Newton's Second Law

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The acceleration of an object is directly proportional to the net force acting upon it. The constant of proportionality is the mass.

F=MA A=F/M

F = Force = A push or pull

M = Mass

A= Acceleration = speeding up, slowing down or changing direction

 

Mass = 5 kg

Letís assume that the wheels of a 5-kg car apply 10 N of force. What is the net force if friction and drag are negligible?

 

The net force would equal 10 Newtons.

 

 

What is the acceleration of the car?

Force

= MA

 

10

= 5A

 

Acceleration =

2 m/s2

 

 

 

Mass = 6 kg

What is the net force if the wheels of the 5-kg car apply 10 Newtons but a 1-kg parachute applies 3 Newtons in the other direction?

 

The net force would equal 3 Newtons. The total mass = 6 kg.

What is the acceleration of the car?

Acceleration

= F/M

 

 

Acceleration

= 3/6

 

 

Acceleration =

0.5 m/s2

 

 

 

 

Mass = 10 kg

A rocket is added to the car and applies an additional force of 10 Newtons. The wheels still apply 10 N. What is the net force if the parachute continues to apply 7 Newtons in the other direction? The total mass of the car, rocket and parachute is 10 kg.

 

The net force would equal 13 Newtons. The total mass = 10 kg.

What is the acceleration of the car?

Acceleration

= F/M

 

 

Acceleration

=13/10

 

 

Acceleration

=1.3 m/s2

 

 

Big masses are hard to accelerate. Big masses require big forces to change speed.

 

Small masses are easy to accelerate. Small masses require small forces to change speed.

Objects move in the direction they are pushed or pulled.

Objects accelerate more quickly when a greater force is used.

 

Assume that both steam engines below apply the same amount of force.

 

A heavy train has a difficult time accelerating. Big masses require big forces to change speed.

Acceleration =

Force / Mass

Acceleration =

100% / 100%

Acceleration =

1

 When the same force is applied to a less massive train its acceleration is greater. Small masses require small forces to change speed.

Acceleration =

Force / Mass

Acceleration =

100% / 10% of the big train

Acceleration =

10 times greater than the big train

 

Law 1: Inertia

Law 3: Action/Reaction

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