Physics Large Group Newton’s Laws of Motion February 6, 2009 turn to the 3.1

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Physics Large Group Newton’s Laws of Motion February 6, 2009 turn to the 3.1. 1 st Law of Motion (Law of Inertia). Inertia is the tendency of an object to resist a change in motion. more mass = more inertia The heavier something is, the more inertia it has. What is mass?. not volume

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### Physics Large GroupNewton’s Laws of MotionFebruary 6, 2009turn to the 3.1

1st Law of Motion (Law of Inertia)

Inertia is the tendency of an object to resist a change in motion.

more mass = more inertia

The heavier something is, the more inertia it has.

What is mass?
• not volume
• not weight

Weight = the force of gravity on an object

• measured in newtons (N)

Mass = amount of matter

• measure of inertia
• measured in kilograms (kg)

weight (N)

mass (kg)

On Earth…

2.2 lbs = 1 kg = 9.8 N

gravity (m/s2)

If Billy has a mass of 15 kg, what is his weight?

Weight = mass x gravity

w =15 kg x 9.8 m/s2

w = 147 kg*m/s2

147 N

True or False?
• The more mass something has the more inertia it has.
• The less inertia something has, the easier it is to change its motion.
• If something has a lot of inertia and is at rest, it is hard to move.
• It is difficult to change the motion of moving objects that have a lot of inertia.
1st Law

Unless acted upon by an unbalanced force, this golf ball would sit on the tee forever.

But once it is airborne, unless acted on by an unbalanced force (gravity and friction from the air) it would never stop!

Then why in everyday life do we observe objects in motion slowing down and stopping when we don’t see any forces acting on them?

It’s a force we sometimes cannot see – friction.

Friction – force that resists motion of objects in contact

Static friction – resistance that must be overcome to start an object in motion

Kinetic friction – resistance between objects already in motion

• Sliding friction: ice skating
• Rolling friction: bowling
• Fluid friction: air or water resistance
Friction Force equation

Ff = μ Fn

force of friction (N)

normal force (N)

coefficient of friction (no units)

Normal force – force that balances gravity - always perpendicular to the surface

Fnet = 400 N ↓

• no net force = no change its motion.
• net force = the object will accelerate

Fnet = 20 N ←

Fn

Ff

Fg

Fg = force of gravity

Fn = normal force

Ff = force of friction

Fn

Ff

Fg

Example Problem

Alex likes to slide through the kitchen in his socks. Alex weighs 850 N and the coefficient of sliding friction between his socks and the floor is 0.120. What is force of friction that opposes Alex’s motion?

Ff = μ Fn

Ff = .120 · 850 N

Ff = 102 N

Slide a book across a table and it will stop. The book comes to a rest because friction with the table.

In the absence of a force of friction, the book would continue in motion with the same speed and direction - forever!

Newtons’s 1st Law and You

Don’t let this be you. Wear seat belts.

Because of inertia, objects (including you) resist changes in their motion. When the car going 80 km/hr is stopped by the brick wall, your body keeps moving at 80 km/hr.

The more massive an object is, the bigger the force.

Newton’s 2nd Law

force (N)

mass (kg)

acceleration (m/s2)

F=?

m=1400 kg

a= 2 m/s2

F = m x a

F = 1400kg x 2 m/s2

F= 2800 kg*m/s2 or 2800 N

1. What acceleration will result when a 12-N net force applied to a 3-kg object? A 6-kg object?

• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
•  3. How much force is needed to accelerate a 66-kg skier 1 m/s2?
• 4. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/sec/sec.?
• 1. What acceleration will result when a 12-N net force applied to a 3-kg object?

12 N = 3 kg X 4 m/s2

• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.

16 N = 3.2 kg x 5 m/s2

• 3. How much force is needed to accelerate a 66-kg skier 1 m/sec/sec?

66 kg-m/sec2 or 66 N

• 4. What is the force on a 1000-kg elevator that is falling freely at 9.8 m/sec/sec.?
• 9800 kg-m/sec2 or 9800 N
Reminders
• Work on 34s
• POTW due Monday