AP Physics 1- Unit 2

Newton's 1st Law

An object at rest stays at rest and an object in motion stays in constant motion (with the same speed and in the same direction) unless acted upon by an unbalanced/external force.

Newton's 2nd Law

Acceleration is produced when a force acts on a mass. The greater the mass (of the object being accelerated) the greater the amount of force needed (to accelerate the object).

Newton's 3rd Law

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

Relationship of Net force= Mass x Acceleration

Net Force and acceleration are in the numerator, so they must be directly proportional and since mass is a scalar quantity (can't be negative) while net force and acceleration are scalar quantities, direction of acceleration dictates the direction of the net force.

Weight/Force due to gravity

Results from the interaction of 2 objects with mass and is always down. Fg= mass x gravity. The unit for weight is Newton (N).

Normal Force

Results from the interaction of an object with a surface. It is always perpendicular to the surface.

Tension

Results from the presence of a string/rope/cable. Same at each ends of one continuous rope. It is always in the direction the string is pulling on the object.

Static Friction

Results from the microscopic imperfections in two surfaces when an object is not moving. It always opposes the potential direction of motion.

Kinetic Friction

Results from the microscopic imperfections in two surfaces when an object is moving. It always opposes the potential direction of motion.

Applied Force

Results from a person or a thing. It is always in the direction of the push or pull.

Air Resistance/Drag

Results from the interaction of the microscopic imperfections in an object and air molecules. It is always opposing the direction of motion.

Force Tool Kit

1) Draw multiple objects in motion

2) Define a line of action (multi-dimensional coordinate system)

3) Draw multiple free body diagrams (1 per object)

4) Explicitly write that Net Force= Mass x Acceleration

5) Net Force for the system as a whole (in line of action) AND net force of forces in each dimension for each object (horizontal and vertical).

Elevator Rides

When you are in an elevator, your actual weight (mg) never changes. However, you feel lighter or heavier during the ride because your apparent weight increases when you are accelerating up, decreases when you are acceleration is down, and is equal to your weight when you are not accelerating at all. Your apparent weight is then a normal force (or what the scale reads).

How does the acceleration and mass of an object compare in an action reaction pair?

Since acceleration is indirectly proportional to mass, the acceleration of a smaller object with less mass is greater than a heavier object with more mass. Due to Newton's 3rd Law, there is an equal and opposite reaction force so mass times acceleration of the larger object equals the mass times acceleration of the smaller object.

When is the normal force equal to the weight?

When the object is on a flat, horizontal surface and their is no external force acting on the vertical component of the object (gravity is not an external force).

How are you feeling when the elevator begins to speed up in the positive direction?

Heavy because velocity is greater than or equal to zero and the acceleration is greater than zero so the normal force is greater than your weight.

Why is Newton's 1st Law also called Law of Inertia?

Because mass is a measure of inertia so more massive objects are harder to put into motion and harder to stop once in motion.

How does the sum of the forces (net force) acting on an object affect the motion of that object?

If the net force is zero, then the object will be motionless. And if the net force equals mass times acceleration, then the object will be in motion.

Spring/Elastic Force

Results from the presence of a spring/elastic "thing". It is always in the direction the spring/elastic is pulling on the object.

Why do we use free body diagrams?

We use FBDs to show forces acting on an object. Using arrows to show direction and length of the arrows to show the size of the force (force=vector quantity). Also, we represent an object as a dot and draw the arrows away from the dot connecting arrows to the dot and labeling all of the arrows.