Goal: To understand Newton’s 2nd law.

Objectives:1) To understand Newton’s Force Law2) To learn about how Weight works

3) To learn about the basics of Friction4) To understand Some applications for

Friction5) To understand Freefall

6) To understand Non-free fall motions

Law 2: Force Law

Force = mass * acceleration

Force is how much you are pushing or pulling on something

Mass is how much stuff you have (NOTE: mass is NOT weight!!!)

Acceleration is the rate at which you change your speed.

Net force = mass * net acceleration

F = m * a

• If my mass is 100 kg and the gravitational acceleration on the surface of the earth is 9.8 m/s2 then what is my “weight” in Newtons on the surface of the earth?

• The moon has 1/6th the gravity as the earth. What is my weight on the moon and what is my mass on the moon?

Friction

• One type of force is friction.

• Nothing is perfectly smooth.

• Everything has microscopic pits and parts that stick out.

• The smoother the surface the less of these they have.

• Friction is the colliding of these two rough surfaces.

Advantage friction

• Friction can be a hindrance – but it can help also.

• When you walk, you are using friction to your advantage!

• You are setting it up so that you get a force applied to you – but more on that in chapter 5.

2 types of friction

• There are 2 types of friction.

• The first is static friction.

• When objects are at rest then tend to settle a bit and make it hard to push them apart.

• This type of friction has a great force.

Sliding friction

• Once an object starts moving it can go over most of the valleys of the other surface, but still gets slowed a bit.

• This type of friction is slower.

• Concept question:• Which object will have a greater static friction

force on it:• A 5 lb hockey puck sliding across a floor at 50

miles per hour or a 5 lb hockey puck sliding across the same floor at 100 miles per hour?

Air resistance

• Sort of like friction – basically it is friction with the air (liquids do something similar, but at a greater scale).

• It slows you down by applying a force to you.

• The more your air resistance the slower you fall (the force depends on air velocity).

Earth vs. space

• Ignoring friction or resistance, what is the acceleration on a 10 kg wheel being pushed with a force of 100 N in the horizontal direction?

• What is the weight of the wheel?

• What would the wheel weigh on the moon (1/6th gravity)?

• Now for the clincher, if you applied a 100 N force to the wheel on the moon in the horizontal direction then what would its acceleration be?

Demo time!

• Need a volunteer.

• We will push off of each other on skateboards (hopefully with equal force).

• Now, some questions?

• Which one of us will reach the wall first?

• Will one of us have a faster speed? If so which one of us.

Acceleration

• Acceleration = net force / mass

• So, with equal forces, the smaller the mass, the bigger the acceleration.

• Half the mass means double the acceleration.

But

• Um but didn’t you say that gravity pulls everything at the same rate?

• Yes!

• So, what does that tell you about the force of gravity?

• If object A has gravitational force F applied to it, then what is the gravitational force on object B if B has twice the mass as A?

Follow up

• An astronaut is in orbit around the earth.• The mass of the astronaut is 100 kg and the

mass of his spaceship is 5000 kg.• What are the “weights” of the astronaut and

spacecraft (hint what is the net force on them?)?• Suppose the astronaut applies a 1000 N force

on the spacecraft. What will the acceleration of the spacecraft be?

• If the same force were applied to the astronaut, what would the acceleration of the astronaut be?

• Why are the two accelerations so different?

Free fall

• If you can ignore wind resistance then as you fall you experience “free fall”.

• That is just a fancy way of saying that you are accelerated at the full rate of gravity (10 m/s2).

• If a stone of mass 1 kg is in free fall then what is the net force being applied to it?

Not free fall

• But what happens if you have wind resistance?

• The wind resistance decreases the net acceleration.

• How does it do this (hint, think net force and think direction)?

Forces add

• If you have more than 1 force on something, they add.

• If the forces are in opposite directions, you still “add” them, but one is a negative number, so you are adding a negative number.

• So, 5 N + (-2N) = 3N• That is, if you had an object weighing 5 N and it

was falling with wind resistance of -2N in the down direction on it (or +2N Force in the up direction) then its NET force would be 3N.

Net force vs acceleration

• 5 N + (-2N) = 3N

• If the object weighs 5N on the surface of the earth, what is its mass?

• If the net force is 3N down then what is the acceleration on the object?

Now lets do it with friction

• You push an object with a force of 299 N.• It does not budge.• You push it just a little harder (300 N) and it

starts to accelerate at a rate of 1 m/s2.• If the mass of the object is 100 kg then find:• A) The force of kinetic friction• B) What is the Net force on the object (no it is

not 300 N)?• C) What is the force of sliding friction on this

object?

Conclusion

• We have seen from Newton’s 2nd law that force = mass * acceleration

• Weight is just the net downwards force. If gravity is the only force other than the force from the ground pushing back then your weight is the same as your gravitational force.

• Your acceleration depends on your mass – smaller masses have bigger accelerations with the same force.

• If you are in freefall you have no weight• If you are not in freefall then your net

acceleration depends on your net force.