Newton's Three Laws of Motion

Over the past 2 weeks, we have been deriving Newton's three laws of motion by performing the hover disk lab and the fan cart lab.

Newton's First Law

• An object at rest or traveling at a constant speed will continue to do so, unless a net force acts on it.

• Any object moving at a constant speed or at rest, has no net force acting on it. (Fnet = 0). The hover disk lab allowed us to see how this law works. When the hover disk is at rest or moving at a constant speed, it will not have a change in velocity, or accelerate, because there is no net force acting on it.

• Although there is no net force acting on the object because the object is at rest, there are still many forces acting on everything. In the interaction diagram, you can see that there is a gravitational force acting down on person 1, person 2, the earth, and the hover disk. But what allows them to not fall through the ground is the normal force acting up on them.

• Here is an example where the object remains at a constant speed. Here we see that there is so net force on the object; however, there are still gravitational and normal forces acting on oerson 1, person 2, the disk, and the earth.

Newton's Second Law

• Force = mass (acceleration)

• From the fan cart lab, we derived this equation. We measured the acceleration of a 0.3 kg fan cart when turned on high with a constant force of 0.15 newtons. We then added weights of different masses to the fan cart to find different accelerations. From this we found that acceleration is the change in velocity divided by the change in time.

Collected Data

0.3 kg cart: 0.450 m/sec^2

0.4 kg cart: 0.380 m/sec^2

0.6 kg cart: 0.208 m/sec^2

1.1 kg cart: 0.146 m/sec^2

1.4 kg cart: 0.123 m/sec^2

1.7 kg cart: 0.105 m/sec^2

• To derive this equation, we found that as the mass increases, the acceleration decreases, meaning that mass and acceleration are inversely proportional. We also had to take into account that the force remains constant; therefore, we came up with the equation F = ma.

Newton's Third Law

• When two objects interact, they exert equal and opposite forces on each other.

• These forces are:

• equal in magnitude

• opposite in direction

• same type of force

• The hover disk lab helped us see how this law is true. When the hover disk is moving with a net force and is stopped by person 1's hand, the disc and the hand with exert and equal but opposite force on each other, which will be a normal force. 

Real World Connection

Here is a website that tells how Newton's laws of motion relate to a dancer.

http://physicsofballet.homestead.com/newtonlaws.html