18/05/2017

1

Starter: 1 label the forcesA B 1. Label the forces

2. Were the forces balanced or unbalanced?3. What force did Daniel not have enough of?

Teapot racing!• 2a label the forces• 2b Describe the motion• 2c Show working to

calculate the net force

2N

2N

0.5N0.5N

Force, Mass and Acceleration

“The net force (Fnet) acting on an object can be calculated using its mass and acceleration. Force can be greater if you are bigger or faster.”

F=ma or

• What units?

Big rig: 43998.5 kg Race car: 699.9kg

If F =ma, at the same acceleration which has the greatest force?

Do:

• Page 39 , 40, 41 n your scipad

18/05/2017

2

• Glider https://www.youtube.com/watch?v=BamQnF6fguU

• That sinking feeling https://www.youtube.com/watch?v=iZqfrPW4Gic

• Glass doors https://www.youtube.com/watch?v=7z7AEpOVeNA

• Falcon https://www.youtube.com/watch?v=ewr-vzw4ZWo

Golden falcon stats:a = 108m/s²m = 5kgForce =

Fox stats:a = 14 m/s²m = 3.2kgForce =

Big rig: 43998.5 kg Race car: 699.9kg

If F =ma, the truck is accelerating at 2m/s² and the car and truck hit a wall with the same force, what would the acceleration of the racecar have to be?

Big rig: Race car:

m = 43998.5 kg m = 699.9kg

a = 2m/s² F = 87997N

F = m x a a = F / m

F = 43998.5 kg x 2m/s² a = 87997N / 699.9kg

F = 87997N a = 12.6m/s²

What is a force?

A force is a

push, pull

or twist.

! Recap

Say if the following are pushes, pulls or twists:

Writing on paper

Typing on a keyboard

Tug-of-war

Unscrewing the top off a bottle

Posting a letter through a door

Putting on a pair of socks

Turning a door handle

Pull

Pull

Push

Push

Push

Twist Twist

Twist Twist

18/05/2017

3

Forcemeters (spring balances) are used to measure the size of forces.

Forces are measured in newtons (N)

Types of force you need to know……

Friction Gravity

MagnetismAir resistance

Tension

Compression

Electrostatic

Nuclear

Describing the forces

Magnetism

Friction

Pressure

Tension

The force you get when two

material rub together

The force you get when a

material is being stretched

The force you get when a

material is being squashed

The force that enables

compasses to work

Describing the forces

Gravity

Nuclear

Air resistance

Electrostatic

The force that holds the particles

in a nucleus together

The force you get when an

object moves through air

The force that exists between all

objects with mass

The force you get between two

charged objects

Describing the forces

Drag

Boyancy

Support

Lift/Uplift

The force that opposes gravity

from liquids

The force that opposes gravity

from solids

The force that resists movement

through air and water

The force that opposes gravity

from gases

What do forces do?

Forces try to change an

objects motion

18/05/2017

4

Forces can change an objects ...

Forces on a speeding fish?

Upthrust

FrictionThrust

Weight

What is thrust?

A forward push

What is upthrust?

The upwards force on a body in a liquid or a gas

Label the forces acting on the moving ship shown:

buoyancy

Thrust

Air

resistance

Drag

Gravity/Weight

Balanced or Unbalanced?

Constant Speed

of 50Kmh-1

Ships thrust = Air resistance and Friction

BalancedNo change in motion!

Balanced or Unbalanced?

Ship hits a rock

Air resistance, Friction and force from

rock out match thrust from ship

UnbalancedBoat decelerates (Slows

down)!

18/05/2017

5

Force Questions

Which of the following is not a force?

A. Heat

B. Gravity

C. Tension

D. Friction

For a man standing up, in which direction does gravity act?

A. B.C. D.

© Boardworks Ltd 2003

For a fish in water, what do we call force Z?

A. Friction

B. Air resistance

C. Gravity

D. Upthrust

Z

In what units is force measured in?

A. Joules

B. Metres

C. Newtons

D. Seconds

What device is used to measure force?

A. Clock

B. Voltmeter

C. Joulemeter

D. Newtonmeter

18/05/2017

6

Net forces – stick it in your brainNet Force

When forces are balanced a Net Force will be zero.

When forces are unbalanced a Net Force will result.

Net forces can cause an object to accelerate, decelerate or turn.

Direction of a force is important. A negative force will act in the opposite direction.

There are two ways to calculate the net force acting on an object.

• 1. By adding up all the individual forces.

• 2. By using a net acceleration to calculate it.

The sum effect of more than one force is called the Net (or Resultant) force.

You can find out the net force by calculating the difference between opposing

forces.

If the resultant force is greater than zero the car will accelerate.

500 N 400 N

100 N

A resultant force of 100 N

is accelerating the car.

Adding up Forces

5N5N

20N

1.

Find the Net force:

10N

Resultant force = 20N -10N = 10N down

The block will accelerate down.

2.5N

5N

5N

5N

Resultant force = 5N - 0N = 5N right.

The vertical forces are equal in size and

opposite in direction so there is no resultant

force in the vertical direction.

The block will accelerate to the right.

13N

3.

10N

20N

10N

3N

7N

17N

Resultant force = 30 - 13 = 17N right.

The vertical forces are equal in size and

opposite in direction so there is no resultant

force in the vertical direction.

The block will accelerate to the right.

18/05/2017

7

This can be used to determine unknown forces.

Example

Barry stands on a bridge. Barry’s weight is 650N.

a) What is the value of the support force provided by the

bridge on Barry?

650 N

Weight force

650 N

Support force

Weight = 650N

A Net force will cause a Net acceleration. We can then use

Newton’s second law to calculate the Net Force

Using Net Acceleration

F = m x a

Example.A 600kg car is accelerating at 4.5 ms-2 away from an intersection. What is the net force acting on the car at this time?

F = m x a

= 600 x 4.5

= 2700N

Sometimes you will be required to use both methods to solve a problem.

Using both

Friction Force

= 15 – 11.4

= 3.6N

Thrust Force = 15N

A 3kg fish is accelerating through the water at a rate of

3.8ms-2. If the fish can generate a thrust of 15N, How

much friction is the water applying to it?

Net Force = mass x acceleration

= 3 x 3.8 = 11.4N

Net Force = 11.4N

Pages 42 and 43 in your scipad

Mass and Weight

• https://www.youtube.com/watch?v=_Z0X0yE8Ioc

In common language the terms mass and weight are used interchangeably.

“Mass is the amount of matter in an object, and is measured in kilograms.

Weight is a measure of force. It is the force on an object caused by gravity acting on that object. Weight is measured in ….”

Mass = 200kg

Weight force = 200 x 10 Weight force = 2000N

18/05/2017

8

Fw=mgWeight force = mass x gravity

Mass = 200kg

Acceleration due to gravity (on earth) = 10ms¯²

Weight force = 200kg x 10ms¯²

Weight force = 2000N down

Remember – forces have directions!

Do: Pages 42, 43

Label forces on a fighter pilot

https://www.youtube.com/watch?v=qJoLJAe_ihU

Fw = mg

Finding the weight force

This is the same as using F=ma, except we use a couple of different symbols.

Weight Force

(N)

Mass (kg)

Acceleration

due to gravity

(10 ms-2 on Earth)

Go through answers/ process for: Pages 42, 43

18/05/2017

9

Copy the table and use the formula F=ma and the value for gravity to calculate your weight force on these planets.

F = your mass x acceleration due to gravity

Planetary body Acceleration due to gravity in ms¯² Your weight force

Sun 274.13

Venus 8.87

Earth 9.81

Mars 3.77

Jupiter 25.95

Saturn 11.08

Neptune 14.07

So…

• Going back to our fighter pilot – 7g’s = what kind of force?

• How does this compare to our weight force on other planets?

• Which ones could you survive on without passing out from lack of blood to the brain?

What next:

1. Friction page in Scipad2. Air resistance page in scipad3. Make sure your definitions – 34 – are up to date4. Choose: Falling& gravity OR Parachuting

5. Create a 1 page summary of the ideas we have covered so far.

6. First pre-test = Friday!