Forces 2Forces 2

Pressure

• You can not wear studded football boots indoor because the damage the floor

• This is because the studs have a smaller surface area than your foot

• Pressure is measured in N/m2

Objects Exerting High Pressure

• People have been stabbed with high heel (stiletto) shoes

• Razor blades have a small surface area to exert a lot of pressure

• Drawing pins have a sharp end that can exert a lot of pressure and stick into boards

Spreading Out Weight

• Caterpillar tracks spread out the weight of a tank

• Snow boots prevent people from sinking into the snow

Caterpillar Tracks

The t-55 Soviet tank

The Snowcat

Pressure Questions

• Why does a JCB have caterpillar tracks?• To spread it’s weight out and prevent it sinking

into mud (Reduce it’s pressure on the ground)• If a women has a weight of 60 N and each shoe

has a surface area of 10 cm². What pressure does she exert on the ground?

• She has two feet so her combined surface area is 20cm².

• Pressure = Force / Area• 60÷ 20 = 3 N cm²

Pressure Questions

• A footballer weighs 80 N and he exerts a pressure of 10 N/ cm². What is the surface area of one boot?

• Area = Force/ Pressure

• Area = 80 / 10 = 8 cm² ( but this is for both boots) The answer is 4 cm²

Pressure Questions

• The pressure the t-55 tank exerts on the ground is 91,500 N/ m². Each of the tanks caterpillar tracks are 2 m². How much does the tank weight?

• Force = Pressure x Area• Force = 91,500 x 4 (A tank has two tracks)• Force = 366,000 N

Speed

• Speed is measured using this equation:

• Units are metres/ second (m/s) or kilometres/ hour (km/h)

The Blackbird (SR-71)• This stealth plane could

travel at speeds of mach 3• Mach 3 means three

times faster than the speed of sound

• It holds the record for the fastest plane (2,193mph)

• It could travel from London to New York in under 2 hours

• (An ordinary jet would take 6 hours)

Speed Questions

• If a sprinter runs 100 m in 10 seconds, what is his speed?

• 100 ÷ 10 = 10 m/sec• If a car is travelling at 60 miles per hour for 3

hours how far does it travel?• 60 x 3 = 180 miles• If a car is travelling 30 miles per hour and it

travels 120 miles how long did the journey take?• 120 ÷ 30 = 4 hours

Speed Questions

• A cheetah can run 460 m in 20 seconds. How fast can it run?

• 460 ÷ 20 = 23 m/sec• Cheetah’s prey on gazelles. A gazelle can run

for 14 km in 15 minutes. How fast is the gazelle and can it escape the cheetah?

• 15 x 60 = 900 seconds• 14 km = 14, 000 m• 14,000 ÷ 900 = 15.6 m/sec• The cheetah will catch the gazelle

Distance/ Time Graphs

• The gradient (or slope) of the graph tells you the speed of an object

• In this graph the object is not moving

Distance/ Time Graphs

• This graphs shows an object moving at a steady speed

Distance/ Time Graphs

• This graph shows an object accelerating (getting faster)

Speed/ Time Graphs

• These graphs show whether an object is accelerating, decelerating or travelling at a steady speed

• This is a an object travelling at a steady speed

Speed/ Time Graphs

• This is an object accelerating

Speed/ Time Graphs

• This is an object decelerating

Questions

• What is happening in this graph?

• Two objects are travelling the same distance, but the yellow line is faster

Questions

• What is happening in this graph?

• An object is travelling at a steady speed, then it increases to a faster steady speed

Questions

• What is happening in this graph?

• An object travels at a steady speed, then stops, then moves off at a faster steady speed

Questions

• What is happening in this graph?

• Two objects are accelerating to the same speed, but the yellow one is accelerating quicker

Questions

• What is happening in this graph?

• Both objects are decelerating to a stop, but the red line is decelerating faster