Chapter 6 - Forces in equilibrium

Question 1

What is the difference between a vector and a scalar?

Answer 1

A vector is a physical quantity that has magnitude and direction but a scalar only has magnitude.

Question 2

Give examples of a vector

Answer 2

• Displacement
• Velocity
• Acceleration
• Force

Question 3

Give examples of scalars

Answer 3

• Distance
• Speed
• Mass

Question 4

How do you draw vectors?
And how do you find the resultant?

Answer 4

• Free body diagram or tip-to-tail diagram
• Find the resultant using pythagoras/ trigonometry. (find angle using trigonometry)

Question 5

Find the magnitude and direction of the resultant force of 10N acting vertically up and 12N horizontally.

Answer 5

resultant^2 = 10^2 +12^2
resultant = 15.62N
direction - tan angle = O/A
angle = 39.8 degrees

Question 6

What 2 components can a force be resolved into?

Answer 6

Horizontal and vertical

Question 7

How do you work out the horizontal component?

Answer 7

H x cos angle

Question 8

How do you work out the vertical component?

Answer 8

H x sin angle

Question 9

What does in equilibrium mean in terms of forces?

Answer 9

When forces acting on each other are equal and opposite.

Question 10

If 3 forces that act on an object are balanced (resultant = 0) what does this tell you?

Answer 10

Resultant of any two forces equal is equal and opposite to the 3rd force.

Question 11

How do you check that the combined effect of 3 forces is zero?

Answer 11

• Resolve each force along the same parallel and perpendicular line and balance the components along each line.

Question 12

Define the moment of a force

Answer 12

Force multiplied by the perpendicular distance from the line of action to the pivot.

Question 13

Give the equation for moment and give unit

Answer 13

moment = force x perpendicular distance from line of action of force to pivot.
unit :Nm

Question 14

State the principle of moments

Answer 14

For an object to be in equilibrium, the sum of the anticlockwise moments = the sum of the clockwise moments.

Question 15

What is the centre of mass?

Answer 15

• Point through which a single force on the body has no turning effect.
• Point where we consider the weight of the body to act.

Question 16

How do you find the centre of mass of a 2D triangular card?

Answer 16

Suspend piece of card and thread of a plumb line on a clamp stand. Draw pencil line along plumb line. The centre of mass is where the lines drawn on the card cross.

Question 17

Where will the centre of mass of a uniform object be?

Answer 17

Exactly at the middle

Question 18

What must the support force equal when an object in equilibrium is supported at one point only?

Answer 18

Equal and opposite to the total downward force acting on the object.
Balances moments from pivot

Question 19

What does Sx equal?

Answer 19

Sx = wdy/D

Question 20

What is a couple?

Answer 20

Pair of equal and opposite forces acting on a body, but not along the same line (but coplanar)

Question 21

How do you find the moment of a couple?

Answer 21

Moment = force x perpendicular distance between lines of action of forces

Question 22

What happens to a body in stable equilibrium when it’s displaced and released? And why?

Answer 22

Returns to equilibrium position as centre of mass of the object is directly below the point of support when the object is at rest. However, when displaced the line of action of the weight no longer passes through the point of support, so the weight returns the object to equilibrium.

Question 23

What causes a tilted object to topple?

Answer 23

If the line of action of weight (from the centre of mass) passes beyond the base of the object

Question 24

Is a object more or less stable with a lower centre of mass?

Answer 24

More stable. Less likely for line of action of weight to go beyond base.

Question 25

What do 3 forces in equilibrium form in terms of tip-to-tail diagrams.

Answer 25

Closed triangle. Resultant = 0