Physics

Motion

Mechanics is the study of motion, and the forces and energies that affect motion.

-Kinematics describes how objects move

-Dynamics describes how forces cause changes in the

motion of objects

-everything is in motion

-when we describe an object’s motion, it is in relationship

to something else…we put it into a reference frame.

A frame of reference is saying where the observation is being made from.

A toy wagon is pulled in a forward direction along the ground at a velocity of 12 cm/s. The bug is crawling on the wagon, toward the front of the wagon at 2 cm/s.

Show the velocities with vectors.

What is the reference frame for the velocity of the wagon?

What is the reference frame for the velocity of the bug?

What is the bug’s velocity with respect to the ground?

A toy wagon is pulled in a forward direction along the ground at a velocity of 12 cm/s. The bug is crawling on the wagon, toward the front of the wagon at 2 cm/s.

If the bug turns around and crawls at 3 cm/s towards the rear of the wagon, what is its velocity with respect to the ground?

If a box is thrown out of the plane, what will be its motion relative to the ground? (Ignore air resistance.)

If a box is thrown out of the plane, what will be its motion relative to the plane? (Ignore air resistance.)

This train is moving at 90 mph to the left. If a baseball pitcher, on the train, throws a ball at 90 mph towards the back of the train, how fast is the ball moving relative to a person standing

on the ground?

This train is moving at 90 mph to the left. If the golfer hits a ball at 90 mph to the left, how fast will the ball be moving relative to

the train?

Distance vs. Displacement

Distance = how far an object has traveled. Units = m

Displacement = change in position of an object. Units = m

These two are often the same number, but not always. Displacement also involves a direction and is represented by a vector.

Distance vs. Displacement

A vector is an arrow that represents both magnitude and direction.

(Magnitude is how big something is. It is a number.)

A scalar is just a number (with or without units), no direction is mentioned.

Distance is a scalar. Displacement is a vector.

What distance do you travel on a normal day, from the time you get up until the time you go to bed?

For most people, most days, their displacement is zero. Why? What would be an exception to this?

Δ (delta)

stands for how much something has changed.

Δx would mean the change in position (displacement). You would find it by subtracting the beginning position from the ending position.

Δx = xf – xi Note: ‘f’ is the final position and ‘i’ is the initial position.

0 1 2 3 4-1-2-3-4

Speed is how fast an object is moving. It is always a positive number. Speed is a scalar quantity. Units = m/s

Velocity is the speed of an object with a direction. It is a vector quantity. It can be positive, zero, or negative.

To find the velocity use:

passed has much time how

position initial-position final

t

xx

t

xv if

Or more simply time

ntdisplaceme

t

dv

0 1 2 3 4-1-2-3-4

passed has much time how

position initial-position final

t

xx

t

xv if

A mouse starts at -2 and moves to 4. It takes it 3 seconds to do this. Find its velocity.

0 1 2 3 4-1-2-3-4

passed has much time how

position initial-position final

t

xx

t

xv if

A mouse starts at 4 and moves to -2. It takes it 3 seconds to do this. Find its velocity.

Acceleration is a vector quantity that describes how fast the velocity is changing. Units = m/s2

-When acceleration has the same sign as the velocity, the object is speeding up.

-When acceleration has the opposite sign as the velocity, the object is slowing down.

-When acceleration is zero….???

Average acceleration can be calculated from the following:

passed time

velocityinitial - velocityfinal

t

vv

t

va if

A car accelerates from rest up to 60 mph (96.5 km/hr or 26.8 m/s) in 8 seconds. Find its average acceleration.

When an object is falling to the ground on earth, its acceleration due to gravity is -9.8 m/s2. It doesn’t matter how heavy the object is!

A formula that is often useful in these situations and any other time that acceleration is constant is 2

2

1atd

A gumdrop is released from rest at the top of the Empire State building, which is 381m tall. Disregarding air resistance, calculate the displacement and the velocity of the gumdrop after 2.00s.

http://phet.colorado.edu/simulations/sims.php?sim=Motion_in_2D