29K Speeding Up Distance, time and speed Balanced and unbalanced forces Contents9K Speeding UpDistance, time and speedBalanced and unbalanced forcesFrictionSummary activities
3Distance, time and speed To work out the speed of an object you need to know:the distance travelled;how long it took to travel that distance.
4Calculating average speed Average speed is calculated using this equation:dsxtformula triangletotal distancetotal timeaverage speed =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.
6Speed calculation example 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.dsxtd (distance in km)average speed(in km/h)t (time in h)=10 km1 h== 10 km/hCover the quantity to becalculated - s (speed)
7Speed calculation example – units check 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.dsxtd (distance in m)average speed(in m/s)t (time in s)=3600 s10,000 m=1x60x60= m/sCover the quantity to becalculated - s (speed)
8Speed calculation – 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.dsxtdistance (km) = speed (km/h) x time (h)= 3.6 km/h x 2 h= 7.2 km
9Speed calculation – question 1 How long would it take a woman to walk 10 km if her average speed is 5.4 km/h ?time =distancespeeddsxt10 km5.4 km/h== hours
10Car 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.
16Car 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 beginsto 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 thatthe speed is changing. The curve is upwards as thecar accelerates at the start of the journey.
17Gradient 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.
18Gradient 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.It took from 16s to 36s to travel this distance = 20s.So the speed at this point = 600m/20s = 30m/s.
19Speed experiment – instructions 1. Time how long it takes you to run 100m.2. Then calculate your average speed for the run.total distancetotal timeaverage speed =3. Repeat the experiment for each member of your group.4. What was the fastest average speed for your group?
20Speed experiment – results Name distance (m) time (s) averagespeed (m/s)100ConclusionThe fastest member of the group with an average speed of ________ was __________.
219K Speeding Up Distance, time and speed Balanced and unbalanced forces Contents9K Speeding UpDistance, time and speedBalanced and unbalanced forcesFrictionSummary activities
22What is a force?A force is a push or a pull. A force cannot be seen but you can see how a force affects an object.
23Balanced and unbalanced forces 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.50 mph500 N500 N
24Balanced and unbalanced forces A crosswind acting on the car produces a sideways force.50 mphcross-windThe 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?
25Balanced and unbalanced forces 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.>50 mph500 N400 NThe car will increase in speed (accelerate) until the forces are balanced again.60 mph500 N500 N
26Balanced and unbalanced forces – summary 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 tomove 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:The speed will change.The direction of motion will change.This is called acceleration.
27A resultant force of 100 N is accelerating the car. The sum effect of more than one force is called the resultant force.The resultant force is calculated by working out the difference between opposing forces.A resultant force of 100 N is accelerating the car.100 N500 N400 N
28Resultant force – question 1 1. What is the resultant force on the block?5NResultant force = 20N –10N= 10N downThe block will acceleratedownwards.20N10N
29Resultant force – question 2 2. What is the resultant force on the block?5N5N5N5NResultant force = 5N – 0N= 5N rightThe 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.
30Resultant forces – question 3 3. What is the resultant force on the block?7N3N10N17N13N20N10NResultant force = (20N +10N) – 13N= 17N rightThe 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.
329K Speeding Up Distance, time and speed Balanced and unbalanced forces Contents9K Speeding UpDistance, time and speedBalanced and unbalanced forcesFrictionSummary activities
33FrictionFriction 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.)
34What are the sources of friction? Label all sources of friction that can act on this bike.brake pad and rimpedal bearingwheel bearingwheel bearingOne more? Probably the most important…air resistance or “drag”links in chaintyre and road
35Air resistance and drag 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.400 NAir resistance depends on:the size of the car;the shape of the car;the speed of the car.300 N
36Other 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 the road is affected by the:inflation pressure of the tyres;road surface;surface condition caused by the weather (rain, ice, etc).
389K Speeding Up Distance, time and speed Balanced and unbalanced forces Contents9K Speeding UpDistance, time and speedBalanced and unbalanced forcesFrictionSummary activities
39acceleration – A change in speed. Glossaryacceleration – 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 donot 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 thatchange its speed or direction.