U4 - Mechanics
When is an object in When there is NO RESULTANT FORCE and MOMENTS BALANCE.
equilibrium?
Thus when ≥ 3 forces act on a body in equilibrium, the resultant of all
but one will be equal and opposite to the last. Similarly, the sum
horizontal and vertical components of all forces is 0.
What is Newton’s First Law “Every body continues at rest or with a constant VELOCITY unless
of Motion (Law of Inertia)? acted upon by a resultant force.”
VELOCITY is key as an object moving in a circle experiences a
centripetal and is thus accelerating. Let’s say you had a cup on a
table, although it looks stationary, considering the Earth/Table
perspective, it isn’t as it’s weight is slightly greater than the normal
force as it’s experiencing a centripetal force due to the Earth’s
rotation. However, we assume they’re the same.
What is Newton’s Second “The resultant force is proportional to the rate of change of
Law of Motion? momentum and acts in the same direction as the change in
momentum.”
F = ma is a special case of where mass is constant. This cannot be
used for a rocket.
Remember that the change in momentum has to be constant. E.g.,
going from 5 kgms-1 to 0 kgms-1 exerts some force whose size is
dependent on the time in which this change occurs. Thus cushions
work better than harder surfaces such as steel.
, What is Newton’s Third Law “For every action, there is an equal and opposite reaction.”
of Motion?
This is a result of electrostatic repulsion between electrons. When you
“touch” stuff, you’re not actually touching it. Your hand is slightly
hovering above it.
What must the forces in Of the same type.
Newton’s Third Law be?
Give an example of Newton’s ● By walking, you push the earth backwards yet the earth
Third Law and explain the pushes you forward (somewhat like a car)
importance of frictional ● This is a result of frictional forces. Without them, you couldn’t
forces walk (like on ice) or even hold a ladder against a wall.
What happens when a ball ● It will experience a change in momentum thus exerting a
hits a wall with reference to force on the wall.
Newton’s Laws? ● The wall will exert an equal and opposite force on the wall.
What is a moment defined A force multiplied by the perpendicular distance between the pivot
as? and the line of action of the force.
In essence, we’re looking at the turning effect of a force. When we
take moments about a point, we can ignore the force acting at the
point. So never pick a point where you’d have to consider all distances
and forces.
What is the principle of “For any system IN EQUILIBRIUM, sum of clockwise moments = sum
moments? of anticlockwise moments.”
What is a couple defined as ● A pair of equal and opposite coplanar forces.
and how is the moment of a ● Force × perpendicular distance between the lines of action of
couple calculated? the forces.
When is an object in When there is NO RESULTANT FORCE and MOMENTS BALANCE.
equilibrium?
Thus when ≥ 3 forces act on a body in equilibrium, the resultant of all
but one will be equal and opposite to the last. Similarly, the sum
horizontal and vertical components of all forces is 0.
What is Newton’s First Law “Every body continues at rest or with a constant VELOCITY unless
of Motion (Law of Inertia)? acted upon by a resultant force.”
VELOCITY is key as an object moving in a circle experiences a
centripetal and is thus accelerating. Let’s say you had a cup on a
table, although it looks stationary, considering the Earth/Table
perspective, it isn’t as it’s weight is slightly greater than the normal
force as it’s experiencing a centripetal force due to the Earth’s
rotation. However, we assume they’re the same.
What is Newton’s Second “The resultant force is proportional to the rate of change of
Law of Motion? momentum and acts in the same direction as the change in
momentum.”
F = ma is a special case of where mass is constant. This cannot be
used for a rocket.
Remember that the change in momentum has to be constant. E.g.,
going from 5 kgms-1 to 0 kgms-1 exerts some force whose size is
dependent on the time in which this change occurs. Thus cushions
work better than harder surfaces such as steel.
, What is Newton’s Third Law “For every action, there is an equal and opposite reaction.”
of Motion?
This is a result of electrostatic repulsion between electrons. When you
“touch” stuff, you’re not actually touching it. Your hand is slightly
hovering above it.
What must the forces in Of the same type.
Newton’s Third Law be?
Give an example of Newton’s ● By walking, you push the earth backwards yet the earth
Third Law and explain the pushes you forward (somewhat like a car)
importance of frictional ● This is a result of frictional forces. Without them, you couldn’t
forces walk (like on ice) or even hold a ladder against a wall.
What happens when a ball ● It will experience a change in momentum thus exerting a
hits a wall with reference to force on the wall.
Newton’s Laws? ● The wall will exert an equal and opposite force on the wall.
What is a moment defined A force multiplied by the perpendicular distance between the pivot
as? and the line of action of the force.
In essence, we’re looking at the turning effect of a force. When we
take moments about a point, we can ignore the force acting at the
point. So never pick a point where you’d have to consider all distances
and forces.
What is the principle of “For any system IN EQUILIBRIUM, sum of clockwise moments = sum
moments? of anticlockwise moments.”
What is a couple defined as ● A pair of equal and opposite coplanar forces.
and how is the moment of a ● Force × perpendicular distance between the lines of action of
couple calculated? the forces.
