Mechanics Flashcards
What is a Couple
A couple is a pair of coplanar forces (meaning they are forces within the same plane), where the two forces are equal in magnitude but act in opposite directions.
How to calculate the moment of a couple
Moment of a couple = Force X Perpendicular distance between the lines of action of forces
State the Principle of Moments
For an object in equilibrium, the sum of anticlockwise moments about a pivot is equal to the sum of clockwise moments.
What is the center of mass of an object
The point at which the objects mass acts
When does terminal speed occur
When the frictional forces acting on an object and driving forces are equal, therefore there is no resultant force and so no acceleration so the object travels at a constant speed.
How does a skydiver reach terminal velocity
As they leave the plane they accelerate because their weight is greater than the air resistance. As the skydiver’s speed increases, the magnitude of air resistance also increases. This continues until the force of weight and air resistance become equal, at which point terminal velocity is reached.
How does air resistance affect a projectiles motion
As speed increases, air resistance increases It affects both horizontal and vertical motion The maximum height is reached earlier, and the vertical and horizontal distance traveled decreases.
What is Newton’s 1st Law
An object will remain at rest or traveling at a constant velocity until it experiences a resultant force.
What is Newton’s 2nd Law
The acceleration of an object is proportional to the resultant force experienced by the object: F = ma
What is Newton’s 3rd Law
For each force experienced by an object, the object exerts an equal and opposite force.
How is the calculation of impulse used in car safety features
Crumple zones that crumple on impact Seat belts that stretch on impact Airbags that expand on impact These all decrease the impact of the passenger, decreasing the force applied on the passenger
What is Hookes Law
Extension is directly proportional to the force applied, given that the environmental conditions (e.g temperature) are kept constant.
How is an object following Hookes Law shown in a F/E graph
Straight line through the origin
What is the Limit of Proportionality
The point after which Hooke’s Law is longer obeyed
What is the Elastic Limit
If you increase the force beyond this point, the material will plastically deform
What does a material’s spring constant represent
Measure of the stiffness of a spring
What is Tensile Stress
Force applied per unit cross-sectional area
What is Tensile Strain
Extension over the original length
What is elastic strain energy, how do you calculate it
Work done on a material to stretch / compress it Area under an F/E graph 0.5xFxchange in length
What is breaking stress
The value of stress at which a material will break apart
What is a plastic / ductile material
Where a material will experience a large amount of extension as the force is increased beyond the elastic limit
What is a brittle material
Where a material will extend very little, likely to break after little extension
What is the F/E graph of a material that has passed its elastic limit. What does the area between the curves represent
The lines will be parallel as the spring constant is the same Work done to permanently deform the material
What is a materials ultimate tensile stress
The maximum stress a material can withstand The highest point on a stress / Strain graph
What is Youngs Modulus (E)
Measures the stiffness of a material It is constant up to the limit of proportionality The gradient of a stress / strain graph
How to calculate Youngs Modulus
E = FL ÷ Change in L x A
Stress ÷ Strain
Describe the Stress / Strain graph for a Brittle, Ductile and Plastic material
How to find the density of an alloy
The total mass of all materials in the alloy (Sum of the masses of each material)
÷
Total Volume
How to find the force needed to stretch 2 springs in parallel
The sum of the force applied to stretch each spring
Their extensions will be the same
How to find the extension of 2 springs in series due to a weight W
Extension = W ÷ Effective Spring Constant
Effective Spring Constant: k-1 = k-1 (p) + k-1 (q)
