 # SACE Stage 1 Conceptual Physics

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SACE Stage 1 Conceptual Physics
Vectors

Vector and Scalar Quantities
Quantities that require both magnitude and direction are called vector quantities. Examples of vectors are Force, Velocity and Displacement.

Vector and Scalar Quantities
Quantities that require just magnitude are known as Scalar quantities. Examples of scalar quantities are Mass, Volume and Time.

Vector Representation of Force
Force has both magnitude and direction and therefore can be represented as a vector.

Vector Representation of Force
The figure on the left shows 2 forces in the same direction therefore the forces add. The figure on the right shows the man pulling in the opposite direction as the cart and forces are subtracted.

Vector Representation of Velocity
The figure on the left shows the addition of the wind speed and velocity of the plane. The figure on the right shows a plane flying into the wind therefore the velocities are subtracted.

Vector Representation of Velocity

Vector Representation of Velocity

Consider a pair of horses pulling on a boat. The resultant force is the addition of the two separate forces F1 + F2.

The resultant vector (black) is the addition of the other 2 vectors (blue + green)

When we add vectors mathematically, we use a vector diagram. This may include using Pythagoras’ Theorem.

Pythagoras’ Theorem, in a right angled triangle, the square of the hypotenuse is equal to the sum of the squares of the other two sides. a2 + b2 = c2

Example – An 80km/hr plane flying in a 60km/hr cross wind. What is the planes speed relative to the ground.

Solution Use Pythagoras’ Theorem to find R Draw a vector representation of the velocities involved.

As velocity is a vector, we need to find the direction of the vector. Can do this by finding an angle (a) with in the vector diagram. Use trigonometry to find the angle.

The answer should include both the size and direction of the vector. The velocity of the plane relative to the ground is 100km/hr at 36.9o to the right of the planes initial velocity.

Equilibrium Combining vectors using the parallelogram rule can be shown by considering the case of being able to hang from a clothes line but unable to do so when it is strung horizontally, it breaks!

Equilibrium Can see what happens when we use the spring scales to measure weight. Consider a block that weighs 10N (1Kg), if suspended by a single scale it reads 10N.

Equilibrium If we hang the same block by 2 scales, they each read 5N. The scales pull up with a combined force of 10N.

Equilibrium What if the 2 scales weren’t vertical but were attached at an angle. We can see for the forces to balance, the scales must give a reading of a larger amount.

Components of Vectors The force applied to the lawn mower may be resolved into two components, x for the horizontal and y for the vertical.

Components of Vectors The rule for finding the vertical and horizontal components is simple. A vector is drawn in the proper direction and then horizontal and vertical vectors are drawn from the tail of the vector.

Components of Weight Why does a ball move faster on a steeper slope?
We can see what happens when we resolve the vector representing weight into its components.

Components of Weight Vector A represents the amount of acceleration of the ball and vector B presses it against the surface. Steeper the slope, more A.

Projectile Motion A projectile is any object that is projected by some means and continues in motion by its own inertia. An example is a cannon ball shot out of a cannon or a stone thrown in the air.

Projectile Motion The horizontal component of the motion is just like looking at the horizontal motion of a ball rolling freely on a horizontal surface.

Projectile Motion The vertical component of an object following a curved path is the same as the motion of a freely falling object as discussed in section 2.

Projectile Motion A multi-image photograph displaying the components of projectile motion.

Projectile Motion The horizontal component of the motion is completely independent of the vertical motion of the object and can be treated differently. Ph14e – projectile motion

Projectile Motion In summary, the a projectile will accelerate (change its speed) in the vertical direction while moving with a constant horizontal speed. This path is called a parabola.

Upwardly Moving Projectiles
Imagine a cannon ball shot at an upward angle in a gravity free region on Earth. The cannon ball would follow a straight line. But there is gravity, the distance the cannon ball deviates from the straight line is the same distance that is calculated from a freely falling object.

Upwardly Moving Projectiles

Upwardly Moving Projectiles
The distance from the dotted line can be calculated using the formula introduced previously.

Upwardly Moving Projectiles
The following diagram shows the vectors that represent the motion of the projectile. Only the vertical component is changing, the horizontal component has remained the same.

Upwardly Moving Projectiles
The horizontal component of the motion will determine the range (how far horizontally the projectile will travel).

Upwardly Moving Projectiles
The following diagram displays the different angle of a projectile launched with the same initial speed.

Upwardly Moving Projectiles
Angles that add up to 90 degrees and launched with the same initial speed have the same Range. Ph14e – projectile motion

Air Resistance on a Projectile
Air resistance affects both the horizontal and vertical components of the motion negatively.

Air Resistance on a Projectile
Need to consider how air resistance effects the horizontal and vertical motion separately. Continuously slows down horizontally and maximum height is reduced.

Physics in Surfing