Forces in Action

A pair of equal and opposite forces acting on a body but not in the same straight line

The product of one of the forces of a couple and the perpendicular distance between the forces

Torque = F * d

The moment of a couple

• The resultant of forces F and T must be equal in magnitude to the third force W but in the opposite direction                                                                                                                            
• Both the resultant vertical force and resultant horizontal force must be zero

• Arrows are drawn to represent each of the 3 forces end-to-end                                                     
• The triangle is closed because the net force is zero and so the object is in equilibrium

The mass per unit volume of a substance

ρ = m / V

1.3 kgm^-3 

The normal force exerted per unit cross-sectional area

p = F / A

1.0x10⁵ Pa

The gravitational force acting on an object through its centre of mass

The force that arises when two surfaces rub against each other

The resistive force on an object travelling through a fluid

The force within a stretched cable or robe

An upward buoyancy force acting on an object when it is in a fluid

A force arising when one object rests against another object

Force parallel to the slope = mgsinθ Force perpendicular to the slope = mgcosθ

Net force = m * a

Net force = normal contact force - weight

• Cross-sectional area
• Speed
• Shape
• Roughness/ texture of object
• Density of fluid

Smooth, streamlined shape

Increases the top speed of the car

Reduces the amount of fuel used on a journey

The constant speed reached by an object when the drag force (and upthrust) is equal and opposite to the weight of the object

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

Moment = F * x

The turning effect of a force about some axis or point

When a body is in equilibrium, both the net force and net moment acting on it is zero

The sum of the anticlockwise moments about any point = the sum of the clockwise moments about that same point

t = 0

net force = mg

a = g

mg > D

net force = mg - D

a = (mg - D) / m

mg = D

net force = 0

a = 0

An imaginary point at which the entire weight of an object appears to act

A point through which any externally applied force produces straight-line motion but no rotation

1) Make small holes along the edges of the object made from card

2) Insert a pin through one of the holes and hold the pin firmly in a clamp

3) Allow the object to swing freely before coming to rest with its CoG vertically below the pin

4) Hang a plumb-line from the pin

5) Draw a line along the vertical string of the plumb line

6) Repeat this process for the other holes

7) CoG = the point of intersection of the lines

A substance that can flow

W = m * g

W = (ρ * V) * g

W = ρ * A * h * g

p = (ρ * A * h * g) / A

p = h * ρ * g

• The upthrust exerted on a body immersed in a fluid (fully/ partially submerged) = the weight of the fluid that the body displaces                                                                                                              
• An object will sink if the upthrust is less than the weight of the object                                                        
• For a floating object the upthrust = the weight of the object = the weight of the fluid it displaces

Force at the top surface = h  ρ * g * A

Force at the bottom surface = (h + x) * p *  g * A

Resultant upward force (upthrust) = ((h + x) * p *  g * A) - (h * ρ * g * A) = x * ρ g * A