Science 8: Unit 2 FluidsCh. 9 Forces

Science 8 Unit 2: Fluids

Force: Anything that causes a change in the motion of an objectA push or pull on an object

Science 8 Unit 2: Fluids

Force: Anything that causes a change in the motion of an objectA push or pull on an object

• Forces can be balance or unbalanced

Science 8 Unit 2: Fluids

Balanced forces: • Two or more forces equal in strength• Opposite in direction• Results in no movement

Science 8 Unit 2: Fluids

Unbalanced forces: • Forces are not equal and opposite• Results in movement (change in speed or direction)

Science 8 Unit 2: Fluids

Mass Vs. Weight• To understand the force of gravity we must understand the

difference between mass and weight

Mass: The amount of matter in an object

Weight: The pull or force of gravity acting on an object

Science 8 Unit 2: Fluids

Mass• Stays the same anywhere

in the universe• Measured in kg

Weight• Will change with different

gravities• Measured in Newtons (N)

Science 8 Unit 2: Fluids

Buoyancy: • The upward force exerted on an object in a fluid

Science 8 Unit 2: Fluids

Buoyancy: • The upward force exerted on an object in a fluid

Comparing buoyant force to the force of gravity will tell us whether an object will sink or float

Science 8 Unit 2: Fluids

Archimedes Principle: The buoyant force acting on an object equals the weight of the fluid displaced by the object

Science 8 Unit 2: Fluids

Archimedes Principle: The buoyant force acting on an object equals the weight of the fluid displaced by the object

• A 1 L cube displaces 1 L of water

• Buoyant force = weight of 1L water

Science 8 Unit 2: Fluids

• Cubes are same volume• Displace same amount of water• Same buoyant force

Why do the wood and metal cubes sink further than the sponge?

Science 8 Unit 2: Fluids

• Cubes are same volume• Displace same amount of water• Same buoyant force

Ex. Metal• More dense• Force of gravity is greater

than buoyant force• Sinks

Science 8 Unit 2: Fluids

Basic rule:

If the density of the object is greater than the density of the fluid it will sink

Science 8 Unit 2: Fluids

Read from “How Buoyancy and Density are Related” on page 339 to page 342 and answer the questions below:

1. Do we float more easily in salt water or in fresh water? Why?

2. How do life jackets keep a person floating?

3. How do blimps work?

Science 8 Unit 2: Fluids

Pressure

Pressure is the force acting on a given area.

• The larger the force, the greater the pressure.• The smaller the area, the greater the pressure.

Science 8 Unit 2: Fluids

Calculating pressure :

P(Pressure) = F (force )A (Area)

UnitsThe unit for pressure is Newton/ Metre2 which is called a Pascal (Pa)

FP A

Science 8 Unit 2: Fluids

Examples:Finding Pressure

Force= 10 N Pressure= F = 10N = 2 PaArea= 5 m2 A 5m2

Science 8 Unit 2: Fluids

Examples:Finding Pressure

Force= 10 N Pressure= F = 10N = 2 PaArea= 5 m2 A 5m2

Finding AreaForce = 50 N Area = F = 50N = 10m2

Pressure = 5Pa P 5Pa

Science 8 Unit 2: Fluids

Examples:Finding Pressure

Force= 10 N Pressure= F = 10N = 2 PaArea= 5 m2 A 5m2

Finding AreaForce = 50 N Area = F = 50N = 10m2

Pressure = 5Pa P 5Pa

Finding ForcePressure = 6Pa Force = P x A = 6 Pa x 2m2 = 12NArea = 2m2

Science 8 Unit 2: Fluids

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