Physics - 8 Forces in Balance

Displacement

Distance without change of direction

Vector quantity

has a magnitude and a direction

Scalar quantity

has magnitude only

How can a vector quantity be represented?

Using an arrow

Is a force vector or scalar?

vector

What can a force do? [3]

• change the shape of an object

• change its state of rest

• change its motion

What are contact forces?

forces that have an effect only on objects that they touch

What are non-contact forces?

pushes and pulls that result without the direct touching of objects acting at a distance

Contact forces [5]

• applied force

• spring force

• drag force

• frictional force

• normal force

Non-contact forces [3]

• magnetic force

• electric force

• gravitational force

Newton's Third Law of Motion

For every action there is an equal and opposite reaction

Driving force

the force that is pushing or pulling something.

Friction

A force that opposes motion between two surfaces that are in contact

Resultant force

The resultant of a system of force is the vector sum of all forces.

Newton's First Law of Motion

if the forces acting on an object are balanced, the resultant force on the object is zero

Zero resultant force at rest

remains at rest

Zero resultant force in motion

remains at the same speed and direction

If the forces on an object are unbalanced...

...the motion of the object depends on the size and direction of the resultant force

If an object is acted on by two unequal forces acting in opposite directions, the resultant force is: [2]

• equal to the difference between the two forces

• in the direction of the larger force

Centre of mass

the point at which the mass of an object can be thought to be concentrated

Suspended equilibrium

A state after when an object is released and comes to rest with its centre of mass below the point of suspension.

Freely suspended

If a suspended object returns to its equilibrium position after the turning force is taken away

Where is the centre of mass for a symmetrical object?

the intersection of the axis of symmetry

How to find the centre of mass?

1. Make a small hole in the object and hang it up so that it is free to swing without obstruction.

2. Hang a plumb line (a piece of string with a weight hanging from it) from the same suspension point. This lets you mark the vertical line directly below the suspension point.

3. Drill another hole at a different location within the object.

4. Again hang a plumb line to determine the vertical and mark it on.

5. The point at which the two marked lines cross is the centre of mass.

Examples of force multipliers [4]

• crowbars

• gears

• bottle opener

• pair of scissors

How do gears work?

their teeth interlock so that turning one causes another to turn in the opposite direction

Axle

a shaft on which a wheel rotates

Low gear gives...

...low speed and high turning effect

High gear gives...

...high speed and low turning effect

In low gear:

a small gear turns a large gear, so that it travels at low speed but with a high turning effect

In high gear:

a large gear turns a small gear, so that it travels at high speed but with a low turning effect

Moment

the turning effect of a force

Load

the weight of the object

Effort

the force applied to a simple machine

Force multiplier

A lever where the load is bigger than the effort.

Formula for moment

moment, M = force, F x distance from pivot, d

When a seesaw is balanced... [2]

• anticlockwise moment = clockwise moment

• W1d1 = W2d2

Principle of moments

The sum of the clockwise moments is equal to the sum of the anticlockwise moments (when a body is balanced)

Measuring the weight of a beam [3]

• balance it off-centre using a known weight

• W1d1 = W0d0

• d0 = distance from pivot to centre of mass of beam

All the forces acting on an object that don’t pass through a fixed point…

…can turn an object about that point