Transcript Forces - Images

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A force is defined simply as a push or a pull on
an object
A force is a VECTOR quantity
Units: lbs or Newtons (N)
 1 lb = 4.45 Newtons
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What is your weight in Newtons? _________
Net Force – the sum of all the forces acting on
an object.
Force
Symbol for
Force
Direction of Force
Friction
Ff
Parallel to surface and
opposing motion
Normal
FN
Perpendicular and away from
surface
Tension
FT
Along the rope away from
object
Weight
FW
Straight down towards the
center of Earth
Applied
F
What ever direction you push
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Internal Forces – we wont do math with these
 Electromagnetic (electroweak) Interactions –
holds atoms & molecules together, responsible for
all contact forces
 Strong Nuclear Interactions – hold nucleus
together
 Weak Nuclear Interactions – radioactive decay
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A drawing that shows all of the forces on an
object
EXAMPLES:
 A flowerpot falls freely from a windowsill. (Ignore
air resistance.)
 A cable pulls a crate at a constant speed across a
horizontal surface. The surface DOES have friction
 A rope lifts a bucket at a constant speed. (Ignore
air resistance.)
An object that is at rest will remain at rest or an
object that is in moving will continue to move in a
straight line with constant speed, if and only if the
net force acting on that object is zero.
 Law of inertia – inertia = resistance to change in
motion or direction
 Equilibrium – net force = 0, moving at a constant
velocity
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Restate Newton’s 1st law IN YOUR OWN WORDS.
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The force on an object is equal to the object’s
mass times its acceleration
FNET = m*a
 Units:
▪ F=N
▪ m = kg
▪ a = m/s2
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What is the net force acting on a 1.0 kg ball in
free-fall?
 9.8 N
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Two people are pushing a stalled car. The mass
of the car is 1850 kg. One person applies a force
of 275 N to the car, while the other applies a
force of 395 N. Both forces act in the same
direction. A third force of 560 N also acts on the
car, but in a direction opposite to that in which
the people are pushing. This force is caused by
friction between the tires and the pavement.
Find the acceleration of the car.
+0.059 m/s2
All forces come in pairs. The two forces in the pair
act on different objects and are equal in magnitude
and opposite in direction: FA on B = - FB on A
 Example:
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Accelerating a Car – the wheels exert a backwards force on
the road. (friction) The road responds with a forward force
on the car.
 When a ball has been thrown, the force of the
hand that threw it remains on it. No, the force of the
hand is a CONTACT force.
 A force is needed to keep an object moving. No net
force = constant velocity. Coasting car.
 Inertia is a force. No it’s a property of matter!
 Air does not exert a force. Air pressure is VERY high.
But exerted on all sides.
 The quantity ma is a force. No, its just been proven
experimentally that the two quantities in F = ma are equal.
Directly proportional.
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What is mass?
 The mass of an object is a quantitative measure of
inertia
 How much matter an object contains
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What is weight?
 The downward force on an object’s mass caused
by gravity.
 Points directly towards the center of the earth.
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Weight still exists when you are not on Earth,
but decreases the further away you get
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Mass is measured with a balance. Mass does
not change no matter where you measure it.
Weight is measured with a scale. Weight
changes as gravity changes.
 Scales – contain a spring which exert an upward
force to create equilibrium. The force of the spring
= the force due to gravity.
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Mass –
 Calculated with density, D = m/V
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Weight –
 Calculated with Newton’s 2nd Law, F = m*a
▪ Where a = g = 9.8 m/s2
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(p 97 #16) Kamaria is learning how to iceskate. She wants her mother to pull her along
so that she has an acceleration of 0.80 m/s2. If
Kamaria’s mass is 27.2 kg, with what force
does her mother need to pull her? (Neglect
friction)
22 N
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Changes in weight due to other forces acting
on a body
When the net force is downwards, your
apparent weight DECREASES
When the net force is upwards, your apparent
weight INCREASES
Would you ‘appear’ lighter or heavier?
 Riding a roller coaster downwards at a high speed
 When an elevator first starts to move upwards
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APPARENT weightlessness
 Because you do still have a weight!
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When the acceleration downwards equals the
acceleration due to gravity (9.8)
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The component of force that a surface exerts
on an object with which it is in contact
The normal force is ALWAYS perpendicular to
the surfaces in contact.
This is the force that keeps you from falling
through your chair.
Normal Force = weight
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Static Friction – no relative motion
 Pushing a heavy box and it doesn’t move
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Static Friction Force
 The Ff(static) is more than zero, but less than μsFN
 Ffs = μsFN
▪ μs = coefficient of static friction. This is a constant that
depends on the surfaces in contact.
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(p 128 #18) You need to move a 105 kg sofa to
a different location in the room. It takes a
force of 102 N to START it moving. What is
the coefficient of static friction between the
sofa and the carpet?
102N = μs * (105kg *9.8m/s2)
0.0991
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Kinetic Friction – between two surfaces in
motion
Our Model for Friction – We assume (pretend)
that friction depends only on the material of
the 2 surfaces in contact.
Ff(kinetic) = μkFN
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(p 128 #17) A girl exerts a 36 N horizontal
force as she pulls a 52 N sled across a cement
sidewalk at constant speed. What is the
coefficient of kinetic friction between the
sidewalk and the metal sled runners?
36N=μk * 52N
0.69
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Tension force is the force applied through a
rope
We assume that the force applied at one end
of the “massless” rope is transmitted
completely to the other end of the rope
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Air resistance/drag is a type of friction force
This force DOES depend on the speed of an
object and the size/shape of the object
The drag force increases as an object’s speed
increases
When the drag force = g, then there is no net
force and no more acceleration. This is called
TERMINAL VELOCITY.
This is the highest velocity a falling object
reaches!
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Exists between ALL objects
Depends on the objects masses and the
distance between them
Law of Universal Gravitation