Forces in Equilibrium

Question 1

Define a scalar

Answer 1

A scalar is any physical quantity that is not directional

Question 2

Give examples of vector quantities

Answer 2

Displacement
Velocity
Acceleration
Momentum
Force 
Pressure

Question 3

When drawing polygons to add vector quantities, what represents the resultant vector?

Answer 3

The arrow that closes the polygon of vectors

Question 4

What can be said about the object if the polygon of vectors acting upon it is closed?

Answer 4

There is no resultant vector

Question 5

Define a moment

Answer 5

The force applied to an object, making it rotate, multiplied by the perpendicular distance to the line of action of the force from pivot

M = F x d

Question 6

State the principle of moments

Answer 6

If an object is NOT rotating, the sum of al the moments about any point must be equal to ZERO

Question 7

Define an object’s centre of mass

Answer 7

The point from which all of the weight of an object appears to act from.

Question 8

State the 2 conditions and object must have for maximum stability

Answer 8

A large base area

A low centre of mass

Question 9

Define a point object

Answer 9

An object that is small enough that all the forces acting on it can be considered to act at one point.

Question 10

Define coplanar forces

Answer 10

Forces that all act in the same plane

Question 11

Give the 2 conditions required for the static equilibrium of coplanar forces

Answer 11

1) The force vectors acting upon the point object must form a closed polygon (therefore the resolved components of all the force vectors in any two perpendicular directions must be zero).
2) The resultant moment about any point in the plane of the forces must be zero.

Question 12

State the principle of concurrency

Answer 12

The lines of the forces actions must all pass through one point

Question 13

Name the condition required for the static equilibrium f three coplanar forces

Answer 13

The forces acting on the object must be concurrent

N.b. if the forces are concurrent, condition 1 is enough to ensure static equilibrium

Question 14

Define a vector

Answer 14

A vector is any physical quantity that has a direction as well as a magnitude

Question 15

Define a couple

Answer 15

A couple is a pair of equal and opposite forces acting on a body, but not along the same line

Question 16

Define stable equilibrium

Answer 16

If a body in stable equilibrium is displaced then released, it returns to its equilibrium position
(e.g. a coat hanger or a hanging basket)

Question 17

Define unstable equilibrium

Answer 17

If a body in unstable equilibrium is displaced, the displacement will be increased by the centre of mass of the body
(e.g. a plank on an oil drum)

Question 18

Describe when a tilted object will topple

Answer 18

A titled object will topple if it is tilted too far. If the objects line of action of its weight (acting from the centre of mass) passes beyond the pivot, the object will topple over.

Question 19

What is the furthest point an object can be tilted without it toppling over?

Answer 19

The position where the line of action of the objects weight passes through the pivot

Question 20

Explain why a taller uniform object will topple over at smaller angled slopes than smaller uniform objects

Answer 20

Because the lower the centre of mass of an object, the more stable it is
This is because an object with a higher centre of mass doesn’t need to be titled as far for its line of action of its weight to pass through the point it would pivot around

Question 21

For a point object acted on by a three forces, describe the condition required for the object to be in equilibrium

Answer 21

The vector sum of all 3 forces must = 0

F₁ + F₂ + F₃ = 0

Question 22

Give the 2 conditions required for a body to be in equilibrium

Answer 22

1) The resultant force must be 0. If there are only 3 forces, they must form a closed triangle
2) The principle of moments must apply (i.e. the moments of the forces about the same point must balance out)