TAKS Objective 5

Motion , Forces

and Energy

Motion can be described as

a change in an object’s position

Average velocity (speed) is the change of position of an object over time

Velocity Graphs V = distance

time

Velocity (v) is the slope (rise over run) of a position (d) vs. time (t) graph

Velocity

0

20

40

60

1 3 5 7 9 11 13 15

Time (sec)

Dista

nce (

m) Series1

Series2

40 The diagram represents the total travel of a teacher on a Saturday. Which part of the trip is made at the greatest average speed?F QG RH SJ T

How do we work this one?

Calculate v = d/t for each segment.

Acceleration Graphs Acceleration (a) is

the slope of a velocity (v) vs. time (t) graph

Plotted on a distance vs. time graph, acceleration is an exponential curve

Acceleration

0

20

40

60

1 3 5 7 9 11 13 15

Time (sec)

Velo

city

((m

/s)(m

)

Acceleration is a change in an objects velocity (speed or direction)

When an object’s speed changes over time it is accelerating (or decelerating)

A = vfinal – vinitial time Units for acceleration

m/s/s or m/s2

Definition of a Force

A Force is a push or a pull

Balanced Force

A force that produces no change in an object’s motion because it is balanced by an equal, opposite force.

4 The picture shows the position of a ball every 0.25 second on a photogram. Using a ruler, determine the velocity of the ball.

F 3.5 cm/s

G 10.5 cm/s

H 14.0 cm/s

J 28.0 cm/s

Use the ruler on the side of the chart and the equation for velocity. The answer was H.

Measure from the center of ball 1 to the center of ball 2 and multiply by 4.

Unbalanced Forces

Are forces that results in an object’s motion being changed.

+

Friction

A force that acts in a direction opposite to the motion of two surfaces in contact with each other.

Friction

Friction causes an object to slow down and stop.

Since the amount of energy stays constant, the energy becomes heat.

Newton’s 1st Law of Motion

Object in motion stays in motion

Newton’s 1st Law of Motion

And Objects at rest stay at rest

Newton’s 1st Law of Motion

Until they are acted upon by unbalanced forces.

Inertia or Newtons 1st Law

Tendency for an object to stay at rest or moving in a straight line at a constant speed.

The mass (m measured in kg) of an object determines its inertia

Newton’s 2nd Law of Motion

Force = Mass X AccelerationF=maWeight (pull of gravity) is a commonly measured force,

calculated by F=mg, g is the acceleration due to gravity 9.8 m/s2

Newton’s 2nd Law of Motion

The greater the mass of an object, the greater the force required to change its motion.

Newton’s 2nd Law of Motion

The greater the acceleration of an object, the greater the force required to change its motion.

A 0.2 N B 0.8 N C 1.5 N D 6.0 N

11 The frog leaps from its resting position at the lake’s bank onto a lily pad. If the frog has a mass of 0.5 kg and the acceleration of the leap is 3 m/s2, what is the force the frog exerts on the lake’s bank when leaping?

Formula chart says F=ma, m is mass in kg, a is acceleration in m/s2.

So, .5 kg x 3 m/s2= 1.5 N

Newton’s 3rd Law of Motion

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

Newton’s 3rd Law of Motion

All forces come in action-reaction pairs

Ex: feet push backward on floor, the floor pushes forward on feet

27 A ball moving at 30 m/s has a momentum of 15 kg·m/s. The mass of the ball is —

A 45 kg

B 15 kg

C 2.0 kg

D 0.5 kg

Formula Page says that Momentum = Mass x Velocity

So 15 kg.m/s = M x 30 m/s solving for M it is:

Work Work: using a force

for a distance W = F x d The work done by forces on an object

= changes in energy for that object. Work and Energy are measured in

Joules 1 Joule=1 Newton • meter

42 How much work is performed when a 50 kg crate is pushed 15 m with a force of 20 N?

F 300 JG 750 JH 1,000 JJ 15,000 J

Use the formula Work = Force x distance

Force of 20 N x 15 meters = 300 Joules Answer:

Why use a machine?

In an ideal (perfect) machine the work put into the machine (Win) = the work put out by that machine (Wout)

Machines make work easier

The ideal mechanical advantage of a machine (IMA) of a machine is the number of times the output force is larger than the input

force IMA=Fout/Fin A machine can only make this

happen by moving the input force through a farther distance than the output force

Fin • din=Fout • dout

48 The diagram shows an electric motor lifting a 6 N block a distance of 3 m. The total amount of electrical energy used by the motor is 30 J. How much energy does the motor convert to heat?

F 9 J

G 12 J

H 18 J

J 21 J

Work Output = Resistance Force x Resistance Distance

Workout = 18J = 6N x 3m

Work Input = 30J done by the motor

The difference is lost as heat due to friction, which is 30J – 18J = 12J

Answer G

Real Machines use Energy No real machine is

100 % efficient. i.e. none put out more work than is put in

Efficiency of a machine is work output/work input X 100 %

Eff = Wout X 100%

Win

Machines use power Power: the rate at

which energy is used (work is done)

P=Work/time Power is measured in

H.P. or watts 1 watt = 1 Joule

1 sec

A accelerated rapidly

B remained motionless

C decreased its velocity

D gained momentum

45 If a force of 100 newtons was exerted on an object and no work was done, the object must have —

Work = Force x Distance

Work = 0 Force = 100 N so

0 J = 100 N x d

distance must be 0

It did not move!

6 Types of simple machines

Some Simple Machines:

Inclined planes Screws Pulleys Wheel and axle Levers Wedge

Universal Law of Gravitation

All objects in the universe attract each other by the force of gravity

Universal Law of Gravitation

Gravity varies depending on two factors:

1) the mass of the object doing the pulling, and

2) the distance from the center of that object

On Earth gravity = 9.8 m/s/s

For every second that an object falls its speed increases by 9.8 m/s

Weight= Mass (m) X gravity (g)

Weight Unit of mass = kg Unit of acceleration =

m/s/s Unit of weight = Newton 1 Newton= about ¼ pound

USE THE FORMULA PAGE

Some of the problems require you to grid in an answer. Make sure you pay attention to the decimal point in the square in the middle.

.

0 0 0 0 0 0

1 1 1 1 1 1

2 2 2 2 2 2

3 3 3 3 3 3

4 4 4 4 4 4

5 5 5 5 5 5

6 6 6 6 6 6

7 7 7 7 7 7

8 8 8 8 8 8

9 9 9 9 9 9