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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.
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
• 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
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
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