Newton’s Laws of Motion Lecture

1. Three laws of motion: Isaac Newton developed these to describe how motion behaves

A. First law: an object at rest will remain at rest, an object in motion will stay in motion until an outside forces acts on the motion

to change it1. also known as the law of inertia2. inertia: to resist a change in

motion (either to a start or a stop of motion)

3. the greater the mass, the greater the inertia: push a Prius or Hummer? Stand in front of a flying golf ball or paper airplane?4. friction acts as an outside force: if

no friction then object will not stop: what environment like this already exists ?5. examples: coin/card, jumping in a bus

2. Second law: the net force of an object is due to its mass and how much it is accelerating

1. Force = Mass x Acceleration

F = M x A (mass in kg, accel in m/s2) (FeMA)

A FALLING OBJECT:

Acceleration (m/s2)

Force (N) 1kg

5 10

10

5

Why is this line at this at this angle?

Where would a 2kg line go?

The heavier the objectrequires more force to start or stop

3. Third law: every action has an equal and opposite reaction

(thus, forces come in pairs: acting on TWO different objects: the action object and the reaction object)

1. walking: push against floor and the floor pushes you

2. bouncing ball: force down creates a force up

3. rocket: gases forced out creates a thrust on the rocket

4. Newton’s cradle

4. Momentum

A. describes how much MOTION an object has

B. it is the inertia of motion

C. can be transferred to another object: pool balls

D. momentum = mass x velocity

(P = M x V)

ie: a bowling ball vs golf ball traveling at the same velocity, which has a greater momentum?

5. remember: the only way an object can be in motion is for the forces acting on it are unbalanced. The opposite is also true (?)

Momentum

• Momentum is the mass of a object times its

velocity.

• The units for momentum are kilogram-meter per second (kg·m/s).

Momentum

• The law of conservation of momentum states that as long as the interacting objects are not influenced by outside forces (like friction) the total amount of momentum is constant or does not change.

Momentum• The result of a

skateboarder throwing a 1-kg ball at a speed of -20 m/sec is that he and the skateboard with a total mass of 40 kg move backward at a speed of +0.5 m/sec (if you ignore friction).

mv = mv

1kg • 20m/sec = 40kg • ?

? = 0.5m/sec

We use positive and negative numbers to show opposite directions.

Collisions• When a large truck hits

a small car, the forces are equal.

• The small car experiences a much greater change in velocity much more rapidly than the big truck.

Which vehicle ends up with more damage?

Solving Problems

• If an astronaut in space were to release a 2-kilogram wrench at a speed of 10 m/s, the astronaut would move backward at what speed if the astronaut’s mass is 100 kilograms?

1. unknown: – … the velocity 2 of the astronaut (backward)

2. Known:– …velocity1 = 10 m/s; mass1= 2 kg;

– ...mass2 = 100 kg;

3. formula: (conservation of momentum)– m1v1 = P = m2v2

4. Work:– Draw a free body diagram.– 2 kg • 10 m/s = 100 kg • ?– ? = 20/100 = 2/10 = 0.2 m/s

Solving Problems