Forces

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Forces

K Warne

http://www.sciencecafe.org.za/







Effects of Forces

Identify a force acting in each picture.

• what effect the force is having.

Force: …………………..….

Effect: …………….……….

Force: …………………..….

Effect: …………….……….

Force: …………………..….

Effect: …………….……….

Force: …………………..….

Effect: …………….……….

Force: …………………..….

Effect: …………….……….








Types of Motion

Investigation• Aim: To investigate the

four different types of motion.

• Apparatus: – Wooden block– Spring balance or elastic

band– Flat surface.

Pull Gently

Pull Harder

No movement

Constant speed

Pull a lot Harder

ACCELERATE!

Pull ??

DECELERATE!








Types of Motion - Conclusion

Pull Gently

Pull Harder

No movement

Constant speed

Pull Harder

ACCELERATE!

Pull ……….

DECELERATE!

Expt. Motion Force (N)

1 No movement 8.4

2 Constant speed 6.5

3 Acceleration 14.3

4 Deceleration 5.2

Conclusion:

A force seems to oppose the motion.

This force is greater when the object is stationary.








FRICTION

• Friction is a force that acts …………………. to the ……………………… whenever objects that touch each other are in ………… ……….

• Friction is …………………. when the object is ………. MOVING!!

………………

…………








Balanced Forces

• Two forces act on an object• Forces are ……………………………..• OPPOSITE in DIRECTION.• Forces are said to be …………………………• The object’s motion will ………………………... (It ill be

at ………….. or have a …………………… velocity.)

Newton’s First Law -…………

”A body continues in its ……………….. or of ………… motion in a ……………….., unless acted on by a ……………… force.“








Unbalanced Forces

• Two forces act on an object• Forces are ………………………….. in SIZE• Forces are said to be ………………………• The object’s motion ……………. (……………...)

Newton’s Laws Second Law

A ………………….. force on an object causes it to ………………… in the ………………. of that force.








Inertia

1. A body has a ............................................ any changes to its state of motion.

• This resistance is known as ........................

• If the card in the picture is flicked ................. ........................................ Inertia keeps the peg stationary when the card is moved quickly.

• The peg’s Inertia is overcomes .................. ................ which try to keep it’s position on the card.

2. The moon was moving past the earth in a straight line but became ................. by the ............................

• Gravity does not act against the direction of motion (90o) so the motion continues because ............................................... to .....................

• (The question is who threw it in the first place!)

Every object in a state of uniform motion tends to remain in that state of motion unless an external (unbalanced)force is applied to it.

Gravity

MotionEarth

1.

2.

Motion

Inertia

Friction








Newton’s Laws Second LawA resultant force on an object causes it to accelerate in the

direction of that force.

Fres = m x a

F m

a

F

m

T

W

These are the two most basic scenarios.








Newton’s 3rd Law"For every action there is an equal and opposite reaction.”

The gasses experience a force backwards out of the rocket, this has an equal but opposite reaction force which drives the rocket forward!

The force exerted by the hand on the head is the same magnitude as the force exerted by the head on the hand!

The head hits the hand just as hard as the hand hits the head!








An apple on a table. Non contact forces

Gravity

• Exerted by the earth on the apple

• Reaction by the apple on the earth.

Contact forces: Table

Apple exerts a force down on the table.

Table exerts a Normal or Reaction force on the apple.

•

All these forces are equal so there is no resultant force.

• Forces on apple: gravity (down) & reaction (up)








Moving Car• What forces act on the car when it moves with

constant speed?

•

Weight

• The force of gravity exerted by earth on the car.

Road holding car up

Thrust (engine pushing forward)

Friction (Drag)








Mass & WeightMass (kg):• This is a measure of

the …………………. (stuff) that makes up a body.

Weight (N):

• This is the ……….. of …………… exerted on a body’s mass by the earth (planet).

Fg

1 kg








Mass & Weight

Mass (….):• Measured in ……………..

using a ……………….

Weight (….):

• Measured in …………….. using a ……………….. or …………….. meter.

0.5 kg








Mass & Weight - tableInvestigation: To determine the relationship between an object’s mass and weight.

Method: We are going to weigh a number of different masses on a spring balance.

We will record the mass and weight in each case. (In a table.)

We will then analyse the results to see if any patterns (relationship) can be seen.

Mass/kg Weight/N








Mass vs Weight - GraphMass & Weight

0

0.5

1

1.5

2

2.5

3

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

Mass/kg

Weig

ht/

N

Weight/N

From the graph we can see that the weight rises as the mass is increased. The straight line indicates a direct proportion between mass and weight. (Double the one the other will also double).

Line corrected for systematic error scales were not zero’d correctly.

Graph should pass through (0;0)








Mass & Weight - Results

Mass/kg Weight/N

0.05 0.3

0.10 0.8

0.15 1.3

0.20 1.8

0.25 2.3

0.30 2.8

From the table we can see:

• As the mass increases the weight increases.

• The weight is (roughly) equal to the ten times the mass of the object.

W = mass x 10Conclusion: The weight of an

object in Newtons is equal to the object’s mass in kilograms multiplied by ten.








Hi -

This is a SAMPLE presentation only.

My FULL presentations, which contain loads more slides and other resources, are freely available on my resource sharing website:

www.sciencecafe.org.za

(paste into your browser if link above does not work)Have a look and enjoy!

Keith Warne









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