Motion

Session 2

Acceleration and Force

Learning Objectives

• TLW know concepts of force and motion evident in everyday life (TEKS 4)

• TLW be able to assess the relationship between force, mass, and acceleration, noting the relationship is independent of the nature of the force (TEKS 4.D)

• TLW demonstrate relationships of force, mass, and acceleration using moving toys, falling objects, etc. (TEKS 4.D)

• TLW perform calculations to determine force, mass, acceleration of everyday objects (TEKS 4.D)

I. AccelerationA. Acceleration - any change in

velocity; speed or direction

1. acceleration is NOT just speed but also direction

2. In a car, when a driver steps on the gas pedal, the car goes faster or accelerates

3. But, when the brakes are applied – the car slows down

**this is also acceleration =

because there is a change in velocity

4. When a car makes a turn, even if the speed does not change,

it is accelerating

B. Acceleration information

1. If acceleration is small = speed increases gradually2. If acceleration is large = speed increases rapidly3. Positive acceleration = velocity increases or speeds up4. Negative acceleration = velocity decreases or slows down (decelerationdeceleration)

5. Acceleration of an object is zero if velocity is constant

~ NO CHANGE ~

in speed or direction =

**** NO ACCELERATION ****

6. Formula for calculating accelerationacceleration = (final velocity – initial velocity)

timea = v/t

final velocity = the speed at which the object ends

initial velocity = the beginning speed of the object

7. Ex. A water balloon falls off a second-story windowsill. The water balloon starts from rest and hits the sidewalk 1.5 s later with a velocity of 14.7m/s. What is the average acceleration of the water balloon?

a = (final velocity – initial velocity) time

a = (14.7 m/s – 0 m/s) / 1.5 s

a = 9.8 m/s2

8. The units for acceleration is meters per second squared or m/s2.

9. Ex. A bike rider was traveling north 13 m/s and after 3 seconds his speed was 3.25 m/s.

What is the rider’s acceleration?

a = (final velocity – initial velocity)/t

a = (3.25 m/s – 13 m/s) / 3 s

a = (-9.75 m/s) / 3s

a = -3.25 m/s2

* a negative # (-3.25) means acceleration is slowing down

Group Practice

4.Marisa’s car accelerates at an average rate of 2.6 m/s2. Calculate how long it takes her car to accelerate from 24.6 m/s to 26.8 m/s. Use this formula: time = (final v – initial v)/ acceleration.

t = unknown initial v = 24.6 m/s

final v = 26.8 m/s

acceleration = 2.6 m/s2

Group Practice

5. A cyclist travels at a constant velocity of 4.5 m/s westward, then speeds up with a steady acceleration of 2.3 m/s2. Calculate the cyclist speed after accelerating for 5.0 s.

Use this formula: final v = initial v X a X t

final v = unknown a = 2.3 m/s2

initial v = 4.5 m/s t = 5.0 s

Group Practice

1. Natalie accelerates her skateboard along a straight path from 0 m/s to 4.0 m/s in 2.5 s. Find her average acceleration.

Group Practice

2. A turtle swimming in a straight line toward shore has a speed of 0.50 m/s. After 4.0 s, its speed is 0.80 m/s. What is the turtle’s average acceleration?

Group Practice

3. Find the average acceleration of a northbound subway train that slows down from 12 m/s to 9.6 m/s in 0.8 s.

Independent Practice

• Acceleration - 1

II. Force

A. Force – the cause of acceleration or change in an object’s velocity.

Force equals mass times acceleration

F = m x aOn earth to calculate F multiply mass (kg) by

acceleration due to gravity (9.8 m/sec2)Force is measured in kg m / sec2 which

equals a Newton (N)

Example – if a student weighs 50 kg what is the force he exerts?

50 kg x 9.8 m/sec2 = 490 kg m / sec2 or 490 N

Group Practice - #1

• What is the force exerted by a 4 kg rock on the earth? (g = 9.8 m/s2)

Group Practice - #2

• What is the acceleration of a 10 kg cat exerting a force of 50 N?

Group Practice - #3

• A car accelerates from a stop to 50 m/s in 5 s. At this point it is exerting a force of 10,000 N. What is the mass of the car?

B. Types of forces

1. Net force – the combination of all forces on an object

a. An object accelerates in the direction of the net force

b. It won’t accelerate if the net force is zero

2. Balanced forces – forces on an object that

creates a zero net forcea. An object doesn’t move with

balanced forcesb. Balanced forces cancel

each otherc. Ex. Tug-of-War between

equal sized teams

3. Unbalanced forces – action on an object that creates a positive net force

a. An object moves if forces are unbalanced

b. The object moves in the direction of the greater force

C. Forces that affect motion1. Friction – a force that acts

against the direction of motiona. Because of friction – a

constant force must be applied to an object to

keep it movingb. Ex. a car - no gas - will

stop

c. Amount of friction depends on the surfaces

1. Both surfaces rough –

greater friction

2. Both surfaces smooth –

less friction

What’s easier to run on – a beach or a paved road?

2. Air Resistance – a form of friction

a. Amount of air resistance depends on objects:

1. Speed

2. Shape

3. Size

b. The more aerodynamic – the less air resistance

3. Gravity - the attraction between two particles of matter due to their masses

a. Gravity is a force

b. Every object exerts a gravitational force

c. All objects are pulled towards the center of Earth due to the force of gravity

Calculations

• Independent Practice Set – velocity, acceleration, force

Lab

• One, some, or all of the following labs will be performed from IPC Manual– How is the speed of the car changing? (p. 14 & 15)– What is the relationship between force, mass, and

acceleration? (p. 16 – 19)– How does increasing the mass of the car affect its

acceleration? (p. 20 – 21)

• Periodic Groups will– Read lab procedures together– Set up labs according to scientific method– Identify potential hazards, precautions, and PPE (if needed)