Do now! Can you continue the questions you started last lesson? (Page 27 questions 6, 7 & 8. Page 29 questions 1 & 2) Bunny suicide #1

Last lesson Motion graphs

No movement distance time speed time

Constant speed distance time speed time Area = distance travelled

Constant acceleration distance time speed time Area = distance travelled Gradient = acceleration a = (v-u)/t

You did some questions! Page 27 questions 6, 7 & 8. Page29 questions 1 & 2.

Today’s lesson 1.7 Know what a force is 1.8 Types of force 1.9 & 1.10 Vectors and scalars 1.11 Adding forces 1.13 Newton’s laws (2 nd law)

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 and vectors

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 scalars vectors Magnitude (size) No direction Magnitude and direction temperaturemass speed velocity force acceleration

Scalars and Vectors scalars vectors Magnitude (size) No direction Magnitude and direction temperaturemass speed velocity force acceleration Copy please!

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 For example; 6 N4 N 2 N 4 m/s 5.7 m/s Resultant force Resultant velocity

Adding vectors For example; 6 N4 N 2 N Resultant force Copy please!

An interesting example Think of a dog (dead) orbiting the earth with constant speed (in a circle).

An interesting example At this point, what is its velocity? velocity?

An interesting example velocity

An interesting example velocity? What is its velocity here?

An interesting example velocity As you can see the velocity has changed as it is now going in another direction.

An interesting example velocity 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!

Resultant force

Newton’s Laws of Motion That’s me!

Newton’s 1 st Law If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero).

Newton’s 1 st 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 1 st Law If there is no resultant force acting on an object, it will move with constant velocity. (Note the constant velocity could be zero). Can you copy it whilst you think about it?

Newton’s 1 st law Newton’s first law was actually discovered by Galileo. Newton nicked it!

Newton’s first law Galileo imagined a marble rolling in a very smooth (i.e. no friction) bowl.

Newton’s first law If you let go of the ball, it always rolls up the opposite side until it reaches its original height (this actually comes from the conservation of energy).

Newton’s first law No matter how long the bowl, this always happens

Newton’s first law No matter how long the bowl, this always happens. constant velocity

Newton’s first law Galileo imagined an infinitely long bowl where the ball never reaches the other side!

Newton’s first law The ball travels with constant velocity until its reaches the other side (which it never does!). Galileo realised that this was the natural state of objects when no (resultant ) forces act. constant velocity

Other examples Imagine a (giant) dog falling from a tall building

Other examples To start the dog is travelling slowly. The main force on the dog is gravity, with a little air resistance gravity Air resistance

Other examples As the dog falls faster, the air resistance increases (note that its weight (force of gravity) stays the same). gravity Air resistance

Other examples Eventually the air resistance grows until it equals the force of gravity. At this time the dog travels with constant velocity (called its terminal velocity) gravity Air resistance

Oooops!

Another example Imagine Mr Porter cycling at constant velocity.

Newton’s 1 st law He is providing a pushing force. Constant velocity

Newton’s 1 st law There is an equal and opposite friction force. Constant velocity Pushing force friction

Newton’s second law Newton’s second law concerns examples where there is a resultant force. I thought of this law myself!

Let’s go back to Mr Porter on his bike. Remember when the forces are balanced (no resultant force) he travels at constant velocity. Constant velocity Pushing force friction

Newton’s 2nd law Now lets imagine what happens if he pedals faster. Pushing force friction

Newton’s 2nd law His velocity changes (goes faster). He accelerates! Pushing force friction acceleration Remember that acceleration is rate of change of velocity. In other words acceleration = (change in velocity)/time

Newton’s 2nd law Now imagine what happens if he stops pedalling. friction

Newton’s 2nd law He slows down (decellerates). This is a negative acceleration. friction

Newton’s 2nd law So when there is a resultant force, an object accelerates (changes velocity) Pushing force friction Mr Porter’s Porche

Newton’s 2 nd law There is a mathematical relationship between the resultant force and acceleration. Resultant force (N) = mass (kg) x acceleration (m/s 2 ) F R = ma It’s physics, there’s always a mathematical relationship!

Newton’s 2 nd law There is a mathematical relationship between the resultant force and acceleration. Resultant force (N) = mass (kg) x acceleration (m/s 2 ) F R = ma Can you copy this too?

An example What will be Mr Porter’s acceleration? Pushing force (100 N) Friction (60 N) Mass of Mr Porter and bike = 100 kg

An example Resultant force = 100 – 60 = 40 N F R = ma 40 = 100a a = 0.4 m/s 2 Pushing force (100 N) Friction (60 N) Mass of Mr Porter and bike = 100 kg

Newton’s 3 rd 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)

Don’t worry! We won’t do Newton’s 3 rd law until next year!

That ’ s all folks!

Let’s try some questions! Complete Physics for IGCSE Read pages 38 and 39 and then try page 37 questions 1, 2 and 3 and page 39 Questions 1 and 2