5-14 Science

Science Lab
Light and sound
Plants and Animals
Periodic Table
Electricity and Heat
Living Body
Earth in Space
Energy Changes
Changing Materials
Cells and Reproduction
Forces

Forces

• A force can:
• Change the speed of an object
• Change the direction of motion of an object
• Change the shape of an object
• The unit of force is the Newton (N)

• A balanced force is when two or more forces cancel out so there is no change in speed, direction or shape:

• The ball will not move or will continue in a straight line at a steady speed.

• An  unbalanced force is when the force(s) in one direction are bigger than the force(s) in another direction so there is a change in speed, direction or shape:

• The ball will move to the right, getting faster all the time

• Force can be measured using a Newton meter
• Inside the Newton meter there is a spring
• The force made by a spring increases as it gets stretched
• Eventually the force of the spring balances the force which is on the meter
• At that point the meter stops moving

• Friction is a force that slows objects down:
• Energy cannot be created or destroyed
• When an object has some kinetic (movement) energy – friction will turn some of that energy to heat
• Kinetic energy is changed into heat energy so the object slows
• Friction is generated when one surface moves over another

• Some friction is necessary to us:
• Friction between our feet and the floor prevents us from slipping
• Friction is used in the brakes of a car to slow it down
• Friction between our fingers and a pencil allows us to hold it
• In a machine however friction is generally minimised:
• A machine turns one form of energy into a more useful form
• Any energy that is changed to heat by friction in the machine reduces its effectiveness
• Oil, grease and bearings are used in machines to minimise friction

• Friction also occurs in fluids
• A fluid is anything that flows like air or water
• The force which resists the movement of an object moving through a fluid is called drag
• Drag is undesirable because it slows down boats, submarines and aircraft and increases the energy needed to drive them
• Drag is reduced by streamlining
• Streamlining is done by changing the shape of an object to let it move through the fluid with less drag.
• The best shape for moving through a fluid has been found to be a teardrop
• This shape minimises the disturbance to the fluid flow and so reduces drag

• This has been very obvious in car design over the past 50 years:

Gravity

• Every object has the property of mass
• An object’s mass remains constant wherever it is in the universe
• The force of gravity acts on an object’s mass (in kilograms) to give it weight (in Newtons)
• An object’s weight is different on different planets

• Every planet has its own gravitational field strength (g)
• The weight of an object is the mass times the gravitational field strength

Weight (N) = mass (kg) x g

• So the weight and mass of an object on the earth is the same.
• The gravitational field strength of the plants is shown in the table below – you are not expected to learn these

• The weight of a 70 kilogram person on Jupiter is:
  • 70 kg x 26.04 = 1823 N

• The weight of a 70 kilogram person on Mars is:
  • 70 kg x 3.75 =;262 N

Pressure

• Pressure is the force exerted on an area
• Pressure is measured in units called Pascals (Pa)

• To calculate pressure you divide the force in Newtons by the area in m².
• As the area gets smaller the pressure on the area increases

if you have a person of 70 kg then the force they exert due to the gravitational field strength = 70 x 9.91 = 687 N and that person is wearing boots with a heel of area 25 cm2 first convert the area to m2 = 25 ÷ 10,000 = 0.0025 m2 the pressure is 274,800 Pa
if you have a person of 70 kg then the force they exert due to the gravitational field strength = 70 x 9.91 = 687 N and that person is wearing boots with a heel of area 1 cm2 first convert the area to m2 = 25 ÷ 10,000 = 0.0001 m2 the pressure is 6,870,000 Pa

In other words stiletto heels damage floors.

This idea of spreading the force over a large area can be quite useful

• wide soft tyres on quad bikes and tracks on heavy plant do not sink into soft, boggy ground as much as car tyres.

Vocabulary

	Newton		balanced force
	unbalanced force		friction
	fluid		drag
	streamlining		mass
	weight		gravitational field strength
	Pascal

Relative sizes of the planets

A nice activity to try using these planet cutouts from Berkeley University.

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