Forces in action

The mass of an object is a physical property and depends on…

The amount of matter it contains

A net (resultant) force acting on an object will make…

The object accelerate in the direction of the net force

Force is measured in…

Newtons

Mass is measured in…

Kilograms

Acceleration is measured in…

Metres per second squared

The mass of an object is…

Constant for a specific object or particle

The magnitude of an objects weight is…

Variable-it depends on location

The weight of an object on the surface of the earth is…

The gravitational force acting on the object

An object in free fall has an acceleration of…

g

You can determine the weight of an object using…

A newtonmeter

The centre of mass of an object is…

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

The centre of mass of an object is a point through which…

Any externally applied force produces straight-line motion but no rotation

The total weight of an object will act through a point which coincides with its centre of mass called…

It’s centre of gravity

A freely suspended object will come to rest…

With its centre of gravity vertically below the point of suspension

A plumb bob is suspended from a piece of string. When the plumb bob comes to rest…

The string is vertical

To find the centre of gravity of an object you can use a…

Plumb-line

The method for the plumb line is…

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

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

Allow the object to swing freely

It will come to rest with its centre of gravity vertically below the pin

Hang a plumb line from the pin and draw a line along the vertical string of the plumb line

Repeat this process for other holes

The centre of gravity will be the point of intersection of the lines

The definition of weight is…

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

The definition of friction is…

The force that arises when two surfaces rub against each other

The definition of drag is…

The resistive force on an object travelling through a fluid ; the same as friction

The definition of tension is…

The force within a stretched cable or rope

The definition of upthrust is…

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

The definition of normal contact force is…

A force arising when one object rests against another object

In a free body diagram each force vector is represented by…

An arrow labelled with the force it represents

In a free-body diagram each arrow is…

Drawn to the same scale

On a sloped free body diagram to resolve the weight parallel to the slope you use…

Wsinx

On a slopes free body diagram to resolve the weight perpendicular to the slope you use…

Wcosx

The component of the weight perpendicular to the slope is equal to…

The normal contact force

The moment of a force is…

The turning effect of a force about some axis or point

The moment is defined as…

Moment = Force x perpendicular distance of the line of action of the force from the axis or point of rotation

The SI unit for the moment of a force is…

Nm

When a body is in equilibrium…

The net force acting on it is zero and its net moment is zero

The principle of moments states…

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

To make an object spin without translational motion you must…

Apply a pair of equal but opposite forces to the object

A couple is defined as…

A pair of equal and opposite forces which must be parallel and along different lines applied on an object

A torque is…

The moment of a couple

The torque of a couple is defined as…

One of the forces x perpendicular separation between the forces

In a triangle of forces…

Arrows are drawn to represent each of the three forces end to end

The triangle is closed because the net force is zero and so the object is in equilibrium