AP Physics U1: Motion and Force

What is a field force? Examples?

"At a distance." A force that is caused from a mass exerting an invisible influence on another object through space. Works through invisible fields - the second object interacts with the field of the force. ex: gravity, EM, nuclear

What is the relationship between -/+ acceleration and -/+ velocity?

+V: • +a = increase in v • -a = decrease in v -V: • +a = decrease in v • -a = increase in v If the velocity and acceleration are in the same direction, velocity increases. Different directions? Velocity decreases.

What are the two components of projectile motion and what are their properties?

1) Horizontal speed of a projectile is constant 2) Vertical motion is accelerating at g and its displacement in the y-direction is identical to that of a free falling body

How do we add vectors?

1) Split vectors into components taking into account + and - direction. 2) Add the x components and the y components separately. 3) Use the pythagorean theorem to find the resultant vector from its vector components. 4) Use inverse tan to find the angle but be careful - it needs to be in the first or fourth quadrants. If it's in the other quadrants, add 180˚.

What is a contact force? Examples?

A force that is caused from physical contact between two objects. ex: push or pull, normal force, tension, air resistance, and friction

What does positive displacement mean? What does negative displacement mean?

A positive displacement means it was displaced it the positive x-direction and a negative displacement means it was displaced in the negative x-direction (depending on your on your coordinate system).

*What is a force? Include unit.*

A push or pull on some object. Measure in Newtons.

What is the difference between a vector and scalar quantity? How are vectors denoted?

A vector has both magnitude and direction. They are symbolized with boldface type and arrow over the top or, when there's only two directions, a plus or minus sign. A scalar has magnitude, but no direction.

What is a net force? Include variable.

Aka the unbalanced force, it is the sum of all forces acting on an object which causes acceleration. If EF = 0, velocity is constant.

What is projectile motion?

An object affected only y gravity that forms in curved parabola shape.

What is Newton's First law?

An object moves with a velocity that is constant in magnitude and direction unless a non-zero net force acts on it. An object stays in its current state of motion (rest or movement) unless a non zero force acts on it.

*What is a free falling object?*

Any object moving freely under the influence of gravity alone (no air resistance), regardless of its initial motion.

What is the difference between speed and velocity?

Average speed is the path length over the elapsed time. If two cars went from point a to to point b but one went along a winding path, while the displacement is the same, since the path lengths are different the speed is different.

What happens when we drop a ball straight down versus when we throw a ball forward with the same initial velocity?

Both balls hit the ground at the same time because the horizontal and vertical motion is independent. They both have the same distance to fall which is what affects the time.

What is the acceleration of free falling objects? Variable?

Denoted by g, it is 9.8 m/s^2. Depends on mass of earth and radius from earth's center - which means the value decreases as the altitude increases.

What is the difference between displacement and distance?

Distance is not a vector while displacement is. This means that if an object goes 5 meters forward and then 5 meters backwards, while its distance is 10 m,the displacement is 0 m because *it did not change position.* One of the distances is -.

What does "lim" mean in instantaneous velocity?

It means you took ∆t and made it smaller and smaller to 0 - the limit of ∆t is approaching zero.

*What are the four equations of motion (kinematics equations)?*

Note: only used when acceleration is constant.

*What is 2-dimensional motion?*

Objects that move in both the x and y directions simultaneously under constant acceleration?

What does +/- acceleration mean?

Positive acceleration means acceleration in the positive direction - a push towards the positive direction. Negative acceleration means a push to the negative direction.

*What is velocity? Include unit and variable.*

Rate of displacement. V, m/s

What is the average velocity on a position vs. time graph?

The average velocity of an object during the time interval ∆t is equal to the slope of the straight line joining the initial and final points on a graph of the object's position versus time.

What is average velocity? Include variable and equation.

The change in displacement over the change in time. Average speed does not show you the speed at each point, just the average. *Another way to find average velocity is just to find the average of instantaneous velocities!!!*

*What is displacement? Include unit, equation, and variable.*

The change in position. It is a vector, which it means it has direction and magnitude. ∆x, ∆x = x𝑓 - x𝑖, meters (m)

What is a frame of reference?

The choice of coordinate ages that defines the starting point for measuring an quantity, an essential first step in solving any problem in mechanics. Set up a coordinate system!

*What is position? Include unit and variable.*

The coordinates of the object at any time. x, meters (m)

*What is motion?*

The displacement of an object from one place in space and time to another.

What does the area under the graph of v versus t represent?

The displacement ∆x of the object.

What is the normal force?

The force a surface exerts on an object on the surface that is perpendicular to the surface.

When does using inverse tan to find the angle of vector components not work?

The formula gives the right answer only if the the vector is in the *first or fourth quadrants.* If it's in the other quadrants, we add 180˚.

How do we solve projectile motion problems?

The horizontal and vertical parts are independent from each other. In fact, projectile motion can be described as a superposition of two independent motions in the x- and y-directions. We must divide the parts into its horizontal motion and its vertical motion, which, when combined, makes the parabolic shape. Once we split them up, we can apply the kinematics equations separately to both components. *Remember to watch the positive and negative signs!*

How are the three graphs of motion related?

The instantaneous velocity in v vs t is the slope of x vs t and the instantaneous velocity in a vs t is the slope in v vs t. Review and practice all three graphs - it is best to practice physically!

What is Newton's Third Law?

The law of action and reaction forces. If you exert a force on a rubber band of 10 pounds, 10 pounds are also exerted on you. The action force is equal in magnitude to the reaction force and opposite in direction.

What is tension?

The magnitude of the force exerted along the rope - same at all points in the rope.

*What is acceleration? Include unit, equation, and variable.*

The rate of change of velocity. a, m/s^2 = ((m/s)/s)

Why does the scale reading on an elevator change when something is accelerating?

The scale reading reads the normal force so when it accelerates, the normal force increases

What is instantaneous velocity on a position vs. time graph?

The slope of the int tangent to the position vs. time curve.

What is instantaneous velocity? Include variable and equation.

The velocity of our car at a particular instant in time. That means the instantaneous velocity is the limit of the average velocity as the time interval ∆t becomes infinitely small.

How are free-body diagrams and solving for force problems related?

The way to solve for force problems is using newton's second law.When we do a free-body diagram, we draw all the forces on the object. Therefore, in Newton's EF = ma equation, free body diagrams gives us the net force, or the EF part of the equation. Note that each axis - the horizontal and the vertical, gets its own equation.

*What is a free-body diagram?*

Way to organize force problems which is an important 1st step in problem solving. It is the drawing and identification of all forces acting on an object.

*How do we deal with vectors that do not fall on the axes of the coordinate system?*

We break the vector into its components along a right triangle. Each vector has a x- component and a y -component whose combined effects create the vector we split up. We can use sine and cosine to find the vector magnitudes since it is a right triangle. If we know the components, we can use inverse tan to find the angle and the pythagorean theorem to find the resultant!

What happens when the initial velocity of a projectile is at an angle?

We componet-ize it! The x-component is the initial velocity of the horizontal motion while the y-component is the initial velocity of the vertical motion. Remember: sketch out the problem and label what you know/don't know and find appropriate equations.

What happens when, in the free body diagram, the object is at an angle?

We move the x and y axes to be along the path of acceleration and then component-ize forces not along the axes.

When acceleration is constant, can we assume average acceleration = instantaneous acceleration and why?

Yes because when an object moves with constant acceleration, the instantaneous acceleration at any point in a time interval is equal to the value of the average acceleration over the entire time interval. Therefore, average acceleration does equal instantaneous when a is constant and we can eliminated the bar.

Do the kinematics equations apply to free falling objects?

Yes! Acceleration is constant!

Is the acceleration for a free falling object going upwards and downwards the same?

Yes! Even though its going upwards, it still has an acceleration of -9.8 m/s^2, which means it will slow down, stop for a very short time, than come back down to earth.

Do all objects fall with the same acceleration? Why?

Yes. Two different objects of different masses are not pulled with the same force. While an object with a mass of 100 kg may attract 100x the force than a 1 kg object, since the 100 kg object also has 100x the mass, it cancels out in a = f/m because of inertia.

If a car were to go straight for 3 m, then turn right and goes 4m, how would you calculated the displacement?

You would find the hypotenuse!

What is Newton's Second Law?

mass in kg

How do we find weight? Include units.

w = mg Weight in Newtons.

*What are the three graphs of motion?*

x vs t, v vs t, and a vs t.