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KS3 Physics. 9K Speeding Up. Contents. 9K Speeding Up. Distance, time and speed. Balanced and unbalanced forces. Friction. Summary activities. Distance, time and speed. To work out the speed of an object you need to know:. the distance travelled;
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KS3 Physics 9K Speeding Up
Contents 9K Speeding Up Distance, time and speed Balanced and unbalanced forces Friction Summary activities
Distance, time and speed To work out the speedof an object you need to know: • the distance travelled; • how long it took to travel that distance.
Calculating average speed total distance average speed = total time Average speed is calculated using this equation: d s x t formula triangle 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.
Speed calculation example d (distance in km) average speed (in km/h) = t (time in h) d 10 km = 1 h s x t 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. = 10 km/h Cover the quantity to be calculated - s (speed)
Speed calculation example – units check d d (distance in m) average speed (in m/s) = t (time in s) s x t 10,000 m = 1x60x60 3600 s Sometimes the units have to be changed in a speed calculation. Here is the same problem but with different units: 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. = 2.8 m/s Cover the quantity to be calculated - s (speed)
Speed calculation – question 1 d s x t Question 1 A family set off from home and walk at an average speed of 3.6 km/h. How far will they travel in two hours? Give your answer in km. distance (km) = speed (km/h) x time (h) = 3.6 km/h x 2 h = 7.2 km
Speed calculation – question 1 distance time= speed d 10 km = s x t 5.4 km/h Question 2 How long would it take a woman to walk 10 km, if her average speed was 5.4 km/h? =1.85 hours
Car graphing activity – instructions This graphing experiment shows an animation of a car travelling along a straight road. 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.
Car graphing activity – results Car graphing activity – results
Car graphing activity – results graph analysis 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 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 car is going fast but at a constant speed. The graph is straight in this part of the journey. 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.
Gradient of a distance/time graph 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 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. Ittook from 16s to 36s to travel this distance = 20s. So the speed at this point = 600m/20s = 30m/s.
Speed experiment – instructions total distance average speed = total time 1. Time how long it takes you to run 100m. 2. Then calculate your average speed for the run. 3. Repeat the experiment for each member of your group. 4. What was the fastest average speed for your group?
Speed experiment – results 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 __________.
Contents 9K Speeding Up Distance, time and speed Balanced and unbalanced forces Friction Summary activities
What is a force? A force is a push, pull ortwist. A force cannot be seen but you can see how a force affects an object.
Balanced and unbalanced forces 50 mph 500 N 500 N Think of a car travelling at a constant speed of 50 mph. The engine provides sufficient force to just overcome all the frictional forces that are acting to decrease the speed.
Balanced and unbalanced forces 50 mph A crosswind acting on the car produces a sideways force. cross- wind The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. If the car turns right so that the wind is now behind the car, what will happen to the speed?
Balanced and unbalanced forces >50mph 500 N 400 N 60mph 500 N 500 N 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. The car will increase in speed (accelerate) until the forces are balanced again.
Balanced and unbalanced forces – summary • If the forces on an object are balanced: • If the object isstationary, it will remain stationary. • If the object ismoving, it will continuetomove 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. • If the forces are unbalanced, two things can happen: • Thespeed can change. This is called acceleration. • Thedirectionof motioncan change.
Resultant force 100 N 400 N 500 N The sum effect of more than one force is called theresultant force. The resultant force is calculated by working out the difference between opposing forces. A resultant force of 100 N is accelerating the car.
Resultant force – question 1 5N 5N 10N 20N 1. What is the resultant force on the block? Resultant force = 20N –10N = 10N down The block will accelerate downwards.
Resultant force – question 2 5N 5N 5N 5N 2. What is the resultant force on the block? 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 forces – question 3 7N 3N 10N 17N 20N 13N 10N 3. What is the resultant force on the block? 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.
Contents 9K Speeding Up Distance, time and speed Balanced and unbalanced forces Friction Summary activities
Friction Frictionalways tries to slow moving objects 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 calledair resistanceordrag. NOTE: 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.
Air resistance and drag 400N 300N Air resistance is a type of friction caused when objects move through the air. 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.
Other sources of friction in cars One of the most important sources of friction in cars is that between the tyres and the road. When the car brakes, the maximum possible amount of friction is desirable so that the car does not skid. • The friction between the tyres and road is affected by the: • inflation pressure of the tyres; • road surface; • surface condition caused by the weather (rain, ice, etc).
Contents 9K Speeding Up Distance, time and speed Balanced and unbalanced forces 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.
