Physics Unit 1: Section 1 - Mechanics
The man has gone for a swim in a river flowing at 0.35m/s. He swims at 0.18m/s at right angles to the current. What is his resultant force?
0.39
Define the centre of gravity
A point at which you can consider all the weight of an object or body can be be concentrated and to act.
Give three examples of vector quantities
Any from displacement, force, velocity, acceleration or momentum
What is the only force acting on projectiles?
Gravity
A tile falls from a roof 25 m high. Calculate its speed when it hits the ground and the time it takes to hit the ground?
In this question it is easier to take downwards as positive so use 9.81 not -9.81. s: 25 m u: 0 m/s since it want travelling at first v: 22.1 m/s a: 9.81 m/s² t: 2.3 s
What is the SI unit of weight?
N (newtons)
State newtons first law.
Newtons first law says that a force is needed to change a velocity, newtons first law of motion states that the velocity of an object will not change unless a resultant force is acted upon it.
A car is travelling at a speed of 10 m/s with a force of 500N What is its power?
P = FV So 500 x 10 = 5000 W
What does resolving a force mean?
Splitting it into its components e.g. vertical and horizontal
When doing a question on projectile motion what do you have to do?
Think of horizontal and vertical motion seperately
A car accelerates steadily from rest at a rate of 4.2 m/s² for 6 seconds, find its final speed.
Use v = u + at => v = 0 + 4.2 x 6 = 25.2 m/s s: ? u: 0 m/s since it want travelling at first v: 25.2 m/s a: 4.2 m/s² t: 6 s
What does a curved displacement time graph show?
acceleration or deceleration
What is the formula used to calculate density?
p = m/v p being density m being mass v being volume
How do you calculate the volume of a sphere?
v = πr²h
What is given by the gradient of a distance time graph?
velocity
If his mass is 60 kg then what is his weight on earth?
w = mg so 588.6N
What is the initial vertical velocity of an object projected horizontally with a velocity of 5 m/s?
0 m/s
If the projectile starts of at an angle what is the method to use?
1. Resolve the initial velocity into both horizontal and vertical velocities using trigonometry. 2. Use the vertical component to work out how long it was in the air and/or how high it goes. 3. Use the horizontal component to work out how far it goes in the air (s). Sometimes you may have to split the vertical component into two.
What do you know from the previous question?
1. That you need to use SUVAT (Uniform acceleration equations) because the question said 'steadily' 2. That u = o m/s as the question says 'from rest'
What are the three main advantages of using data loggers and ICT to draw displacement / time and velocity / time graphs?
1. The data is more accurate - you don't have to allow for human reaction time, this removes human error involved. 2. Automatic systems have a much higher sampling rate than humans leading to more accurate results. 3. You can see the data displayed in real time.
What are the two main ways of finding the centre of gravity of an object?
1. symmetry, an object with more than one line of symmetry has a centre of gravity where the two lines cross over. 2. Using a plumb line and hanging the object next to it draw a line where the object hangs, after this repeat the process in another place on the object, where the tow lines cross is its centre of gravity.
If a man walks 12m north and 18m east what would be the magnitude of his displacement?
21.63m
At what angle is his resultant force?
27.2 degrees
What would be the direction of his displacement?
56.31 degrees
What is gravity on earth?
9.81 N
What is the force of gravity on earth?
9.81 N
What is weight?
A force experienced by a mass due to a gravitational field.
Define Scalar
A scalar quantity has only magnitude/size
Define Vector
A vector quantity has both magnitude/size and direction
What are the units of acceleration and velocity?
Acceleration: m/s² Velocity: m/s
What do free body diagrams show?
All the forces acting on a single body. Remember a free body force diagram shows the sizes of the forces depending on the sizes of the arrows used.
What is a resultant force?
An overall force acting on something.
Give three examples of scalar quantities
Any from mass, temperature, time, length, speed or energy
Why is this value negative?
Because it was a deceleration
Why was gravity negative in the previous question?
Because when using uniform acceleration equations you usually take upwards as positive and downwards as negative. Despite this as long as you state at the start of your answer whether your going to take downwards as positive like in the tile question then you can do it that way.
What is uniform acceleration?
Constant acceleration
What are car safety feature designed to do?
Devices such as seat belts, airbags, crumple zones and safety cages are designed to slow you down more gradually so you decelerate at a slower rate causing less harm to the driver.
What formula is used to calculate elastic potential energy?
Eel = 1/2ke² k being stiffness constant e being extension
What formula is used to calculate gravitational potential energy?
Egrav = mgh m being mass g being gravity h being height
What formula is used to calculate kinetic energy?
Ek = 1/2mv² m being mass v being velocity
What is the principle of conservation of energy?
Energy cannot be created or destroyed. Energy can be transferred from one form to another but never lost, for example a motor may convert electric energy into heat, sound and kinetic energy.
If a robot working in a factory moves an object of mass 150 kg a distance of 300 m in 5 minutes how much work is it doing?
F = ma so 150 x 9.81 = 1471.5 N W = Fs = 1471.5 N x 300 = 441450 J 441450/1000 = 441.45 kJ
A tree trunk is pulled along the ground by a car exerting a force of 1200 N to the tree trunk at an angle of 25 degrees to the horizontal. Calculate the components of this force in the horizontal and vertical directions.
Horizontal = 1200 x cos 25 = 1088 N Vertical = 1200 x sin 25 = 507 N
How high the centre of gravity tells us what?
How stable the object is.
What is work done measured in?
J (Joules)
What is mass?
Just an amount of something
What is the SI unit of distance?
M (metre)
State newtons second law.
Newtons second law says that acceleration is proportional to force (or f = ma) it says that the more force you have the more acceleration you will get and the more mass you have the less acceleration you get.
State newtons third law.
Newtons third law says that for every action, there is an equal and opposite reaction. The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object.
Calculate its power
P = W/t 5 x 60 = 300 s 441450 W/300 s = 1471.5 W
What is the formula for power?
Power = Work done/time Or Force x Velocity
What is the SI unit of time?
S (second)
What quantities can you not use pythagorus or trigonometry for?
Scalars
What increases thinking distance?
Speed, tiredness, alcohol, drug use, illness or distractions.
What did Aristotle believe about free fall and projectile motion?
The heavier the object the quicker it would fall, for example if two objects are dropped at the same time the heavier one would hit the ground first.
What did Galileo believe about free fall and projectile motion?
The weight of the object is irrelevant, he said that all objects in free fall had the same acceleration. This theory was correct.
What is the formula used to calculate the stoping distance of a car?
Thinking distance + Breaking distance = Stopping distance
What is the formula for thinking distance?
Thinking distance = Speed x Reaction time v = s/t so vt = s
When using pythagorus how would you draw the vectors?
Tip to tail
A motorcyclist slows down uniformly as he approaches a red light. He takes 3.2 seconds to stop and travels 40 m in this time. What was his acceleration?
Use a = v - u/t = -7.81 m/s² s: 40 m u: 25 m/s v: 0 m/s as it says 'to stop' a: -7.81 m/s² t: 3.2 s
How far did she fall in this time?
Use either s = (u+v)/2 x t OR s = ut + 1/2 at² s: -122.625 m u: 0 m/s since she started at rest v: 49.05 m/s a: -9.81 m/s² t: 5 s
A motorcyclist slows down uniformly as he approaches a red light. He takes 3.2 seconds to stop and travels 40 m in this time. What was his initial speed?
Use s - v x 2/t = 25 m/s s: 40 m u: 25 m/s v: 0 m/s as it says 'to stop' a: ? t: 3.2 s
A car accelerates steadily from rest at a rate of 4.2 m/s² for 6 seconds, find its final speed. Find the cars distance travelled in 6 s.
Use s = (u+v)/2 x t OR s = ut + 1/2 at² s: 75.6 m u: 0 m/s since it want travelling at first v: 25.2 m/s a: 4.2 m/s² t: 6 s
Michael throws a stone horizontally with a velocity of 100 m/s from 1.5 m above the ground. How long does it take to hit the ground? Assume theres no friction.
Use s = 1/2 gt² => t² = 2 x -1.5 / -9.81 = 0.55 s s vertical: -1.5 m u vertical: 0 m/s v vertical: a vertical: -9.81 t vertical: 0.55 s
A stream provides a constant acceleration of 6 m/s². A boat is pushed against the current and then released 1.2 m upstream from a waterfall. Its speed just before it hits the waterfall was 5 m/s. Calculate maximum distance upstream the boat reaches
Use t = v - u/a = 1.38 s And then use s = v² - u²/2a s: 2.08 m u: 0 m/s v: 5 m/s a: 6 m/s² t: 1.38 s
Michael throws a stone horizontally with a velocity of 100 m/s from 1.5 m above the ground. How far does the stone travel before hitting the ground? Assume theres no friction.
Use v = s/t and rearrange to vt = s = 55 m s horizontal: 55 m u horizontal: v horizontal: 100 m/s a horizontal: o m/s² t horizontal: 0.55 s
A skydiver jumps from an aeroplane when it is flying horizontally. She accelerates due to gravity for 5 s. What is her maximum vertical velocity
Use v = u + at = 49.05 m/s s: u: 0 m/s since she started at rest v: -49.05 m/s a: -9.81 m/s² t: 5 s
A stream provides a constant acceleration of 6 m/s². A boat is pushed against the current and then released 1.2 m upstream from a waterfall. Its speed just before it hits the waterfall was 5 m/s. What was its initial velocity?
Use v² = u² + 2as => u² = v² - 2as = -3.26 m/s s: 1.2 m u: -3.26 m/s v: 5 m/s a: 6 m/s² t:
What is the unit of power
W (Watts)
What is free fall?
When there is only gravity and nothing else. It is defined as 'the motion of an object undergoing an acceleration of g'
What is the formula for work done?
Work done = force x distance (W=Fs)
What is his acceleration?
a = v/t so 5 m/s²
How do you calculate the area of a sphere?
a = πr²
What is given by the gradient of a velocity time graph?
acceleration
What is the formula for acceleration?
acceleration = velocity/time
What do you change between the equations of uniform acceleration and the equations of free fall?
acceleration becomes gravity and you usually remove the initial velocity as it usually starts from rest.
A tree trunk is pulled along the ground by a car exerting a force of 1000 N to the horizontal and 600 N to the vertical find its resultant force.
a² + b² = c² 1000² + 600² = 1166.2 N
What does a straight line on a distance time graph show?
constant velocity
What is given by the area under a velocity time graph?
distance travelled
A skydiver has reached terminal velocity falling show the forces acting on him.
drag up weight (mg) down equal length
What increases breaking distance?
friction, grip of tires, quality of brakes or worn out tires.
What is the SI unit of mass?
kg (kilograms)
What is the unit of density?
kg/m³
How do you calculate the volume of a rectangle?
length x width x height
Define density.
mass per unit volume
Does density vary with size or shape?
no, it just depends on the material it is made of
What does a curved velocity time graph show?
non-uniform acceleration
What does each of the five letters represent?
s: Displacement u: Initial velocity v: Final velocity a: Acceleration t: Time
In free fall and projectile motion which signs are positive and which are negative?
s: either u: either v: either a: usually negative as it goes downwards t: always positive as it is a scalar quantity
What are the equations typically used for free fall calculations?
v = gt v² = 2gs s = 1/2 gt² s = vt/2
Calculate the mass of a cube of aluminium of side 5 cm
v = lwh = 5 x 5 x 5 = 125 cm² p = 2.72 g/cm² m = pv = 2.72 x 125 = 340 g 340/1000 = 0.34 kg
If a man travels 20 m in 2 s then what is his velocity
v = s/t so 10 m/s
If there is a initial velocity involved, for example Michael throws a stone downwards then what equations do you need to use?
v = u + at s = (u+v)/2 x t s = ut + 1/2 at² v² = u² + 2as
What are the four main uniform acceleration equations?
v = u + at s = (u+v)/2 x t s = ut + 1/2 at² v² = u² + 2as
A cylinder of aluminium has a radius of 4 cm and a height of 6 cm and a mass of 820 g. Whats its density?
v = πr²h = π x 4² x 6 = 301.6 cm² p = m/v = 820 g / 301.6 cm² = 2.72 p = 2.72 g/cm²
What is the formula for velocity?
velocity = distance/time
A lion has a mass of 200kg, what would be its mass and weight on earth (g = 9.81) and the moon (g = 1.6)
w = mg Weight on earth: 1962 N Mass on earth: 200 kg Weight on moon: 320 N Mass on moon: 200 kg
A picture of mass 8 kg is suspended from a hook by two strings each at an angle of 50 degrees to the horizontal, calculate the tension in each string.
w = mg = 8 x 9.81 = 78.48 N 78.48/2 = 39.24 N 39.24 N/sin 50 = 51.2 N
What is the formula used to calculate weight?
weight = mass x gravity (w=mg)