Download Forces - Physics-S3

Forces
Types of Forces
Balanced / Unbalanced
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Different forces
A force, (a push or a pull) occurs when 2 objects interact with each other.
Forces always act in PAIRS, whenever one object exerts a force on another
it always experiences an EQUAL and OPPOSITE force in return.
Forces are VECTOR quantities, they have a Magnitude (SIZE) and a DIRECTION.
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Forces.
• GRAVITY (weight) – two masses are attracted to each
other
• AIR RESISTANCE (drag) – the air tries to slow down a
skydiver by pushing upwards against him/her. The skydiver
pushes the air out of the way with an equal and opposite
force.
• Rockets and jet engines – the engine pushes gas
backwards (action) and the gas pushes the rocket forwards
(reaction)
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Measuring magnetic forces
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Object falling freely
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Object resting on a table
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Reaction of surfaces
The bag squeezes the foam until the upward force of the springy foam
on the bag exactly balances the downward gravity force on the bag
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Balanced/Unbalanced forces
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Alternative version
A
B
Advantage:
reduced friction so use a small difference between
the two masses
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Method 1
1. Use a level runway or bench. Connect the light gates to the
data logger to measure velocity at each light gate.
2. Push the trolley from each end to check if the runway or
bench is level; make adjustments as necessary.
3. Fix the bench pulleys to the runway or the bench and
connect string to the trolley and both the mass hangers.
4. With no slotted masses on the hangers release the trolley
and the mass hangers, first when trolley is not moving, then
given a push in either direction.
Comments?
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Method 1
1. Use a level runway or bench. Connect the light gates to the
data logger to measure velocity at each light gate.
2. Push the trolley from each end to check if the runway or
bench is level; make adjustments as necessary.
3. Fix the bench pulleys to the runway or the bench and
connect string to the trolley and both the mass hangers.
4. With no slotted masses on the hangers release the trolley
and the mass hangers, first when trolley is not moving, then
given a push in either direction.
No movement if trolley at rest
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Method 2
1. With no slotted masses on the hangers release the trolley
and the mass hangers, first when trolley is not moving, then
given a push in either direction.
2. Repeat step 4 with 100g on each mass hanger and then
200g.
What conclusion can be made from these results?
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Method 2
1. With no slotted masses on the hangers release the trolley
and the mass hangers, first when trolley is not moving, then
given a push in either direction.
No movement if trolley at rest
Same speed or velocity at each light gate if given a push.
2. Repeat step 4 with 100g on each mass hanger and then
200g.
What conclusion can be made from these results?
With equal masses hanging at each end;
if the trolley is initially stationary it remains so,
if it is given a push then its velocity remains the same.
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Action of gravity and the pulley
• The force of gravity is pulling the masses downwards and the
pulleys change the direction of these forces to be horizontal
and parallel to the runway or bench.
• Strings can ONLY PULL so the two forces acting on the trolley
are in opposite directions.
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Conclusion
• The force of gravity is pulling the masses downwards and the
pulleys change the direction of these forces to be horizontal
and parallel to the runway or bench.
• Strings can ONLY PULL so the two forces acting on the trolley
are in opposite directions.
With equal forces acting in opposite directions on the trolley;
if the trolley is initially stationary it remains so, and
if it is given a push then its velocity remains the same.
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Method 3
3. Now add 100g to one of the mass hangers, with no extra
masses on the other, i.e. the total mass is 200g one end and
100g the other. Release the trolley from the end with the
smaller moving mass, measuring its velocity as it passes the
light gates.
4. Add one 100g mass to each mass hanger, making the hanging
masses 300g and 200g. Release the trolley from the end with
the smaller moving mass, again measuring its velocity as it
passes the light gates.
5. Repeat with total masses of 400g and 300g.
Conclusions ?
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Final Conclusion
• The trolley speeds up in the same direction as
that of the larger falling mass, i.e. in the
direction of the larger force, the driving force
acting on the trolley.
• The larger force is the Driving force
• The smaller force is the Counter force
• The difference is the Resultant force
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Effects of resultant force
Comment on the motion of the
object while the R.F. acts.
a. ?
b. ?
c. ?
d. ?
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Effects of resultant force
Comment on the motion of the
object while the R.F. acts.
a. Initially object stationary, velocity
of object increases, same
direction as the force.
Car at traffic lights
b. Initially object is moving in same
direction as the R.F, velocity of
object increases, same direction
as the R.F
Car preparing to overtake
c. Initially object is moving in
opposite direction to R.F, object
accelerates in direction of R.F.
Car preparing to stop
d. Initially object turning, object
accelerates in direction of R.F,
while still turning
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Forces on a cycle
Tim is cycling along on his bicycle.
The horizontal forces are:
• On each of the following diagrams:
• draw arrows to show the direction of these two forces and
where they act
• the length of the arrow should show the size of each force
• label each arrow to show which force it is
Forces on a cycle – 1 and 2
1.
Tim has just started off. He is pedalling to
make the bike move faster.
2.
Tim is now cycling along at a steady speed.
Forces on a cycle – 3, 4, and 5
3.
He wants to go a bit faster to get to his destination
sooner. So he pedals harder to speed up.
4.
Tim is now going as fast as he can. His speed is
steady again, but faster than it was before.
5.
Tim finds it too tiring pedalling so hard, so he eases off.
He is slowing down to a more comfortable speed.
Muddy problem-2
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Who wins – Red or Blue?
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Who wins – Red or Blue?
= 950 N
R
= 1000 N
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Force diagrams
Size ? Direction ?
The size of the arrow represents the size of force.
The direction of the arrow shows the direction of the force.
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Force diagram-Truck stationary
Size? Direction?
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Force diagram-Truck stationary
Size? Direction?
The truck is moving to the right when the man starts pulling.
Does the truck speed up, slow down or move at the same speed?
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Force diagram-Truck stationary
Size? Direction?
The truck is moving to the left when the man starts pulling.
Does the truck speed up, slow down or move at the same speed?
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Forces acting when an object is at rest
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A falling apple
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Size of force on an apple
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Gravity as seen by Matt in the Telegraph
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Streamlines
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Forces acting on a parachute
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Terminal velocity
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Motion of parachute
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Forces during flight
In order for an object to fly in a
stable manner, it needs to
balance 4 forces:
Lift
Drag
Thrust
Weight
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Forces during flight
In order for an object to fly in a
stable manner, it needs to
balance 4 forces:
Lift
Drag
Thrust
Weight
Weight comes from gravity pulling down on the object.
Thrust is a force that pushes the forward. It can be generated by a propeller, a
rocket, a catapult- anything that makes the object move.
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Forces during flight
In order for an object to fly in a
stable manner, it needs to
balance 4 forces:
Lift
Drag
Thrust
Weight
But Lift and Drag can only arise as air moves past an object.
Lift pushes the object upward, and Drag, a type of air resistance, slows it down.
What causes these forces?
To find out look at the cross section of an aircraft wing-a shape called an aerofoil.
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The causes of Lift
At least 2 forces combine to cause LIFTThe Bernoulli Effect and an example of Newton’s Third Law of Motion.
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Lift: The Bernoulli Effect
A Difference in Pressure
As an aerofoil moves, its shape and angle force oncoming air to curve as it
passes the aerofoil’s top side.
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Lift: The Bernoulli Effect
A Difference in Pressure
Because of this curve, the air above the aerofoil moves farther and faster
than air flowing underneath.
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Lift: The Bernoulli Effect
A Difference in Pressure
As the speed of air increases, its pressure drops.
This is called the Bernoulli Effect, after Daniel Bernoulli, the Swiss
mathematician.
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Lift: The Bernoulli Effect
A Difference in Pressure
The curved airflow generates more pressure below the aerofoil than
above, and the aerofoil is pushed upward.
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Lift: Newton’s Third Law of Motion
Opposing forces
An aerofoil also creates Lift by “bending” or redirecting airflow
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Lift: Newton’s Third Law of Motion
Opposing forces
Oncoming air follows the curved shape of the aerofoil, shifting downward
as it moves past
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Lift: Newton’s Third Law of Motion
Opposing forces
This downward motion causes an opposing force that pushes the aerofoil
up.
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Lift: Newton’s Third Law of Motion
Opposing forces
This is Newton’s Third Law of Motion at work:
Every action has an equal and opposite reaction.
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Drag
As an object moves through air, it encounters a form of resistance
called drag.
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Drag
The amount of drag depends on an object’s size and shape.
A thick, boxy object will cause more drag than a streamlined one.
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Drag
Drag is also influenced by other factors, on of which is the quality of the
object’s surface. A rough surface causes more drag than a smooth
polished one.
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