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 DistanceGravity

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

UNBALANCED FORCESAn 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 MOTIONWhat 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 MOTIONWhat 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

MASS AND WEIGHT1. 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

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 = Δd / Δt

v = 1km/1min

v = 1000m/60s

v = 16m/s

Steepest section is fastest

Δd = 1km

Δt = 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

ACCELERATIONAcceleration is the change in speed in an object in a period of time.

Δ v

a Δ 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 = Δv/Δt 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 = Δv/Δt

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