Forces and Pressures
Force
Ë a push or a pull acting on an object
Ë Force examples:
Ë The SI unit of force: Newton(N)
Ë is measured by a spring balance or dynamometer
Spring balance
Ë The effects of forces:
o Change the shape of an object
o Change the size of an object
o Change the direction of a moving object
o Change the speed of a moving object
o Stop a moving object
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o Make a stationary object moving
Ë Two main groups of forces: contact and non-contact.
Frictional Force
Ë occurs when two objects are in physical contact with each other
Ë a force that acts in the opposite direction and opposes the motion of an object
Ë slows down or stops a moving object
Ë is greater when an object moves on a rough surface than on a smooth surface
Advantages of friction in our daily lives
Disadvantages of friction in our daily lives
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Ways to overcome friction in our daily lives
Gravitational force
Ë Every object on Earth has mass and weight.
Ë Gravity: a force that pulls
Ë The force which acts between two masses without direct in contact with each other
Ë The magnitude of the gravitational force is affected by two factors:
Ë Gravitational field: the region in which an object experiences a gravitational force from
the other object
Ë The gravitational force of the Earth pulls al objects towards its center.
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Ë When an object is placed within the gravitational field of the Earth, it will fall toward its
surface.
Ë Gravitational force causes objects to have weight.
Ë Weight of an object: a measure of the amount of the gravitational force that acts on it
Ë The more mass an object has, the stronger the gravitational force that acts on an it because
the more this object weighs.
Ë Gravitational field strength: the amount of gravitational force acting on a 1 kg mass
Ë The Earth exerts a gravitational force of 10 N on every 1 kg of object.
Ë The symbol for gravitational field strength is g.
w = mg
Ë The weight of an object can be determined by the following expression: w --- weight (unit --> N)
m --- mass (unit --> kg)
weight (N) = mass (kg) × gravitational field strength (N/kg) g --- gravitational field strength
(unit --> N/kg)
Ë The SI unit for weight is the Newton (N).
m=w/g
Calculation involved weight and mass g=w/m
m = 55kg
w=?
g = 10 N/kg
w = mg = 55 x 10
= 550 N #
w = 25.4 N
m=?
g = 10 N/kg
w = mg
m=w/g
= 25.4 N
10 N/kg
= 2.54 kg #
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Difference between mass and weight
Magnetic force
Ë a force of attraction between magnets and magnetic substances
Ë Not all the metals are magnetic substances.
Ë Iron, nickel and cobalt are the common magnetic substances.
Ë Steel, an alloy made of iron mainly, is also a magnetic material.
Ë A typical bar magnet has two poles-North (N) and South (S).
Ë A magnet can exert a force of attraction (pull) or repulsion (push) on another magnet.
Ë Magnetic forces are useful.
o For example, the magnetic needle in a compass indicates the direction to help us
navigate our ways in jungles and oceans.
Pressure
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Force increases --- Pressure increases Area decreases or remain the same ---
Force decreases --- Pressure decreases pressure increases
Force remain the same --- Pressure decrease Area increases --- pressure decreases
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F = 50 N ,
A = 0.4 m^2
P=?
P=F/A
= 50 N / 0.4 m^2
= 500 / 4 Nm^2
= 125 N/m^2
m = 20 kg
P = 80 N/m^2
A =?
g = 10 N/kg
P =F/A
PA = F
A =F/P
F = mg = 20 x 10
= 200 N
A = 200 N / 80 N/
A = 8.0 m^2 , P = 15.0 kN/m^2 , F= ? m^2
P = F / A , F = P x A = 15 kN/m^2 x 8.0 m^2 = 120.0 kN # = 120.0 x 1000 N = 120 000 N = 2.5 m^2
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