Noadswood Science, 2012

To understand the effect of a force on an object

Friday, April 21, 2023

Rockets

Watch the demo of the water rocket - what forces are involved with it?

Can you explain why it accelerates up and why it eventually slows and begins to fall back to Earth?

How do you represent the forces?

Forces

The rocket accelerates upwards as the force propelling it up is greater than the force pulling it back down (the thrust of the rocket exceeds the force of gravity) - as rocket fuel is expelled one way, there is an equal and opposite force on the rocket in the opposite direction (giving it lift)

Eventually when the rocket has expelled all its fuel it will slow (gravity's force becomes proportionally larger) until a point where gravity becomes the dominant force and then it will drop back to Earth (unless it has escaped Earth's gravity)

Force can be represented by pairs of arrows (the bigger the arrow, the bigger the force)

Forces

What is a force?

A force is a push or a pull (measured in Newtons (N)) Gravity Reaction force (e.g. from a surface, pushing you back up) Thrust (push/pull) Drag (resistance or friction which slows objects down) Lift (e.g. from an aeroplane wing (where air flows over the top

of the wing quicker than the bottom causing a pressure difference))

Tension

Forces are represented by arrows (always in pairs) which point the way the force is acting - the bigger the arrow, the bigger the force (balanced forces have equally sized arrows)

Unbalanced forces - acceleration

Balanced forces – steady speed

Newton

Newton developed the idea of the three laws of motion…

First law – balanced forces mean no change in velocity

Second law – a resultant force means acceleration

Third law – reaction forces

Newton’s First Law

First Law - balanced forces mean no change in velocity

As long as the forces are balanced then the object will stay at a constant velocity (this could be 0mph or 100mph)!

To go at a steady speed there must be zero resultant force

Car moving at a steady speed (100mph)

(balanced force)

Car stationary (0mph)

(balanced force)

Newton’s Second Law

Second Law - a resultant force means acceleration

If there is an unbalanced force then the object will accelerate in that direction

The size of the acceleration is determined by Force = mass x acceleration

Acceleration can be in the form of: starting; stopping; speeding up; slowing down; and changing direction

Car accelerating (unbalanced force)

m

F

a

Newton’s Third Law

Third Law - reaction forces

If object A exerts a force on object B, then object B exerts the exact opposite force on object A

Push against a wall, and the wall will push back against you just as hard (opposing forces must be there, else the wall would fall down)!

Place the following in order of how fast they are travelling (from slowest to fastest) - what information do you need to work out how fast something is travelling?

SR-71 Blackbird Lamborghini

Daytona 675

Snail

Olympic CyclistBulletSpace Shuttle

Light

Speed (and velocity) are measurements of how fast you are going (velocity includes direction) – to work out speed you need to know the distance travelled and the time it took…

SR-71 Blackbird2500 mph

Lamborghini200 mph

Daytona 675155 mph

Snail0.05 mph

Olympic Cyclist30 mph

Bullet1500 mph

Space Shuttle17’000 mph

Light670,616,629 mph

Speed(s)

Distance(d)

Time(t)

Speed = Distance Time Time = Distance Speed Distance = Speed x Time

Jack ran 100m in 12 seconds. What speed was he traveling at?

Jack then ran 100m again, but this time it was much more windy, and it took him 15 seconds. What was his new speed, and why was this different?

My car was going at 50mph for 1 hour. How many miles did I travel

My car was going at 50mph, and I traveled 20 miles. How long did this take me?

Speed

Distance

Time

Jack ran 100m in 12 seconds. What speed was he traveling at?

Speed = 100 12 = 8.34m/s

Jack then ran 100m again, but this time it was much more windy, and it took him 15 seconds. What was his new speed, and why was this different?

Speed = 100 15 = 6.67m/s (more air resistance)

My car was going at 50mph for 1 hour. How many miles did I travel

Distance = 50 x 1 = 50 miles

My car was going at 50mph, and I traveled 20 miles. How long did this take me?

Time = 20 50 = 0.4 hours (24 minutes)

Speed

Distance

Time

Distance-Time graphs are used to show three pieces of information: - Distance Time Speed (distance ÷ time)

Complete a rough graph of the following journey (plot the time (seconds) on the x-axis and the distance (meters) on the y-axis (x is across))

I left school to walk to the shop – I walked slowly for 10 minutes, covering a distance of 1500m. At the shop I stopped to talk on the phone for 5 minutes. Having realised I was about to miss my bus I ran for 2 minutes to the bus-stop which was 1800m away…

Distance-Time graphs are used to show three pieces of information: - Distance Time Speed (distance ÷ time)

Key information: - 10 minute (600 seconds) walk covering 1500m 5 minute (300 seconds) stationary covering 0m 2 minute (120 seconds) run covering 1800m

*Total time = 17 minutes (1020 seconds); total distance = 3300m

Work out the speed during the different sections (walking; stationary and running)

Annotate your graph – what does the graph show?

Steady speed (2.5 m/sec) Steady speed (stationary) (0 m/sec)

Steady speed (15 m/sec)

Speed

Distance

Time

1500 ÷ 600 0 ÷ 300

1800 ÷ 120

In a distance-time graph the gradient = the speed A flat section is where the object has stopped moving The steeper the graph the faster the speed

However distance-time graphs can also show acceleration and deceleration: - Steepening curve = speeding up (gradient increase) Leveling-off curve = slowing down (gradient decrease)

The gradient shows the speed in a distance-time graph: -

(vertical ÷ horizontal)

Annotate the following graph, explaining what is being shown – include the distances covered, the speed (was this constant speed / accelerating / decelerating) and the overall time for the journey…

Steady speed (15 m/sec)

Speed

Distance

Time

300 ÷ 20

Steady speed (0 m/sec)

0 ÷ 20Accelerating (5 m/sec)

100 ÷ 20

Decelerating (3.33 m/sec)

100 ÷ 30

Steady speed (16.67 m/sec)

500 ÷ 30

*The gradient shows the speed, e.g. on return journey the gradient = 500 ÷ 30 = 16.67msec-1

How could you collect your own data for a distance-time graph with someone in the class running a 100m race (we want to see the changes in speed during different parts of the race)?

Measure the distance (100m!) and then every 10m have someone standing with a stop watch

Time how long it takes the runner to cover each 10m, then we can graph and note the speed differences between the start and end of the race…