Physics Large Group

Newton’s Laws of Motion

February 6, 2009

turn 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?

Weight = the force of gravity on an object• measured in newtons (N)

Mass = amount of matter• measure of inertia• measured in kilograms (kg)

• not volume

• not weight

On Earth…

2.2 lbs = 1 kg = 9.8 N

mgw weight (N)

gravity (m/s2)

mass (kg)

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.

• Table cloth demo

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.

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 that resists motion of objects in contact

Friction Force equation

Ff = μ Fn

force of friction (N)

coefficient of friction (no units)

normal force (N)

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

Net force – combination of all the forces acting on an object

Fnet = 400 N ↓

Fnet = 20 N ←

no net force = no change its motion.

net force = the object will accelerate

Someone rolls a ball, after they let it go, what forces are acting on it?

Fg = force of gravity

Fn = normal force

Ff = force of friction

Fg

Fn

Ff

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 Fg

Fn

Ff

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 force you apply to something the more it’s motion changes.

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

a~F

m~F

Newton’s 2nd Law

maF

force (N)

acceleration (m/s2)

mass (kg)

How much force is needed to accelerate a 1400 kilogram car 2 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

Check Your Understanding1. 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.?

Check Your Understanding

• 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

Copernicus