3.2 Forces in action

Define 1N

The force required to accelerate a 1kg object at 1ms⁻²

When does equilibrium occur?

When there's zero resultant force and zero movement

Define inertia

The tendancy of an objuect to remain at rest or remain in motion (reluctance to change velicity)

What is the net (resultant) force?

The sum of all the forces acting on an object

What is the formula for the weight of an object?

W = mg

What direction does friction act in?

The opposite direction to the movement causing it

List the factors that increase the size of the drag force exerted on an object

Greater surface area of the object, greater relative speed, less viscous fluid, less aerodynamic shaped object

What direction does the normal contact force act in?

Perpendicular to the surface

What is a free-body diagram?

A diagram that represents the forces or the resultant force acting on an object

Give the formula for the moment due to a force

moment = Fxsinθ

Explain why any force that acts through the chosen pivot does not cause a moment about the pivot

The perpendicular distance from the force to the pivot is zero

Define a couple

A pair of forces that together cause a resultant moment, but not a resultant force

State the conditions for a pair of forces acting on the same object to be a couple

The two forces have to be the same size but acting in opposite directions

What is upthrust?

When an object displaces a fluid, the fluid exerts an upthrust force upwards on the object

What is tension?

Any contact pull

What is the total moment about a given point?

The sum of the individual moments about that point

What is torque?

A resultant moment

What is the equation for torque due to a couple?

torque = Fd

What is the centre of gravity of an object?

The point through which the object’s weight appears to act

How can you find an objects centre of gravity?

1. Suspend the shape from a pin, so it can hang freely

2. Suspend a plumb line from the pin in front of the shape and mark the position of the line on the shape

3. Repeat for 4 other positions of the pin and the centre of gravity is where all the lines cross

What is the centre of mass?

The average position of the object’s mass, and is the same as the centre of gravity if the object is in a uniform gravitational field

State the principle of moments

For an object to be in equilibrium, the sum of the clockwise moments must equal the sum of the anti-clockwise moments exerted on it

An object is in equilibrium if:

• The resultant force exerted on it is 0

• The net movement exerted on it is 0

If there is no acceleration, what is the resultant force?

Zero

What is the horizontal resolved force?

Fsinθ

What is the vertical resolved force?

Fcosθ

What formula is used to calculate acceleration when mass and force is given?

F = ma

What is the formula used to calculate density when mass and volume are given?

ρ = m / v

How can you find the density of an irregular shape?

1. Find the mass of the object on a scale

2. Then fully submerge the shape in liquid in a measuring cylinder

3. The change in volume is the volume of the object

4. This is the displacement method

5. Then calculate the density

What is the calculation for pressure when force and area are given?

P = F / A

What is the formula for pressure in a liquid?

P = hρg

What is h in P = hρg?

Vertical height of where the object is submerged to

When does upthrust occur?

When a fluid is displaced

State Archimedes’ principle

The upthrust exerted by a fluid is equal to the weight of the displaced fluid

What formula is used to calculate upthrust?

U = ρVg

What is V in U = ρVg?

The volume of fluid displaced

If an object is floating, what is the resultant force?

Zero