Forces in action

Define weight of an object and give the equation for weight.

• The gravitational force acting on the object

• ____ = mass x gravity.

Define the mass of an object

• The amount of matter in an object

• Base quantity measured in kg

Define centre of gravity.

A point where the entire weight of the object appears to act.

Define centre of mass.

A point through which any externally applied force produces straight-line motion without any rotation.

Define the newton

A force that will give a 1kg mass an acceleration of 1ms^-2 in the direction of the force

How do you find the centre of mass of an object?

1. Suspend the object freely from a point.
2. Use a plumb-line to draw a straight vertical line on the object.
3. Repeat step 1 using different suspension points.
4. Determine the point of intersection of these straight lines.
5. The intersection point is the centre of mass.

What is drag?

The resistive force experienced by an object moving through fluid (liquid or gas).

What are the factors that affect drag?

• The speed of the object (the greater the speed, the greater the drag)

• The cross-sectional area at right angles to the direction of travel (the larger the area, the greater is the drag)

• The type of fluid / its viscosity.

What happens when an object reaches terminal velocity?

• The speed of the object is constant

• Object’s acceleration is zero. (The net force on the object is zero)

Define moment of a force.

• The product of the force applied and the perpendicular distance of the line of action of the force from the axis or point of rotation.

• The turning effect of a force about some axis or point

What is the equation for moments?

moment = force x perpendicular distance from the pivot.

What is the unit for moment?

Nm (Newton metres)

What are the two conditions for an object in equilibrium?

• Resultant force acting on object = 0

• Resultant moment acting on object = 0

What is a couple?

• Two equal but opposite forces acting on an object.

• The forces must be parallel and along different lines.

What is torque?

The moment of a couple

What is the equation for the torque of a couple?

torque of a couple = one of the forces x perpendicular separation between the forces = Fd

What is the torque of a couple the same as?

The total moment of the two forces about their midpoint.

Define density of a substance.

Its mass per unit volume.

How could you determine the density of a substance?

By calculating the mass of the substance and its volume.

• The mass of a substance can be measured using a top pan balance.
• The volume of a regular solid can be calculated once its dimensions are measured using a ruler, a vernier calliper, or a micrometer. For an irregular solid, its volume can be determined from the volume of water displaced when it is completely submerged. The volume of the displaced water can be measured directly using a measuring cylinder.

Define pressure.

The force exerted per unit cross-sectional area.

What is the unit for pressure?

Nm^-2 or Pa

what do the symbols stand for in p= hρg

• h= height

• ρ=density of the fluid

• g= accel due to gravity.

What is Archimedes' principle.

• The upthrust exerted on an object immersed in a fluid is equal to the weight of the fluid it displaces.

• This is true for partially and fully submerged objects

Principle of moments

For a body in rotational equilibrium, the sum of the anticlockwise moments about any point is equal to the sum of the clockwise moments about that same point.

How can you increase the efficiency of a mechanical system

• Use oil or bearings to reduce heat losses through friction in moving parts

• Use a stiffer / stronger cables to reduce energy loss through stretching