws of Motion pages 119 144 Date __

Chapter 4: Forces and the Laws of Motion pages 119 ­ 144 Date _____
Introduction to Forces
A force is an action exerted on a object which may change the object's state of motion.
Forces can cause objects to In each case the force is responsible for a change in velocity with respect to time which is
___________________________________.
Notice that a force MAY change the object's state of motion. That means that other forces acting on this object may cancel the original force on the object so there may not be a change in motion.
Forces can act through contact or at a distance.
Field forces (Objects NOT in contact)
Contact Forces
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The SI unit of force is newton (N).
When a force acts on a 1 kg mass and it accelerates 1 m/s2, 1 newton of force has been applied. One average apple or the meat in a quarter pounder weigh about 1 newton.
When you push on a toy car, it accelerates. If you push harder, the acceleration is greater. The acceleration of the car depends on the force's magnitude. The direction the car moves depends on the force's direction.
Force is a __________ quantity.
Forces in the same direction ADD.
Forces in the opposite direction SUBTRACT.
Since more than one force may act on an object, the vector sum of all the forces acting on the object is
the net force. Since force is a vector quantity both the ____________
and the ____________ of the net force are important.
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Free body diagrams
Free body diagrams are diagrams to show the relative magnitude and direction of all forces acting on an object.
The tail of the arrow is from the center of the object on which the force is acting.
The size of the arrow reflects the __________________ of the force and the direction of the arrow shows the ________________ which the force is acting.
The object is represented by a box and the force arrow is drawn from the box outward in the direction in which the force is acting.
A laptop is sitting on a table. The force due to gravity (Fg) is pulling the
laptop down. This can't be the only force
or the laptop would be falling to the ground.
The normal force (FN) is acting against the force of
gravity to hold the laptop up. The table is strong enough to hold up the book.
The normal force is exerted upwards by a surface (like a table or a floor) and is perpendicular to the surface. When there is no vertical acceleration, the force due to gravity, Fg, EQUALS the normal force, FN.
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Type of forces.
Fg The force due to gravity on an object.
Fa An applied force on an object like a push or pull.
Ft A tension force transmitted through a string or rope when it is pulled tight.
FN The normal force (or support force) exerted upwards by a surface (like a table or a floor) and is perpendicular to the surface.When an object is sitting on a level surface then the normal force is always equal and opposite of the weight of the object. Ff The resistance to motion that occurs whenever two materials or media are in contact with each other. Fnet The vector sum of all the forces acting on an object.
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A bag of groceries sits on a table. What are the forces acting on the bag?
Draw the free body diagram.
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A horse pulls a sleigh to the right. What are the forces acting on the sleigh?
Draw the free body diagram.
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Observe the forces acting on the shoe when a person holds it by its laces.
Draw the free body diagram.
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Calculating the Net Force
Once the free body diagram is constructed, the net force on the object can be calculated.
Remember, forces in the same direction, ____________ and forces in the opposite direction, ________________.
Ff = 10 N
upward
Fg = 20 N
downward
Fnet = FN = 50 N
upward
Fg =50 N
downward
Fnet = FN = 30 N
upward
Ft = 35 N
right
Ff = 10 N
left
Fa = 60 N
right
Fg = 30 N
downward
Fnet = 8
Balanced Forces: When the net force on an object is ZERO, the object experiences _____________ ___________
and there is NO CHANGE in motion. The object is in equilibrium.
Does an object have to be at rest to be in equilibrium? Why or why not?
No ________________ in motion
Unbalanced Forces: When the net force on an object is NOT ZERO, the object experiences an unbalanced force and the object ______________________.
Block _________________ left
Block _________________ right
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Balanced or Unbalanced Forces?
The object below is moving at a constant velocity to the right when the following forces are applied. Does it experience balanced or unbalanced forces?
5N left
5N right
What is the affect of the forces on the motion of the object?
The object below is moving at a constant velocity to the left when the following forces are applied. Does it experience balanced or unbalanced forces?
3N left
4N left
What is the affect of the forces on the motion of the object?
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Calculating net force
I push my beloved Physics book across the table. I push with 29 N of force to the right and there is a force due to friction of 8 N between the table and the book. Draw the free body diagram and calculate the net force.
What is the affect of the forces
on the motion of the book?
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Draw the free body diagrams for the objects below:
A bowling ball is hanging from a rope.
A stretched spring.
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The SI unit of force is newton (N).
When a force acts on a 1 kg mass and accelerates it 1 m/s2, 1 newton of force has been applied. One average apple or the meat in a quarter pounder weigh about 1 newton.
Force is a __________ quantity.
Since more than one force may act on an object, the vector sum of all the forces acting on the object is
the net force. Since force is a vector quantity both the ____________
and the ____________ of the net force are important.
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We know that there will be a force due to gravity ﴾Fg﴿ pulling the laptop down, but if that
was the only force it should be dropping down towards the ground.
●
.
.
normal force ﴾FN ﴿. A normal force is exerted
●
upwards by a surface ﴾like a table or a floor﴿ and is perpendicular to the
surface.
7/
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Force Diagrams
Free­body diagrams are diagrams to show the relative magnitude and direction of all forces acting on an object.
The tail of the arrow is attached to the object on which the force is acting.
The size of the arrow reflects the __________________ of the force and the direction of the arrow shows the ________________ which the force is acting.
The object is represented by a box and the force arrow is drawn from the box outward in the direction in which the force is acting.
Fapp is the force which is applied to an object.
Ffrict is the force exerted by an surface as the object moves across it or makes an effort to move across it.
Fgrav or Fg is the force of gravity on an object also known as its weight
Fnorm is the support force exerted on an object when it is in contact with another stable object.
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Forces in the same direction add.
Forces in the opposite direction subtract.
The net force is the vector sum of all
the forces acting on an object.
Ftens is the tension force transmitted through a string or rope when it is pulled tight.
Fair is the air resistance force which is a frictional force as an object moves through the air
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Below are four free­body diagrams. For each situation, determine the net force acting on the object.
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Mass versus Weight
Mass (m) is
The mass of an object can be measured on a triple beam balance because the mass of the object is measured compared to known masses. If this balance was taken to the moon the force of gravity on the balance would affect the known mass and the unknown mass in the same manner. The same mass would be measured.
Weight (Fg) is
The weight of an object is measured with a spring scale because the weight
is the force felt by an object caused by another object, typically the Earth. If this scale
was taken to the moon, the force of gravity would be different and the weight would be different.
For an object in free fall (only gravity acting on the object), the weight of an object can be calculated using
The weight of an object depends on its ____________.
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