Newton’s Laws Take 2

take out your 3s

February 13, 2009

Newton’s 2nd Law proves that different masses accelerate to the earth at the same rate, but with different forces.

• We know that objects with different masses accelerate to the ground at the same rate.

• However, because of the 2nd Law we know that they don’t hit the ground with the same force.

F = ma

98 N = 10 kg x 9.8 m/s/s

F = ma

9.8 N = 1 kg x 9.8 m/s/s

Terminal Velocity – force of friction = force of gravity air resistance = weight

– acceleration = 0 m/s2

– Fnet = 0 N

Who has a higher terminal velocity, a person who has a mass of 200 kg or a person with a mass of 100 kg?

200 kg

Video Clip

• fastest-growing sport in Norway 

• http://www.youtube.com/watch?v=Nf2ENQRHKA4

Free Body Diagrams - A picture that shows all of the forces acting on an object

What are the forces acting on this object?http://lectureonline.cl.msu.edu/~mmp/applist/si/plane.htm

FfFn

Fg

Fp

Forces on an inclined plane…

Fn

Fp Fg

mgF g

θn cosFF g

θp sinFF g

nF F f

Billy and his dad are sledding down the hill at an angle of 15.0°. If their combined mass is 55.2 kg, what is the force pulling them down the hill?

θp sinFF g

θp sinmgF

15sin 9.855.2Fp

N 014F p

Example) A car of mass of 2000 kg is on an icy driveway inclined at an angle of 20.0º, determine the force of the plane?

What info does the problem give us?

m=2000 kgӨ= 20.0ºFp= ?

θp sinFF g

θgp sinmF

20sin8.92000F pN 0670F p

Tension = stretching force in a rope

Demos with spring scales

The tension in the rope can be broken down into its horizontal and vertical components. The vertical component of Ft has to equal the Fg.

cos θ = Fg FTFg / FT

θcos/FF gT

Tension Practice Problem

• A stop light with a mass of 25 kg is supported by 2 wires. What is the tension in the wire that is 15˚ from the normal? What is the tension in the wire that is 50˚ from the normal?

15˚ 50˚

Tension Practice Problem• A stop light with a mass of 25 kg is supported by 2

wires. What is the tension in the wire that is 15˚ from the normal? What is the tension in the wire that is 50˚ from the normal?

15˚ 50˚

Fg = m g

Fg = (25 kg) (9.8 m/s2) = 245 N

15cos

245 NF

t

50cos

245 NF

t

NFt

6.253 NFt

2.381

Pressure (N/m2) or Pa

• Awl Demo• Bed of Nails Video

A

FP

Force (N)

Area (m2)length x width

• Bernoulli's Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases

• Pipe Demo

• 2 Hole Water Demo

Creates the ability for planes to fly!

If an airplane is traveling at a constant velocity, how does the force of thrust compare to the

force of drag?

Newton’s 3rd Law

For every action, there is an equal and opposite reaction.

What forces are involved in a tug of war?

Fg

Fn

Ff

Fp

Action / Reaction

Arnold Strongman and Suzie Small are having a tug of war contest. They pull on opposite ends of a rope. Who

exerts a greater force on the rope?

• Rope tension must be the same throughout the whole rope. Therefore, Arnold can pull no harder on the rope than Suzie.

• The most important factor in a tug of war is not how hard you pull on the rope, but how hard you push on the ground.

Suppose two carts, one twice as massive as the other, fly apart when the compressed spring that joins them is released.

1. How does the force exerted by the spring on the 1m-cart compare to the force exerted by the spring on the 2m-cart?

The forces are equal

2. How fast does the 2m-cart roll compared to the smaller 1m-cart?

It would roll half-as fast since it experienced half the acceleration when released from the spring.

Which will experience the greater acceleration, the car or truck if the

forces are equal?

F = F

M a = M a

Action / Reaction

Fgun = Fbullet

M a = M a

What gun and bullet would you want to be shot with?

– A gun with very little mass and a bullet with a lot of mass?

Or

– A gun with lots of mass and bullet with very little mass?

•Fan cart demo

•Fire extinguisher DVD demo

Reminders

• POTW

• Work on 35s Newton’s Laws worksheet #2

• No school Monday

Hundreds Gather to Protest Hundreds Gather to Protest Global WarmingGlobal Warming