Chapter 4: Forces in action

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Physics AS Level - OCR Gateway A need to complete ...

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35 Terms

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Mass

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

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Definition of mass

Measure of an amount of matter in an object

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Weight

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

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Definition of weight

Effect of a gravitational field on mass

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Free fall

- 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

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Force and acceleration

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)

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Geometric centre

The middle of an object

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Centre of mass and geometric centre

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.

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Weight (centre of gravity)

W = mg

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

through the point - centre of gravity (which coincides with

the objects centre of mass)

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Finding the centre of gravity

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

<p>A freely suspended object will come</p><p>to rest with its <span style="color: purple">centre of gravity</span></p><p>below the the point the object is</p><p>suspended by.</p><p>It produces a vertical line which</p><p>is inline with g and the <span style="color: blue">centre of mass</span></p>
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Centre of gravity of another object

<p></p>
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Free-body diagram rules

  • 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

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Different forces

  • weight

  • friction

  • drag

  • upthrust

  • thrust

  • tension

  • normal contact force !!

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Weight (force)

gravitational force produced when a mass in a

gravitational field (g)

  • acts through the objects centre of mass

ALWAYS DRAWN STRAIGHT DOWN

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Friction

opposes motion of an object

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Drag

opposes motion when an object is travelling through a fluid

(gas / liquid)

because the object collides with particles in the fluid

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Upthrust

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

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Tension

the force within a stretched cable / rope

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Normal contact force

when an object rests

against another (is

stationary).

the normal is at 90 degrees where the objects touch

<p>when an object rests</p><p>against another (is</p><p><mark data-color="blue">stationary</mark>).</p><p>the normal is at <strong><span style="color: red">90 degrees </span></strong>where the objects touch</p>
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Compare the force drag and the force friction

- 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

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Factors affecting drag

- ↑ cross-sectional area ↑ drag

- ↑ speed ↑ drag

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Relationship between drag and speed

drag ∝ speed2

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Terminal velocity

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.

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Turning moments

  • the turning effect of a force and can be

    clockwise / anti-clockwise

can occur when forces cause objects to rotate at a point

<ul><li><p>the <mark data-color="blue">turning </mark><strong><mark data-color="blue">effect </mark></strong>of a force and can be </p><p></p><p>clockwise / anti-clockwise</p></li></ul><p>can occur when forces cause objects to rotate at a point</p>
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Moment definition and formula (Nm)

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

the axis / point of rotation

= Fx

Force (N)

Perpendicular distance (m)

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Force applied at an angle (Moments)

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Equilibrium: principle of moments

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

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Translational motion

object moving through space

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Making an object move with no translational motion

Apply equal and opposite forces

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Couples

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

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A couple consists of two forces that are..

  • 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

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Torque of a couple (Nm)

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

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Equilibrium

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

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Coplanar forces in equilibrium

can be represented by vector triangles.

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

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Triangular forces in equilibrium

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