Physics - Moments

Question 1

What type of quantity is weight and what are its units?

Answer 1

Force, Newtons

Question 2

Write down the equation for weight

Answer 2

Weight = mass x gravitational field strength

Question 3

Write down the equation for turning force/moments

Answer 3

Turning force/moment = force x perpendicular distance from the pivot to the force

m (Nm) = F (N) × d_⊥ (m)

Question 4

State the principle of moments

Answer 4

When an object is in equilibrium the sum of the clockwise moments is equal to the sum of the anti-clockwise moments

Question 5

Where are moments used in practical situations

Answer 5

Door handles, spanners, crowbars, cranes, etc.

Question 6

Define the centre of gravity

Answer 6

The centre of gravity of an object as the point at which all of its weight can be considered to act

Question 7

Describe how to experimentally find the centre of gravity of a non-uniform, laminar object

Answer 7

• Place a pin at the corner of the laminar and allow to land.
• Use a thread and bob to hang down and draw a line along the thread.
• Repeat starting at different points.
• Where the lines cross is the CoG

Question 8

Describe how to find the CoG of a uniform object

Answer 8

It lies on the intersection of the line(s) of symmetry

Question 9

Suggest an object where the CoG does not lie within the object itself

Answer 9

Doughnut, bowl

Question 10

How is the centre of gravity different to the centre of mass?

Answer 10

The centre of mass is mean position of the mass in an object

Question 11

Explain qualitatively how the upward forces on a light beam supported at its ends vary with the position of a heavy object placed on the beam

Answer 11

The closer the heavy object to the support, the greater the upwards force on the support. If the heavy object is in the centre, the upwards force is equal

Question 12

What two factors determine the stability of an object and how?

Answer 12

Height of the Centre of Gravity - the lower it is the more stable it is

Width of the base - the wider it is the more stable it is

Question 13

Describe the basic concept of equilibrium

Answer 13

Forces are balanced and torques are balanced (equilibrium in movement has a different concept so beware)

Question 14

What is basic equilibrium?

Answer 14

An object displaced from its equilibrium position feels a force pushing it back to its equilibrium position. (e.g. pushing an object in a bowl)

Question 15

What is unstable equilibrium?

Answer 15

An object displaced from its equilibrium position feels a force pushing it away from its equilibrium position (e.g. pushing an object on a hill)

Question 16

What is neutral rotational equilibrium?

Answer 16

The angular velocity/momentum is conserved and there is no net torque on the object (e.g. pushing an object in space)