OBJECTIVE 5 – MOTION FORCES AND ENERGY (IPC 4)

INFORMATION ORIGINALLY FOUND FROM MIDWAY ISD WEBPAGE

Physics IIITAKS Review

IPC (4) (A) – Calculate speed, momentum, acceleration, work and

power

One way to describe the motion of an object is by its

speed. Speed is the distance an object moves in a given time

interval. As an equation it is

t

dv

The equation is given on the formula chart and the units for speed are m/s or km/h. This equation is for constant

or average speed.

Ex. An alien being driving a VW bug travels 150 km in 2 hours. What is the speed?

Note, km is a measure of distance and hours is a measure of time. Use the

units to help you make substitutions into the formula.

Now, you try this one (it is a little different)

An orangutan runs with a speed of 5 m/s for 10 s. How far does the

orangutan travel?

Another way to describe motion is acceleration.

Acceleration is the change in speed during a given time period. As a formula it is

t

vva if

vf is the variable for final speed and vi is the variable for initial speed (the speed changes – ya dig?) The units

are m/s2. The formula is on the formula chart.

A Cro-Magnon man jogs at a speed of 4 m/s. He spots a saber toothed tiger and increases his

speed to 6 m/s in a time period of 2 s. What is his acceleration?

Try this ‘un. A track sprinter starts from the blocks and

reaches a maximum speed of 12 m/s in 3 s.

What is her acceleration?

Notes on graphs. Which of the following shows an object

moving at a constant speed and which one shows an

object that is accelerating?

d

t t

d

New topic. A Ford Focus and a Hummer collide with

separate, yet identical trees. Which tree receives the most

damage?

Mass and speed can be combined to describe

momentum. Momentum is the product of mass and speed.

The form-you-lah is

mvp

p is momentum, m is mass and v is speed. The unit is kg·m/s

and the formula is on the chart.

So try this – A 0.5 kg baseball is thrown with a speed of 30 m/s. What is the momentum

of the baseball?

Conservation of Linear Momentum is one of four conservation laws in

physics.

It simply means that the momentum a system of

objects has after a collision must be the same momentum

a system has before the collision.

For example, let’s say a bowling ball strikes a stationary bowling pin. The ball has 120 kg·m/s of

momentum before the collision. If the ball has 100 kg·m/s of

momentum after the collision, what is the momentum transferred to the pin?

Nuthernewidea (“Heigh-ho, heigh-ho, it’s off to ? we go”)

Work means many things in everyday usage but has a specific

meaning in physics. Work is a force applied on an object over a

given distance. Symbolically

FdW

On the chart again. The unit for work is a N·m which is also

known as a Joule (J)

Is work done in the following cases? ‘Splain.

A student presses against a stationary chalkboard with her hand.

An intergalactic spacecraft coasts through space.

Three students push a disabled car across a parking lot.

Example – An extremely buff toddler applies a force of 550 N to drag a 220 kg comatose alligator 3

m across the kitchen floor. How much work does superchild do on

the alligator?

A concept that relates work and time is power. If the child pulls the alligator across the floor in 2 s or 20 s, the work

done is the same in each case. However, doing the work in 2 s requires more power than the

same work done in 20 s.

Power is the rate at which work is done. The units are Joules per second which is

also known as a Watt. And the formula is . . .

t

WP

Ex. Find the power required for the child to do the work describe on the alligator (a

few screens ago) in 2 s and 20 s.

IPC (4) (B) – The student investigates and describes applications of Newton’s

Laws

The First Law. Suppose a hockey puck slides across frictionless ice (you can buy this at Sam’s if you know where to look). What are the horizontal forces acting on

the puck? And the answer is . . .

None! The puck will continue to slide indefinitely at its

current velocity until some force (like friction, a hockey stick, a dead cat in the way)

acts upon it.

The same is true if the puck is initially at rest. It won’t move unless some net external force

changes it velocity (from zero to some value that is not zero).

This describes Newton’s First Law. How does the First Law explain

how spacecraft in our solar system (like the Voyager and Pioneer

probes) can travel through space for decades using little or no fuel?

Suppose you kick a soccer ball and a bowling ball (ouch!) with equal

forces. Which one will experience the greatest acceleration (change in

velocity, remember?)?

So, for a constant force, more mass means ____ acceleration.

Now, suppose you have soccer balls of equal mass and you

kick them with different forces. Which one will

experience the greatest acceleration (the one kicked

with the larger or smaller force?)?

Then we can say, for constant mass a larger force produces a _____

acceleration.

Putting the two relationships together

m

Fa

Or more commonly written as

maF Force is measured in kg·m/s2,

which is a Newton (N).

Keerful Podnahs - the force used in Newton’s 2nd Law is the net force.

The net force is the is the sum of the forces acting on an object.

For instance, a book resting on a table is not accelerating, even

though there are two vertical forces acting on it (what are they?).

The forces are equal, but in opposite directions, therefore there is no net force (the sum

is 0) and consequently no acceleration.

Ex. A 2 kg object starts from rest and reaches a final

velocity of 10 m/s in 5 s. What is the acceleration of the

object? What is the net force acting on the object?

Now try this. A force of 20 N to the right acts on a 2 kg object while

simultaneously a force of 8 N acts to the left. What is the net force on the

object? What is the acceleration?

The Third Law. Try this at home without adult supervision. Have a friend, neighbor or total stranger hold a piece of paper vertically.

Punch it with your fist as hard as you can.

Then, go outside and punch the bricks on the side or your house, again, as hard as you

can. One of these experiences is going to hurt and one isn’t.

Guess which one will hurt. Why?

The force your fist experiences when it hits the paper is only

as great as the force the paper can exert on your fist (not

much).

Likewise, the force the brick wall exerts on your fist is the same (only

in the opposite direction) as the force your fist exerts on the wall. So

when you get mad and punch the wall, it gets mad an punches back

at you. So be nice.

These examples describe Newton’s Third Law which says: if one object

exerts a force on a second object, the second object will exert an equal, but

opposite force on the first object. These are called “action-reaction”

pairs

A little tricky – Newton’s Third law describes two forces and two objects.

A net force (several forces) on one object is an example of which law?

Which of the following are action reaction pairs?

The weight of a person pushing downward on a chair and the chair pushing upward on the person.

A student pushing a box across the floor while friction acts against the motion of the box.

A kangaroo doing pushups pushes down on the floor while the floor pushes upwards on the kangaroo

IPC (4) (D) – The student investigates and demonstrates

mechanical advantage and efficiencies of various

machines

We use simple machines everyday. A couple of ways to

describe the usefulness of machines is their efficiency and mechanical advantage.

The ideal mechanical advantage (IMA – it’s called

ideal if we assume the machine is frictionless) is

calculated from

in

out

F

FIMA

Fout is the output force (the force a machine exerts on an

object) and Fin is the input force (the force exerted on the

machine

Ex. A prince uses a pulley system to raise an ogre to a second floor castle window. If the weight of the ogre is

1600 N and the prince applies a force of 80 N on the ropes of the

pulley, what is the IMA?

This formula is used for real machines (not frictionless ones). In the real world we never get as much work out of a machine as

the work we put into it.

Another way to describe the usefulness of a machine is its efficiency. The efficiency of a

machine is found from

100 efficiency % in

out

W

W

Recall, work is found by multiplying the force applied to an object by the

distance.

Ex. A 62.5 N force is applied to one end of a lever over a distance of 0.4 m. This raises a 200 N rock at the

other end of the lever 0.1 m. What is the efficiency of the lever?

Before substituting numbers into a formulas, decide what each of the values describe. A couple of hints solving efficiency problems for machines: the output force is

always greater than the input force and the input distance is always greater than the output

distance.

So back to the example. 62.5 N is the input force and 0.4 m is the

input distance. 200 N is the output force and 0.1 m is the output

distance.

Calculating

100m )4.0)(N (62.5

m) 0.1(N) 200( eff. %

% eff. = 80%

Yerz: A force of 20 N is used to push a 30 N box 2 m along an inclined plane. This raises the block 0.5 m above its initial

height. What is the efficiency of the inclined plane?