Ch. 4.0 Unit B – Systems in ActionIntroduction

Systems in Action

• System - A group of individual parts or procedures that work together as a complex whole to accomplish a desired task

• Component – all the different parts of a system

Categories of Systems

• Mechanical System – composed of physical parts working together– mechanical systems use forces to transfer energy– ie. a city bus, which has physical parts that work

together to provide transportation

Categories of Systems• Non-mechanical System – a set of procedures,

methods, or rules that accomplish a task.– ie. A bus schedule and routes maps.

Ch. 4.1 Force

Force

• force – a push or a pull that acts on an object– measured in newtons (N)

• Classifying Forces– contact forces– action-at-a-distance forces (or non-contact forces)

Contact Forces

• contact forces – force that acts between objects that are touching.– example: hitting a tennis ball; friction between a

tire and the road

Friction• friction – a force that opposes the relative motion of

an object• surfaces can exhibit different amounts of friction on an

object– a hockey puck slid across a wooden floor will slow down and stop

due to the friction between the wooden floor and the puck– that same hockey puck slid across an ice rink (with the same

amount of initial force) will go farther before stopping because the ice applies less friction to the puck than the wooden floor

Friction• Why are tires on street bikes used by Tour de France

competitors so thin?

• Let’s talk about curling, shall we.

Action-at-a-Distance Forces• action-at-a-distance forces – forces that can

push or pull an object without touching it.– WHAT?!?! How’s that even possible?– examples: gravity, static electricity, and magnetism– These are also called non-contact forces

Gravity• What is gravity?

Gravity

• gravity – the attraction between two objects due to their mass – the amount of attraction depends upon the

amount of each object’s mass and the distance between the two objects

– when I let go of a ball, the ball is pulled back down to the ground by the force of gravity, even though nothing is touching it• this is because both the Earth and the ball have mass

Gravity

• What else, other than the Earth, produces gravitational force?

Mass vs. Weight

• Once we have identified the type of force, it is often necessary to measure the amount of force.– before measuring the amount of force, it is

important to know the difference between mass and weight

Mass

• mass – the amount of matter in an object– the metric unit for measuring mass is the gram (g)

or kilogram (kg)– ie. the mass of a bowling ball is greater than the

mass of a tennis ball because the bowling ball contains more matter

Weight

• weight – the amount of force on an object due to gravity– so, weight is the same thing as the force of gravity

Weight vs. Mass

• no matter where an object is located, its mass stays the same, but its weight changes due to the gravitational force

Location Mass (kg) Weight (N)

Earth 50 490

Moon 50 80

Mars 50 160

Jupiter 50 1,140

Units of Force

• “BUT WAIT!” you say. “I weigh myself in kilograms (or pounds), not newtons.”– when you “weigh” yourself, you are not actually

finding your weight (in newtons), but are finding your mass in kilograms

Measuring Force• spring scale (Newton gauge) – most common

force meter; consists of a spring with a hook on the end; as more force is applied to the hook, the spring stretches further

Calculating the Force of Gravity (Weight)

• the mass of an object and its weight are directly proportional– an object with twice the mass will also have twice

the weight

• multiplying any mass by 9.8 N/kg will yield that objects weight on Earth

• 9.8 N/kg is called Earth’s gravitational field strenght (g)

Calculating the Force of Gravity (Weight)

• the force of gravity (Fg) on any mass (m) near the surface of the earth can be calculated by:

force of gravity = (mass of an object) x (the strength of the earth’s gravitational field)

OR

Fg = mg

where mass is in kilograms (kg) and g is 9.8 N/kg

Calculating the Force of Gravity (Weight)

• example: find the weight of a 50-kg student on Earth

Fg = mg

Fg = (50 kg)(9.8N/kg)

Fg = 490 N

Let’s Review

1. Use the words “mass” or “weight” to correctly complete the following sentences:

a) Even if gravity changes, the of an object does not change.

b) The of an object would cange if the gravity changed.

c) Kilogram (kg) is the metric unit for .d) The newton (N) is the metric unit for .

mass

weight

weightmass

Let’s Review

2. The following masses are located on Earth. Calculate the weight of each object:

a) 25 kgb) 40 kgc) 150 kg

3. An object has a mass of 5.0 kg on the surface of the Moon. What would be the object’s:

a) mass on Earth?b) Force of gravity on Earth?

Fg = 25 kg x 9.8 N/kg = 245 NFg = 40 kg x 9.8 N/kg = 392 NFg = 150 kg x 9.8 N/kg = 1,470 N

mass = 5.0 kg no matter where it is located

Fg = 5.0 kg x 9.8 N/kg = 49 N