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## Forces and motion

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**Speed**metres Speed = distance travelled time taken Metres per second (m/s) seconds**Speed**kilometres Speed = distance travelled time taken Kilometres per hour (km/h) hours**d**s t x triangle**No movement**distance time**Constant speed**fast The gradient of the graph gives the speed distance slow time**Getting faster (accelerating)**distance time**A car accelerating from stop and then hitting a wall**Let’s try a simulation distance time**Speed against time graphs**speed time**No movement**speed time**Constant speed**speed fast slow time**Getting faster? (accelerating)**speed Constant acceleration time**Getting faster? (accelerating)**v The gradient of this graph gives the acceleration speed a = v – u t (v= final speed, u = initial speed) u time**Getting faster? (accelerating)**speed The area under the graph gives the distance travelled time**A dog falling from a tall building (no air resistance)**speed Area = height of building time**Forces**• Remember a force is a push (or pull)**Forces**• Force is measured in Newtons**Forces**• There are many types of forces; electrostatic, magnetic, upthrust, friction, gravitational………**Which of the following is the odd one out?**Mass Speed Force Temperature Distance Elephant**Scalars**Scalar quantities have a magnitude (size) only. For example: Temperature, mass, distance, speed, energy. 1 kg**Vectors**Vector quantities have a magnitude (size) and direction. For example: Force, acceleration, displacement, velocity, momentum. 10 N**Scalars and Vectors**Copy please! No direction vectors scalars Magnitude (size) Magnitude and direction temperature mass velocity force acceleration speed**Representing vectors**Vectors can be represented by arrows. The length of the arrow indicates the magnitude, and the direction the direction!**Adding vectors**When adding vectors (such as force or velocity) , it is important to remember they are vectors and their direction needs to be taken into account. The result of adding two vectors is called the resultant.**Adding vectors**Copy please! For example; Resultant force 2 N 6 N 4 N**An interesting example**We have constant speed but changing velocity. Of course a changing velocity means it must be accelerating! We’ll come back to this in year 12! velocity**Newton’s 1st Law**If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero). Does this make sense?**Newton’s second law**Newton’s second law concerns examples where there is a resultant force. I thought of this law myself!**Newton’s 2nd law**There is a mathematical relationship between the resultant force and acceleration. Resultant force (N) = mass (kg) x acceleration (m/s2) It’s physics, there’s always a mathematical relationship! FR = ma**An example**Resultant force = 100 – 60 = 40 N FR = ma 40 = 100a a = 0.4 m/s2 Mass of Mr Porter and bike = 100 kg Pushing force (100 N) Friction (60 N)**Newton’s 3rd law**If a body A exerts a force on body B, body B will exert an equal but opposite force on body A. Hand (body A) exerts force on table (body B) Table (body B) exerts force on hand (body A)**Gravity**Gravity is a force between ALL objects! Gravity**Gravity**Gravity is a very weak force. The force of gravitational attraction between Mr Porter and his wife (when 1 metre apart) is only around 0.0000004 Newtons!**Gravity**The size of the force depends on the mass of the objects. The bigger they are, the bigger the force! Small attractive force Bigger attractive force**Gravity**The size of the force also depends on the distance between the objects.**Gravity**The force of gravity on something is called its weight. Because it is a force it is measured in Newtons. Weight**800 N**Gravity On the earth, Mr Porter’s weight is around 800 N. I love physics!**Gravity**On the moon, his weight is around 130 N. Why? 130 N**Mass**Mass is a measure of the amount of material an object is made of. It is measured in kilograms.**Mass**Mr Porter has a mass of around 77 kg. This means he is made of 77 kg of blood, bones, hair and poo! 77kg**Mass**On the moon, Mr Porter hasn’t changed (he’s still Mr Porter!). That means he still is made of 77 kg of blood, bones, hair and poo! 77kg**Mass and weight**Mass is a measure of the amount of material an object is made of. It is measured in kilograms. Weight is the force of gravity on an object. It is measured in Newtons.**Calculating weight**To calculate the weight of an object you multiply the object’s mass by the gravitational field strength wherever you are. Weight (N) = mass (kg) x gravitational field strength (N/kg)**Gravity = air resistanceTerminal velocity**As the dog falls faster and air resistance increases, eventually the air resistance becomes as big as (equal to) the force of gravity. The dog stops getting faster (accelerating) and falls at constant velocity. This velocity is called the terminal velocity. Air resistance gravity**Falling without air resistance**Can you copy the words please? Without air resistance objects fall faster and faster and faster……. They get faster by 10 m/s every second (10 m/s2) This number is called “g”, the acceleration due to gravity. Where did I come from? gravity**Falling without air resistance?**distance time**Falling without air resistance?**speed Gradient = acceleration = 9.8 m.s-2 time**Falling with air resistance?**distance time