Mechanics

Question 1

What is a vector quantity?

Answer 1

Any physical quantity which has both direction and magnitude.

Question 2

Give eg of a vector

Answer 2

Displacement. Velocity. Force. Acceleration.

Question 3

What is a scalar?

Answer 3

Physical quantity which only has magnitude (not direction)

Question 4

Eg of a scalar

Answer 4

Density. Mass. Temperature. Distance. Age.

Question 5

How do we represent vectors.

Answer 5

With arrows.
Length of arrow = magnitude of vector quantity.

Direction of arrow = direction of vector quantity.

Question 6

When can we add vector quantities?

Answer 6

When both arrows are pointing in the same direction.

Question 7

When can we subtract vector quantities?

Answer 7

When two vectors are pointing un opposite directions.

Question 8

What do we do when vectors are PERPENDICULAR to each other?

Answer 8

1) We must use Pythagorus to find the resultant ( hypotenuse)
= magnitude if the vector

2) trig to find the angle / direction.

= Eg) 200km, 53.1 decrees, north east.

Question 9

What does resultant mean?

Answer 9

The combined effect of two forces.

Question 10

Equilibrium

Answer 10

When a body is at rest/ moving at constant speed.

Forces are balanced

Question 11

For there to be be equilibrium two forces must be:

Answer 11

Equal and opposite.

Question 12

for there to be equilibrium 3 forces must:

Answer 12

Equal to 0

= 2 forces must be equal and opposite to the 3rd force

Question 13

Why do we have to consider the direction forces act?

Answer 13

To find out if two forces cancel each other out

= find the resultant force.

Question 14

What conditions result un the turning effect?

Answer 14

When a force is pushes on an object about a pivot.

Question 15

How can the turning effect/ given force be increased?

Answer 15

The further away the force put on the object is from the pivot the more it will turn.

The longer the leaver the smaller the force needed to turn the object.

Question 16

What is moment?

Answer 16

a turning effect produced by a force acting at a distance on an object

Question 17

What is required to balance a force which produces a turning effect?

Answer 17

When the object is acted on by another force acting in the opposite direction.

Sum of clockwise moment = sum of anticlockwise moment.

Question 18

Equation for moment

Answer 18

M = force x distance.

(Greater distance = greater moment)

(Distance = perpendicular distance from line of the force(weight) to the point)

Question 19

What is the centre of mass?

Answer 19

The point on a body through which a single force on the body has no turning effect.

Question 20

Why is the centre of mass important in equilibrium?

Answer 20

When the centre of mass is perfectly balanced:

force down= support force up.

Therefore there us no turning effect.

Question 21

What is a support force?

Answer 21

Force which is equal and opposite to the total downwards force acting on an object.

Question 22

What is a support force?

Answer 22

Force which is equal and opposite to the total downwards force acting on an object.

Question 23

What is the support force on a pivoted body

Single support problem

Answer 23

Total support down = single support force up

S = w1 + w2 + weight of object.

Question 24

How much force is exerted on each support in a two-support problem?

Answer 24

If CofM is directly between both supports= weigh of beam divided equally between both supports.

If CofM is of centre=
Take moments where x is in contact with the beam and then where y is in contact with the beam.

Question 25

Equations to find support force x/y

Answer 25

[ W x d(x/y) ] / D

Question 26

What is a couple?

Answer 26

Pair of equal and opposite forces which act on a body but not along the same line.

Eg) support force x & y

Question 27

Equation for moment of a couple:

Answer 27

Moment if a couple = force x distance between them.

Question 28

Stable equilibrium

Answer 28

Body will return to original pos. when displaced and released.

Eg. Coat hanger on pole

Question 29

Unstable equilibrium

Answer 29

Body move away from support point if body is displaced.

When displacement of CofM is no longer above the support = weight acts to turn body.

Question 30

Tilting

Answer 30

Object at rest acted on by a force = object raised on one side. Around a pivot.

Question 31

Toppling

Answer 31

Tilted object topples if tilted to far.

Question 32

How do you calculate when an object will topple.

Answer 32

When the clockwise momentum (fxd) > the anticlockwise momentum [(wxd) /2]

Question 33

What is the furthest an object can tilt before toppling?

Answer 33

When the line of action of CofM goes directly through the pivot line.

Question 34

What prevents an object from toppling when on a slope?

Answer 34

When the line of action of CofM lies within the base of the vehicle.

Question 35

Why is a vehicle more stable with a lower CofM?

Answer 35

Because the line of action of CofM has a larger area within the wheel base.

Question 36

What are the conditions of equilibrium?

Answer 36

• the resultant force must = 0
(Vectors make rectangle/triangle)

• principle of moment applies: moment forces about a point must balance.

Question 37

How can we predict the forces acting on a body in equilibrium?

Answer 37

• Draw known vectors to scale
• use protractor the find the correct angle for the second vector.

If the body is equilibrium the 3rd vector will join up the 1st two vectors = triangle.