1.5 Mechanics
1. The basic terms and measurements used in physics 2. Newton's laws of motion 3. Vectors and vector diagrams and how to calculate resultant forces 4. Turning forces, equilibrium and the orders of levers 5 Stress and strain
1. The basic terms and measurements used in physics 2. Newton's laws of motion 3. Vectors and vector diagrams and how to calculate resultant forces 4. Turning forces, equilibrium and the orders of levers 5 Stress and strain
What are the three fundamental quantities to which all measurements in physics are related?
- length (m)
- mass (kg)
- time (s)
What is the basic unit of mass?
kilogram
m = weight / gravity
Briefly explain the difference between mass and weight.
Weight is the effect of the force of gravity pulling an object (mass) toward the centre of the earth
Weight = mass * gravity
Give a simple definition of gravitational force
What is the force of a person with a mass of 75kg?
F= 75kg * 9.81m/s2
F= 735N
Calculate the weight of a person who has a mass of 62kg
F = 62kg * 9.81m/s2
F= 608N
Momentum is a ...(1)... quantity, and has both ...(2)... and ...(3).
- vector
- direction
- magnitude
What is meant by the resultant?
Forces which have direction as well as magnitude are known as vectors
The sum of two or more vectors is a single vector which is called their resultant.
Find by drawing or calculation the resultant of two forces each of 5N acting at a point at an angel of 60° with each other
8.7N
Give another name for a turning force
torque
Describe how a couple works
This is a pair of equal and opposite forces acting at a distance about the centre of rotation of the object to which the moment is applied.
A weight of 300N is placed on one end of a seesaw. Teh distance from the fulcrum to the weight is 2m.
The lever arm at the other side of the seesaw measures 2.5m.
How much force must be applied to lift the weight?
F1 = 300N * 2.0m = 600Nm
F2 = 600Nm / 2.5m = 240Nm
If a weight of 150N is placed on a seesaw 1m from the fulcrum and another weight of 300N is placed at the opposite end 0.5m from the fulcrum, will the system balance?
Yes:
F1 = 150N * 1m = 150Nm
F2 = 300N * 0.5m = 150Nm
Give common example of first-order levers
muscles that extend the neck
Give common example of second-order levers
In performing plantar flexion, the calf muscles act across a second class lever
Give common example of third-order levers
Biceps brachii which flexes the elbow
How is direct stress measured?
By the ration:
load/cross-sectional area
Describe shear stress.
Shear stress results from external forces acting parallel to the plane on which shear occurs
Calculate the contact stress in pascals of a 13N load acting over an area of 0.7m2
p = 13N / 0.7m2
p = 18.5 Pa
A metal bar of 13cm in length is streched out to a length of 14.78cm by a 10kg weight.
What is the measure of strain that it has ungergone?
Direct Strain = change in length / original length
Direct Strain = 1.78cm / 13cm = 0.136
= 13.6%
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Cartes-fiches
What are the three fundamental quantities to which all measurements in physics are related?
- length (m)
- mass (kg)
- time (s)
What is the basic unit of mass?
kilogram
m = weight / gravity
Briefly explain the difference between mass and weight.
Weight is the effect of the force of gravity pulling an object (mass) toward the centre of the earth
Weight = mass * gravity
Give a simple definition of gravitational force
What is the force of a person with a mass of 75kg?
F= 75kg * 9.81m/s2
F= 735N
Calculate the weight of a person who has a mass of 62kg
F = 62kg * 9.81m/s2
F= 608N
Momentum is a ...(1)... quantity, and has both ...(2)... and ...(3).
- vector
- direction
- magnitude
What is meant by the resultant?
Forces which have direction as well as magnitude are known as vectors
The sum of two or more vectors is a single vector which is called their resultant.
Find by drawing or calculation the resultant of two forces each of 5N acting at a point at an angel of 60° with each other
8.7N
Give another name for a turning force
torque
Describe how a couple works
This is a pair of equal and opposite forces acting at a distance about the centre of rotation of the object to which the moment is applied.
A weight of 300N is placed on one end of a seesaw. Teh distance from the fulcrum to the weight is 2m.
The lever arm at the other side of the seesaw measures 2.5m.
How much force must be applied to lift the weight?
F1 = 300N * 2.0m = 600Nm
F2 = 600Nm / 2.5m = 240Nm
If a weight of 150N is placed on a seesaw 1m from the fulcrum and another weight of 300N is placed at the opposite end 0.5m from the fulcrum, will the system balance?
Yes:
F1 = 150N * 1m = 150Nm
F2 = 300N * 0.5m = 150Nm
Give common example of first-order levers
muscles that extend the neck
Give common example of second-order levers
In performing plantar flexion, the calf muscles act across a second class lever
Give common example of third-order levers
Biceps brachii which flexes the elbow
How is direct stress measured?
By the ration:
load/cross-sectional area
Describe shear stress.
Shear stress results from external forces acting parallel to the plane on which shear occurs
Calculate the contact stress in pascals of a 13N load acting over an area of 0.7m2
p = 13N / 0.7m2
p = 18.5 Pa
A metal bar of 13cm in length is streched out to a length of 14.78cm by a 10kg weight.
What is the measure of strain that it has ungergone?
Direct Strain = change in length / original length
Direct Strain = 1.78cm / 13cm = 0.136
= 13.6%
