Chapter 9: Physics; Motion and Energy Flashcards
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Distance
- Represented as d
- Total scalar length an object travels
- Distance does not depend on direction
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Displacement
- Represented as ∆d
- Vector that points from an object’s starting position to its final position
- Displacement depends on direction
- Displacement = final position - initial position
- ∆d = dfinal - dinitial
- Defined using: + or -, NSEW, Angles
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Scalar
- Distance is a scalar quantity, meaning it depends on only magnitude and not direction.
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Vector
- Displacement is a vector quantity as it depends on both magnitude and direction
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Speed
- Represented by s
- Scalar Quantity that refers to the rate at which an object is moving
- Speed = distance/time
- E.g car travels 20 m/s (magnitude)
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Velocity
- Represented by v
- Vector quantity that refers to the rate at which an object changes its position
- Velocity = displacement/time
- Has both magnitude and direction
- E.g car travels 20 m/s east (magnitude and direction)
Velocity Formula
Speed, Distance, Time
speed = distance/time
time = distance/speed
distance = speed x time
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Acceleration
- Represented by a
- Vector quantity, rate of change of the velocity
- To accelerate, change your speed or your direction-or change both
- avg acceleration = (final speed - initial speed)/time or change in velocity/time
- can be written as a = v - u/t
- SI units are m/s/s or ms-2 or m/s2
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How to calculate final speed
- Use formula for acceleration
- Where a = acceleration, v = final speed, u = initial speed, t = time taken
- final speed = initial speed + (average acceleration x time taken)
- v = u + at
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Negative Acceleration
- When the object is moving in the positive direction, but the rate of change of velocity is negative, where a = v-u(neg)/time
- Negative value means that initial speed is greater than final speed, object is decelerating
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Types of Acceleration
An object can be accelerating when it is:
* Speeding up: car speeds up
* Slowing down: car slows down
* Changing direction: car turns, changes velocity (can be at constant speed)
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Acceleration due to gravity
- Falling objects accelerate towards the earth due to gravity
- Gravity = 9.8m/s, therefore acceleration due to gravity = 9.8 m/s/s or m/s2 (g-force)
- Falling objects do not reach enormous speeds due to friction and air resistance
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Terminal Velocity
- When an object falls towards earth, it will eventually reach a point where the downwards force due to gravity is balanced with the upwards force of friction and air resistance. While the object still falls extremely fast, it will not speed up.
- Terminal velocity is the constant speed with which an object falls when air resistance balances the force of gravity.
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Human tolerance and G-Force
- Human tolerance depends on the direction and the part of the body the force acts on
- Humans are more suited to tolerate horizontal forces as a vertical drop for long durations can disrupt blood flow to the brain
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Newton’s First Law
An object at rest will remain at rest and an object in motion stays in motion with the same speed and in the same direction unless it is acted upon by an unbalanced force.
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Inertia
It is the tendency of an object to resist any change to its motion or state, including changes to speed and direction
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Unbalanced Force
- Unbalanced forces cause change in an object’s motion; a stationary object will accelerate, a moving object will change direction or speed
- For example, a book on a table is pushed to the right, friction acts to the left to slow the book down. The unbalanced force changes its state of motion and the book accelerates to the right.
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Examples of Newtons First Law
- A driver of a car brakes suddenly, by intertia, the car and passenger lurches forward
- A runner continues to run past the finish line due to inertia
- A rolling ball on rough ground stops before a rolling ball on smooth ground because the rough surface offers more friction to act upon the moving object
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Newton’s Second Law
- An object will accelerate in the direction of an unbalanced force acting upon it. The size of acceleration depends upon the mass of the object and the size of the force acting.
- If you push something, it moves in that direction
- If you continue to push, it will accelerate
- The heavier the object, the more force required
- The heavier the object, the less it will accelerate
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Force, mass, acceleration (Newton’s Second Law)
Net Force (Newtons) = mass (kg) x acceleration (m/s/s)
m = fnet/a
a = fnet/m
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Newton’s Third law
For every action force on an object, there is an equal and opposite reaction force back on the other object.
Examples of Newton’s Third Law
- A nail is hit by a hammer = The nail exerts an equal force back on the hammer
- A book rests on a table and exerts its weight force onto the table = The table exerts an equal support force upwards on the book
- You stand a skateboard and push against a wall = The wall pushes back on you with equal force and you move away
- Rocket pushes gas out = Gas pushes rocket upwards
How to convert between m/s and km/h
m/s x 3.6 = km/h
km/h ÷ 3.6 = m/s
In terms of motion, when a cheetah is sitting stationary in the forest it has something in common with when it’s running on a straight course at 100km/hr. Explain
No acceleration = 0km/h/h
How to solve for time with acceleration formula
time = final speed - initial speed over acceleration
t = (v - u)/a
How to solve for intial speed
initial speed = final speed - acceleration x time
u = v - at
Distance-Time Graph
- Shows how far an object travels over time
- Slope/gradient is equivalent to the object’s speed over a time interval
- The steeper the gradient, the faster the object is moving
- Speed = rise/run
Speed-Time Graph
- Shows how an objects speed changes over time
- Flat line = constant acceleration
- Increase = increasing speed
- Decrease = decreasing speed
- Acceleration = rise/run
- Area under line gives distance travelled up to that point
Analysing Distance Time Graphs
Converting km m etc.
Converting Time
