Inertia (en-ur-sha) ☺

• Resistance to acceleration
• It’s a property of all matter
• More Matter → More Mass
• More Mass → More Inertia
• Things with more inertia require more force to accelerate them.

Newton’s First Law

• Describes Inertia

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(Mass Resists Acceleration)

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Objects tend to keep moving (or not) unless an outside net force acts on them

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Forces

May be a push or a pull on any object

Many types of forces exist – gravity, friction, electrical, direct contact, etc.

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• Net force on an object is the SUM of all forces acting on it.

Balanced Forces (no net force)

• Result in no acceleration
• Does not mean “no motion”

→ object remains at constant velocity or zero velocity.

• Examples:
• Ping pong ball in tube
• Car cruising on straight highway
• Anything at rest (standing still)

Unbalanced Forces (a NET force)

• Causes acceleration

Examples:

• Sled going downhill
• Ball flying through air
• Car accelerating positively or negatively, or going

around a curve

Newton’s 2^nd Law

• F_net = m • a
• Net Force = mass x acceleration
• accel is proportional to mass and net force

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• No net force → no acceleration

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• Rate and direction of accel. directly proportional to F_net

Force

• Pounds measure force

(in non-SI units)

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• SI units of Force = Newtons

1N = 1kg•m/s²

1N ~ 1/5 pound

On Earth, a 1kg object weighs 10N

Mass Vs. Weight

• Mass = amount of matter in an object
• Weight = force on an object due to gravity

W = m • g

• same as F = m • a
• “g” means the amount of gravitational accel.

Find the weight of a 113g (_?_kg) package of butter.

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�How much does a 0.1 kg keychain weigh?

More weight practice

How much does a liter of water weigh?

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How much would a liter of water weigh on the moon? (gravity is ⅙ as strong)

Scales and Balances

• Scales measure force – using springs
• Balances measure mass

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Your MASS will not change in different gravitational fields

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Your weight depends on the gravity!

Weight

• Can also calculate using F_g=Gm₁m₂

r²

_{G = gravitational constant = 6.67•10^-11 (7x10^-11)}

_{m = mass of each object}

_{r = distance between centers of mass of 2 objects}

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Find the gravitational attraction between you and your pencil

Pretend your mass is 50 kg

The mass of your pencil is about 30g (____kg)

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Your pencil is 0.5 meters from your center of mass.