Chapter 4: Forces in action

Is a property and is measured in kg

Resists change in motion

Greater the mass of a body, the smaller the change produced by an applied force

Measure of an amount of matter in an object

As it is a force on an object due to the pull on gravity,

its measured in Newtons (N)

The weight is equal to:

w = mg

g: changes depending on the planet

Effect of a gravitational field on mass

- it is solely falling under the influence of gravity

- on Earth every object accelerates at 9.81 ms^-2

- in the absence of air resistance - all bodies near the

Earth fall with the same acceleration- regardless of their

mass

according to newtons second law, objects will accelerate if

there is a resultant force exerted on them

this acceleration is in the same direction as the

resultant force

F = ma

RESULTANT FORCE

force is directly proportional to acceleration (when mass is

constant)

The middle of an object

Objects with uniform density - the centre of mass is

the geometric centre of the object

A force being applied at the objects centre of mass will

produce a straight line motion without rotation.

W = mg

Weight (W) - the resultant gravitational force, will act

through the point - centre of gravity (which coincides with

the objects centre of mass)

A freely suspended object will come

to rest with its centre of gravity

below the the point the object is

suspended by.

It produces a vertical line which

is inline with g and the centre of mass

• draw each vector head to tail

• anchor the vectors from the centre of mass

• label each vector arrow

• the length of the arrow represents the size of the force

• weight

• friction

• drag

• upthrust

• thrust

• tension

• normal contact force !!

gravitational force produced when a mass in a

gravitational field (g)

• acts through the objects centre of mass

ALWAYS DRAWN STRAIGHT DOWN

opposes motion of an object

opposes motion when an object is travelling through a fluid

(gas / liquid)

because the object collides with particles in the fluid

an upward buoyancy force acting on an object when it is a

fluid. this is due to the pressure difference between the top

and bottom of a submerged object

the force within a stretched cable / rope

when an object rests

against another (is

stationary).

the normal is at 90 degrees where the objects touch

- both forces oppose the motion of an object

- drag is when an objects motion is opposed by a fluid (liquid or

gas)

- while friction is when an objects motion is opposed by its

surface being in contact with another object

- ↑ cross-sectional area ↑ drag

- ↑ speed ↑ drag

drag ∝ speed²

As the speed of an object increases, the drag does.

This continues until the drag force balances the weight of the

object.

It has now reached terminal velocity.

• the turning effect of a force and can be

  clockwise / anti-clockwise

can occur when forces cause objects to rotate at a point

force x perpendicular distance of the line of action of force from

the axis / point of rotation

= Fx

Force (N)

Perpendicular distance (m)

When a body is in equilibrium, the net forces and the net moments on an object is zero.

• the object is not accelerating - no net forces

• the object is not rotating - no net moment

no net moment means clockwise moment = anti-clockwise moment

object moving through space

Apply equal and opposite forces

Forces that produce rotational motion and no translational

motion

the moment of a couple doesn’t depend on a povot, only the

perpendicular distance between the two forces

• equal in magnitude

• opposite in direction

• perpendicular to the distance between them

• couples produce a zero resultant force - the object doesn’t accelerate

• the size of the turning effect is given by the torque

one of the forces (N) x perpendicular sperating between the forces (m)

A system is in equilibrium when all the forces are balanced:

• no resultant force

• no resultant torque

An object as equilibrium will therefore remain at rest / constant

velocity - DOESN’T ROTATE

can be represented by vector triangles.

In equilibrium - they’re closed triangles - form a closed path