MECHANICS A

Question 1

Define what a couple is.

Answer 1

A couple is 2 equal forces acting in opposite directions

Question 2

Define the principle of moments.

Answer 2

For an object to be in equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments about any point.

Question 3

Do couples cause a resultant linear force?

Answer 3

No, they do not. They only have rotational effect.

Question 4

How do you calculate the moment of a couple?

Answer 4

The force x the perpendicular distance between the lines of action of the forces.

Question 5

How do you determine the centre of mass of a regular object?

Answer 5

Draw on the lines of symmetry
The COM is at the point of intersection.

Question 6

How would you determine the COM of an irregular object?

Answer 6

Suspend the object freely from a point.
Draw a vertical line downwards on the object from the point of suspension, using a plumb bob
Repeat for a different point
The COM is at the point of intersection.

Question 7

Trucks, cars and double-decker buses all have different base areas. What determines if a vehicle will topple over?

Answer 7

An vehicle(object) will topple over when a vertical line drawn downwards from it’s centre of mass falls outside it’s base area.

Question 8

State N1L.

Answer 8

An object will remain at rest, or continue at a constant velocity, unless a resultant force is acted upon it.

Question 9

State N2L.

Answer 9

The acceleration of an object is directly proportional to, and in the same direction as, the resultant force applied to the object, and inversely proportional to it’s mass.

Question 10

State N3L.

Answer 10

When two objects interact, the forces they exert on each other are equal and opposite.

Question 11

When a parachutist jumps out of an airplane, they accelerate at a decreasing rate until they reach a terminal velocity.
Explain why.

Answer 11

• initially, the only force acting on her is her weight downwards, so she accelerates down
• as her speed increases, the resistive force of drag increases, which acts in the opposite direction to the driving force, resulting in a smaller resultant force downwards, and a smaller acceleration downwards
• eventually, the force of drag is equal in size and opposite to her weight, meaning no resultant force and thus no acceleration (N1L)