9 k speeding up (boardworks)

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KS3 Physics

9K Speeding Up


9K Speeding Up

Contents

Distance, time and speed

Balanced and unbalanced forces

Friction

Summary activities


To work out the speed of an object you need to know:


the distance travelled;

how long it took to travel that distance.


Average speed is calculated using this equation:

Speed can be measured in different units, e.g. m/s, km/h, km/s, miles per hour.

The units of distance and time used will give the units to be used for speed.

d

s x t

formula triangle

Calculating average speed

total distance

total timeaverage speed =


Speed formula triangle


A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in km/h.

d

s x t

Cover the quantity to be calculated - s (speed)

= 10 km/h

Speed calculation example

10 km1 h

=

d (distance in km)average speed (in km/h) t (time in h)

=


3600 s10,000 m

=

= 2.8 m/s

Sometimes the units have to be changed in a speed calculation. Here is the same problem but with different units:

1x60x60

Speed calculation example – units check

A boy takes 1 hour to travel from his home to the cinema, a distance of 10 km. Calculate his average speed in m/s.

d (distance in m)average speed (in m/s) t (time in s)

= d

s x t

Cover the quantity to be calculated - s (speed)


= 3.6 km/h x 2 h

distance (km) = speed (km/h) x time (h)

Question 1

A group set off from home and walk at an average speed of 3.6 km/h. How far would they travel in two hours?

Give your answer in km.

d

s x t

Speed calculation – question 1

= 7.2 km


10 km

5.4 km/h =

Question 1

How long would it take a woman to walk 10 km if her average speed is 5.4 km/h ?

d

s x t

time =distance

speed

Speed calculation – question 1

= 1.85 hours


This graphing experiment shows an animation of a car travelling along a straight road.

Car graphing activity – instructions

1. Copy the results table shown on the next slide and complete it as the movie is played.

2. Record the distance the car has travelled every five seconds.

3. Plot a graph of your results.


Results table for distance/time graph

Time/seconds Distance/metres

0

5

10

15

20

25

30

35

40

45

50

55

Car graphing activity – results table layout


Car graphing activity – animation


Results table for distance/time graph

Time/seconds Distance/metres

0 0

5 16

10 76

15 186

20 234

25 484

30 634

35 784

40 904

45 974

50 994

55 994

Car graphing activity – results table


Distance / Time graph for car

0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce /

me

tre

s Car graphing activity – results graph



0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce /

me

tre

s Car graphing activity – results graph



0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce /

me

tre

s

The car has stopped. The graph is flat – the distance of the car from the start point is not changing.The graph is straight – there is no change in speed.

The speed of the car is changing – the graph is not flat. The slope of the graph is less steep as the car begins to slow down.

The car is starting to move. The curve shows that the speed is changing. The curve is upwards as the car accelerates at the start of the journey.

The car is going fast but at a constant speed.The graph is straight in this part of the journey.

Car graphing activity – results graph analysis


The speed of the car can be calculated by looking at the gradient of the distance/time graph.

Speed is “Distance Travelled divided by Time Taken”

These values can be read off the distance/time graph at different points, and this is the same as the gradient of the graph.

Gradient of a distance/time graph



0

200

400

600

800

1000

1200

0 5 10 15 20 25 30 35 40 45 50 55

Time / seconds

Dis

tan

ce

/ m

etr

es

Consider the gradient of this graph at the point shown by the two arrows in a triangle:

The car has travelled from 200m to 800m = 600m.It took from 16s to 36s to travel this distance = 20s.

Gradient of a distance/time graph

So the speed at this point = 600m/20s = 30m/s.


1. Time how long it takes you to run 100m.

Speed experiment – instructions

total distance

total timeaverage speed =

3. Repeat the experiment for each member of your group.

2. Then calculate your average speed for the run.

4. What was the fastest average speed for your group?


Name distance (m) time (s) average speed (m/s)

100

100

100

100

100

Conclusion

The fastest member of the group with an average speed of ________ was __________.

Speed experiment – results


9K Speeding Up

Contents



Friction

Summary activities


A force is a push or a pull. A force cannot be seen but you can see how a force affects an object.

What is a force?


500 N 500 N

Think of a car travelling at a constant speed of 50 mph.

50 mph


The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed.


crosswind

A crosswind acting on the car produces a sideways force.

The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced.

50 mph


If the car turns right so that the wind is now behind the car, what will happen to the speed?


The air resistance will decrease because the car has a “tail wind” (it is being blown from behind). This means the forces acting on the car are no longer balanced.

500 N 400 N>50 mph

500 N 500 N60 mph


The car will increase in speed (accelerate) until the forces are balanced again.


If the forces on an object are balanced:

If the object is stopped, it will remain stopped.

If the object is moving, then it will continue to move at the same speed and in a straight line.

In other words, the object will continue to do what it is already doing without any change.

Balanced and unbalanced forces – summary

If the forces are unbalanced two things can happen:

The speed will change.

The direction of motion will change. This is called acceleration.


The sum effect of more than one force is called the resultant force.

400 N500 N

100 N

A resultant force of 100 N is accelerating the car.

Resultant force

The resultant force is calculated by working out the difference between opposing forces.


10N20N

5N5N

1. What is the resultant force on the block?

Resultant force = 20N –10N = 10N down

The block will accelerate downwards.

Resultant force – question 1


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.

Resultant force – question 2


10N

13N


10N

20N

3N7N

17N

Resultant force = (20N +10N) – 13N = 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.

Resultant forces – question 3


Resultant force activity


9K Speeding Up

Contents



Friction

Summary activities


Friction always tries to slow moving object down– it opposes motion.

Friction is created whenever two touching objects or surfaces move past each other.

Friction also occurs when things move through air. This is called air resistance or drag.

(The size of the frictional force equals the applied force unless the applied force is bigger than the maximum value of the frictional force. If this is the case then the frictional force remains at the maximum possible value.)

Friction


One more? Probably the most important…

Label all sources of friction that can act on this bike.

tyre and road

brake pad and rim

wheel bearingwheel

bearing

pedal bearing

links in chain

air resistance or “drag”

What are the sources of friction?


Air resistance is a type of friction caused when objects move through the air.

400 N

300 N

Air resistance and drag

Cars are designed so that they are streamlined. The flow of air around the body is made as smooth as possible so that air resistance is minimized.

Air resistance depends on:

the size of the car;

the shape of the car;

the speed of the car.


One of the most important sources of friction in cars is that between the tyres and the road.

The friction between the tyres and the road is affected by the:

inflation pressure of the tyres;

road surface;

surface condition caused by the weather (rain, ice, etc).

When the car brakes, the maximum possible amount of friction is desirable so that the car does not skid.

Other sources of friction in cars


Effects of frictional forces


9K Speeding Up

Contents



Friction

Summary activities


Glossary

acceleration – A change in speed. air resistance – A frictional force that acts against an

object moving through air.balanced forces – Forces acting on an object that do

not change its speed or direction.drag – A frictional force, such as air resistance or water

resistance, which slows down a moving object.friction – A force that occurs when two things rub against

each other and so slows down a moving object.speed – How quickly an object is moving. It equals the

distance moved divided by the time taken, often measured in ‘metres per second’ (m/s).

streamlined – A smooth shape which reduces drag.unbalanced forces – Forces acting on an object that

change its speed or direction.


Anagrams


Multiple-choice quiz

9 k speeding up (boardworks)

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Transcript of 9 k speeding up (boardworks)