Jim is walking down the street with a speed of 3 m/s. An angry mob starts chasing him so he accelerates to 6 m/s in 2 seconds. What is Jim’s acceleration? Show ALL of your work.

Thursday, September 03, 2015

Given: Work:

a=

vi=

vf=

t=

Formula: Answer:

Acceleration

I can compare an object’s acceleration and velocity.

Learning Target

When an object’s velocity and acceleration are in the same direction, the object is speeding up.

When an object’s velocity and acceleration are in the opposite direction, the object is slowing down.

Acceleration and Velocity, Directions

Negative acceleration does not necessarily mean the object is slowing down.

If the acceleration and velocity are both negative, the object is speeding up.

The word deceleration has the connotation of slowing down.◦ This word will not be used in the text.

Acceleration

A freely falling object is any object moving freely under the influence of gravity alone.◦ Acceleration due to gravity is 9.80 m/s2

It does not depend upon the initial motion of the object.◦ Dropped – released from rest◦ Thrown downward◦ Thrown upward

Freely Falling Objects

We will neglect air resistance. Free fall motion is constantly accelerated

motion in one dimension.◦ Use model of a particle under constant acceleration

Objects in a free fall near the Earth experience constant acceleration.

Let upward be positive Use the kinematic equations

◦ With ay = -g = -9.80 m/s2

◦ Note displacement is in the vertical direction

Acceleration of Free Fall

Initial velocity is zero

Let up be positive Use the kinematic

equations◦ Generally use y

instead of x since vertical

Acceleration is◦ ay = -g = -9.80 m/s2

vi= 0 a = -g

Free Fall – An Object Dropped

ay = -g = -9.80 m/s2

Initial velocity = 0 With upward being

positive, initial velocity will be negative.

vi≠ 0 ay = -g

Free Fall – An Object Thrown Downward

Initial velocity is upward, so positive

The instantaneous velocity at the maximum height is zero.

ay = -g = -9.80 m/s2 everywhere in the motion

v = 0 vi≠ 0 ay = -g

Free Fall – Object Thrown Upward

Initial velocity at A is upward (+) and acceleration is -g (-9.8 m/s2).

At B, the velocity is 0 and the acceleration is -g (-9.8 m/s2).

At C, the velocity has the same magnitude as at A, but is in the opposite direction.

The displacement is –50.0m (it ends up 50.0 m below its starting point).

Free Fall Example

Falling

Air resistance will increase as it falls faster◦ An upward force on

the object Eventually gravity

will balance with air resistance

Reaches terminal velocity - highest speed reached by a falling object.

Force of gravity is constant

Terminal velocity

air resistance increases as you speed up until the force is equal

Equal forces, no acceleration constant velocity terminal velocity

If touching going in the same direction add the velocities

If touching going in opposite directions subtract the velocities

Relative Velocity – Touching

If not touching going in the same direction subtract the velocities

If not touching going in opposite directions add the velocities.

Relative Velocity – Not Touching

Example

15 m/s + 1 m/s = 14 m/s

A student walking down the aisle on a school bus while bus is in motion.

Example

15 m/s + =1 m/s 16 m/s

A student walking up the aisle on a school bus while bus is in motion.

Suppose you are on a train platform as the train rushes through the station without stopping. Someone on board the train is pitching a ball, throwing it has hard as they can towards the back of the train. If the train’s speed is 60 mph and the pitcher is capable of throwing at 60 mph, what is the speed of the ball as you see it from the platform?

a) 60 mph to the right b) 120 mph to the right c) 0 mph (not moving) d) 60 mph to the left

Relative Motion:

60 mph Train60 mph