Forces Introduction

• Intersections

• smart car crash

• stopping distance

• Crash investigation

Levers

• You can use a

mechanism to move

something more

easily.

• Force Multiplier

• force you produce is

bigger than the force

you apply

• Mechanical

Advantage

• 3 types

= Effort

Load =

Class 1

• The force you apply is

on the opposite side

of the fulcrum to the

force you produce.

Class 2

• The fulcrum is at one

end.

• You apply force at the

other end and the

force you produce is

in the middle.

Class 3

• apply the force in the

middle and the force

you produce is at the

opposite end.

• They reduce the force

you apply, giving you

much greater control.

Inclined plane -ramp

• You use less

force, but you

need to pull/push

a longer distance

• you use the

same amount of

energy in each

case

Pulleys• Two or more wheels and a loop of rope

around them creates a lifting machine.

• Each time the rope wraps around the

wheels, you create more lifting power or

mechanical advantage.

Pulleys

• Pulleys transfer rotation from one shaft to another.

• Same diameter = same speed rotation.

• Large drive pulley makes a smaller pulley rotate faster.

• If the belt is crossed rotation is in the opposite direction.

Wheel

• Wheels can multiply speed/ distance or force.

• The axle turns a short distance (blue arrow)

• leverage of the wheel means the outer rim turns much further (red arrow) in the same time.

FORCES

Forces are pushes or pulls (a combination is a

twist).

Objects are stationary when forces are

balanced

gravity is always acting but we don’t keep

falling due to a support force

Forces can be measured using a Newton meter.

BALANCED FORCESAn unbalanced forces cause changes to objects motion (speed or

direction), or shape.

If a force acts on a stationary object and causes motion, the object

has gained kinetic (movement) energy.

Friction will stop the object moving.

Types of force:

Gravity

Electrostatic

Tension – the force in rope, etcMagnetism

Friction – the force that opposes motion

Support

Bouyancy – in the water

Lift – in the air (planes/birds)

CONTACT FORCESSome forces only act on contact, others can act from a distance.

Which are which?

Contact Distance

Gravity

Electrostatic

Tension

Magnetism

Friction

Support

FORCE PAIRS

Forces act in pairs (e.g.

thrust and friction, gravity

and support).

Force diagrams show the

forces acting on an object

and whether they are

balanced or unbalanced.

Arrow size represents force

size if no measurements are

available.

Force pairs

• What are the

missing terms?

• Buoyancy

• Drag

• Thrust

• Weight

BALANCED OR UNBALANCED?

Explain whether the forces in the following scenarios balanced

or unbalanced.

1. The international space station is orbiting Earth at about

28,000kmhr-1.

2. A can is being crushed.

3. A car is travelling at a constant speed.

4. A skydiver has just jumped from a plane.

5. A car stays at 50kmhr-1 as it turns a corner.

UNBALANCED FORCES

An unbalanced force (a net force) results in acceleration.

The rate of acceleration depends on the mass of the object and

force applied…

Force = mass × acceleration

(F = m × a).

F

m a

FORCE AND MOTION

What happens when you apply (using a Newton meter) a small

constant force to a trolley and time it over a set distance?

Small constant

force

An unbalanced force causes acceleration.

The trolley should accelerate because…

Set distance

FORCE AND MOTION

What happens when you apply (using a Newton meter) a small

constant force to a trolley carrying a 1kg mass and time it over

a set distance?

Small constant

force

The larger the mass the slower the

acceleration

The trolley should accelerate but slower than previously

because…

Set distance

1Kg

FORCES AND ACCELERATION

Given the formula F = ma try the following questions.

1. What are the names and units of F, m and a?

2. Complete the table….

3. The rider and cycle are 150kg:

a. What is the Nett force?

b. What is the cyclist’s acceleration?

4. A bike accelerates at 10ms-2 using a force of 6000N. The

rider is 70kg. What is the mass of the bike?

F m A

a. 9kg 0.5ms-2

6N 0.2kg b.

c. 800g 1.5ms-2

350N d. 15ms-2

e. 1200kg 0.015ms-2

800N150N

WEIGHT FORCE

Weight is a force. It is therefore measured in…

An object’s weight depends on two things…

Newtons (N)

Gravity

• varies depending where you are

• 10ms-2 or 10N/Kg on Earth

Mass

• does not vary

• measured in Kg

•A man with mass of 75Kg on earth weighs 750N

BUT on the moon he weighs 125N

Weight Loss

Planet Mass relative

to Earth

Radius

Mercury 0.06 2400

Venus 0.82 6000

Earth 1 6400

Mars 0.1 3400

Jupiter 318 71500

Saturn 95 60300

Uranus 14 25600

Neptune 17 24800

Pluto 0.002 1200

MASS AND WEIGHT

1. a. What is mass?

b. What is it’s unit?

2. a. What is Weight?

b. What is it’s unit?

3. ON EARTH: 1N = kg 1kg = N

4. How would your mass and weight change if you went to

Jupiter?

FRICTION

Friction is a contact force that opposes motion, it

causes heat, damage, wear and slowing

Friction can be reduced by…

lubrication,

streamlining (aerodynamics),

slowing down,

smoothing surfaces

Reaction Times

• http://www.bbc.co.uk/science/humanbody/

sleep/sheep/reaction_version5.swf

• Click Above for the Reaction Time Game

SPEED

Speed is the distance that an object travels in a period of time.

d

t v

Units are meters and seconds

(and therefore meters per

second). However, sometimes

km/hr is more sensible.

A cyclist travels 25 km in ½ an hour.

What is their speed - in kmhr-1

- in ms-1

= 25km/0.5hr = 50km/hr

= 25000m/1800s = 13m/s

DISTANCE/TIME GRAPHSA car takes 1.5 minutes to travel 500m down a busy road. It stops

at lights for 30 seconds, then continues on for 1 minute as it goes

another 1km.

Plot this on a distance/time graph.

Time (min)

1 2 3

Distance

(km)

0.5

1

1.5

Using the distance/time

graph:

1.What is the total distance

traveled?

2. In what part of the trip is

the car going the fastest?

3. What is the fastest

speed?

= 1.5 km

= part 3

v = ∆d / ∆t

v = 1km/1min

v = 1000m/60s

v = 16m/s

Steepest

section is

fastest

∆d =

1km

∆t =

1min

In a distance/time graph

the slope of the line =

the speed of the object.

SPEED QUESTIONSWhat would these look like on a distance/time graph?

1. stopped

2. slow

3. fast

4. accelerating

ACCELERATION

Acceleration is the change in speed in an object in a period of

time.

∆ v

a ∆ t

Units ms-2

It takes a cyclist 20 seconds to go from

a standing start to 14m/s.

What is their acceleration?

What is 14m/s in km/hr?

a = ∆v/∆t a = 14m/s / 20s a = 0.7ms-2

= 14 × 60s × 60min ÷ 1000m

= 50.4km/hr

SPEED/TIME GRAPHSA runner travels at 4m/s for 10 seconds, then stops suddenly for 5

seconds, then accelerates for 5 seconds to get to 8m/s and

continues for 10 seconds.

Plot this on a speed/time graph.

Time (sec)

10 20 30

Speed

(m/s)

4

8Using the speed/time

graph:

In what part of the trip

is the runner going the

fastest?

What is the

acceleration in part 4?

= part 5

In a speed/time graph the

slope of the line = the

acceleration of the object.

a = ∆v/∆t

a = 8m/s/5s

a = 1.6ms-2

SPEED/TIME GRAPHS

Time (sec)

10 20 30

Speed

(m/s)

4

8

What distance is covered in part 1?

What is the total distance covered?

d = v × t

d = 4m/s × 10s

d = 40 m

Part 1 = 40m

Part 2 = 0m

Part 3 = v × t × = 8m/s × 5s × = 20m

Part 4 = v × t = 8m/s × 10 s = 80m

Total = 40 + 20 + 80 =140m

Part

1

Part

2

Part

3

Part

4

In a speed/time graph the distance covered = the area under the graph.

ACCELERATION QUESTIONSWhat would these look like on a speed/time graph?

1. stopped

2. slow

3. fast

4. accelerating

Crumple Zone

• Absorbs the force of

impact by

• Increasing the time of

impact

• Which decreases the

force of impact

Stopping Distances