P5 - Forces Flashcards
Compare a Scalar quantities and Vector quantities.
Scalar only has magnitude:(mass,temperature,speed,energy)
- *Vector has magnitude and direction. They can be expressed using an arrow whose length represents magnitude.**
- *(displacement, velocity,**
What is a Force?
A force is a push or a pull that acts on an object due to the interaction with another object.
Force is a vector quanitity. Measure in N, Newtons
Forces are either contact or non-contact.
What is a contact Force?
Examples of contact forces include friction, air resistance, tension and normal contact force.
What is a non-contact force?
gravitational force, electrostatic force and magnetic force.
What does the Mass tell us?
Does the mass of an object change depend on the location?
The mass of an object tells us how much matter an object has inside of it. Measure in kg. It is scalar as it has no direction. The Mass of the elephant does not matter where it is. Mass of an elephant on Earth is 5000kg but if it was on the moon is the same.
What does the weight tell us?
Does the weight change in different locations?
Weight is the force acting on an object due to gravity. The force of gravity close to the Earth is due to the gravitational field around the Earth
The weight of an object may be considered to act at a single point referred to as the object’s ‘centre of mass’.
The weight of an object and the mass of an object are directly proportional.
Weight is measured using a newtonmeter
What is the Equation for weight?
weight = mass × gravitational field strength W = m g
weight, W, in newtons,
N mass, m, in kilograms,
g gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given.)
How do we calculate resultant force?
The Resultant Force is a single force that has the same effect as all of the original forces acting together. To work out the resultant force we subtract the smaller force from the larger one.
Example 1)
Resultant Force
Example 2) Resultant force
Example 3) Air resistance
What is a free-body diagram?
Plane is point to the right
Question Example)Two forces are acting on an object. One force has a magnitude of 10N and the other force has a magnitude of 8N. The angle between the two forces is 30 degrees. Calculate the resultant force
Start by making a scale drawing showing the forces involved. Scientists call this a vector diagram.
Lets say 1cm = 1N
Example Question)A 100N force is acting at an angle of 35 degrees with the horizontal. Determine the horizontal and vertical components of this force.
We will use a scale of 1cm = 10N
- Here is object the force is acting on
- We draw faint lines showing the horizontal and vertical axes
- using a protractor we measure 35 degrees from the horizontal
- Then we use a ruler to draw the vector showing 100N force
- Then we draw dotted lines from the head of the vector to the horizontal and vertical axes.
- Now we can draw our horizontal and vertical components.
- Then we use a rule to measure their magnitude.
Resolving Forces
What are the forces acting on a man when he pushes a box?
A man is pushing a box along the floor and the box is moving at a constant velocity. He is applying a forward force but friction is acting between the box and the floor which causes the temperature of the box to increase.
The Chemical energy stored in the muscles is being transferred to thermal energy in the box.
When is work done?
What is the equation for work done?
When a force causes an object to move through a distance, work is done on the object.
So a force does work on an object when the force causes a displacement of the object
- *One joule of work is done when a force of one newton causes a displacement of one metre.**
- *1 joule = 1 newton-metre**
A car is moving at velocity of 20m/s. Then the driver presses the brakes. What happens when the car breaks.
A moving object has kinetic energy. Due the brakes pressing against the wheel the force of friction now acts between the brake and the wheels, The kinetic energy store of the car is transferred to the thermal energy store of the brakes.
The temperature of the brakes increases and at the same time, the car slows down as it loses kinetic energy.
What happens when we stretch an elastic material?
- An example of stretching is placing a mass on the bottom of a vertically hanging spring
- The two forces are:
- The weight of the mass
- The tension in the spring
- These two forces are away from each other
- These opposite forces are a result of Newton’s Third Law
What happens when you squeeze an elastic material?
- An example of compression is placing a mass on top of a spring placed on a flat surface
- The two forces are:
- The weight of the mass
- The reaction force from the surface to the spring
- These two forces are towards each other
What happens when applying three forces to an elastic material?
why, to change the shape of an object (by stretching, bending or compressing), more than one force has to be applied – this is limited to stationary objects only
We need to apply multiple forces; this is because if we only applied one force, we would just move the elastic object instead of changing the shape.
What is the relationship betwen the extension and force applied in a spring?
What is the limit of porportionality
The extension of an elastic object, such as a spring, is directly proportional to the force applied, provided that the limit of proportionality is not exceeded.
When an object is deformed, (stretched, bent or compressed), if it returns to its original shape once the forces are removed, it is an elastic deformation.
If it does not return to its original shape it is an inelastic deformation
How do you calculate the force needed to stretch an elastic object using this equation:
force(N) = spring constant(N/M) × extension (m)
F = k e
force, F, in newtons,
N spring constant, k, in newtons per metre,
extension, e, in metres,
What is the limit of porportionality?
Spring constant is a measure of the stiffness of a spring up to its limit of proportionality or elastic limit. The limit of proportionality refers to the point beyond which Hooke’s law is no longer true when stretching a material. The elastic limit of a material is the furthest point it can be stretched or deformed while being able to return to its previous shape. Once a material has gone past its elastic limit, its deformation is said to be inelastic.
The higher the spring constant, the stiffer the spring. The spring constant is different for different elastic objects. For a given spring and other elastic objects, the extension is directly proportional to the force applied. For example, if the force is doubled, the extension doubles. This works until the limit of proportionality is exceeded.
Describe the difference between a linear and non-linear relationship between force and extension?
- For a linear relationship between force and extension, the gradient of the force-extension graph is equal to the spring constant (1)
- as force and extension are directly proportional (1)
- while for a non-linear relationship between force and extension, the gradient of the force-extension graph is not equal to the spring constant (1)
- as force and extension are not directly proportional (1)
- -the extension increases more for each unit increase in force (1) .
How do particles in gas spread and move rapidly?
What is a fluid?
A fluid can be either a liquid or a gas.
The pressure in fluids causes a force normal (at right angles) to any surface.
How do you calculate pressure?
Pressure in a Fluid
- A fluid is either a liquid or a gas
- Pressure is defined as
The concentration of a force or the force per unit area
- For example, when a drawing pin is pushed downwards:
- It is pushed into the surface, rather than up towards the finger
- This is because the sharp point weight is more concentrated (a small area) creating a larger pressure
What is the atmosphere?
How does density change as the altitude increase?
The atmosphere is a thin layer of air around the earth is most dense at the surface of the Earth. The atmosphere gets less dense and the higher the attitude.
Air molecules colliding with a surface create atmospheric pressure. The number of air molecules (and so the weight of air) above a surface decreases as the height of the surface above ground level increases. So as height increases there is always less air above a surface than there is at a lower height. So atmospheric pressure decreases with an increase in height.
How do you calculate the pressure of column of liquid using the equation?
pressure = height of the column × density of the liquid × gravitational field strength
pressure, p, in pascals, Pa
height of the column, h, in metres, m
density, ρ, in kilograms per metre cubed, kg/m3
gravitational field strength, g, in newtons per kilogram, N/kg
How does pressure explain how object float and sink?
A partially (or totally) submerged object experiences a greater pressure on the bottom surface than on the top surface. This creates a resultant force upwards. This force is called the upthrust.
For an object to float Upthrust must = Object’s weight. Otherwise if upthrust is less than weight of the object the object sinks.
Size of the upthrust acting on the object is the same as the weight of water displaced by the object so if an object can displace its own weight of water then upthrust will = weight.
so an object less dense than water only has to displace a small amount of water before the weight of water displaced is equal to the weight of the object. so now the upthrust equals weight of the object and the object floats.
An object with the same density as water will have to displace its own volume of water in order for weight of water displaced to equal the weight of the object. Once again upthrust = to weight of object and so object floats but a lower height.
An object more dense than water cannot displace a volume of water equal to its own weight. Therefore weight of the object is greater than upthrust so object sinks.
What is the difference between Distance and Displacement?
How do state the displacement of objects?
How do you calculate Speed?
Is speed scalar?
Distance does not involve direction so it is a scalar quantity.
Displacement tells us the distance an object moves moves in a straight line from the start point to the finish point. It must include direction so it is a vector quantity.
Speeds you need to know?
and their factors
- *walking - 1.5m/s**
- *Running - 3m/s**
- *Cycling - 6m/s**
- *Car - 13m/s-30m/s**
- *Train - 50m/s**
- *Cruising airplane - 250m/s**
- *Sound - 330m/s**
However it depends on a variety of factors like: age, terrain, Distance(tiredness)
What is velocity?
How do you state velocity?
Velocity is the speed of an object in a given direction.
Velocity is a vector quantity. e.g. 20m/s south
You have to state the direction.
What happens if an object is moving at a constant speed in a circle?
Its velocity is changing as although the speed is constant, it’s direction is still changing.
How can you work out the speed of the gradient on distance-time graphs?
Calculate the gradient
How do you determine the speed of an object at any given point
- How do we determine the speed of this object.
- To do that we place a dot on the line we want to find out. e.g. 100s
- Then we draw a tangent
- Then we calculate the gradient of the tangent
How do you calculate Acceleration?
Umots
a = ∆v / t
acceleration, a, in metres per second squared, m/s2
change in velocity, ∆v, in metres per second, m/s
time, t, in seconds, s
Velocity-Time Graph
Velocity-Time Graph tells us the acceleration of the object.
The gradient tells us the acceleration of the object
If the Line is horizontal the object is travelling at a constant velocity.
An upward slope shows acceleration and downwards shows deceleration
How do you calculate the distance on a velocity/time graphs?
Equation 2 for acceleration
(final velocity)^2− (initial velocity)^2 = 2 × acceleration × distance
What is newtons first law of motion?
What does this mean if an object travels at a steady speed?
What does it mean if the velocity is changing?
If the resultant force acting on an object is zero and:
- *• the object is stationary, the object remains stationary**
- *• the object is moving, the object continues to move at the same speed and in the same direction.**
So the object continues to move at the same velocity. So, when a vehicle travels at a steady speed the resistive forces balance the driving force.
So, the velocity (speed and/or direction) of an object will only change if a resultant force is acting on the object.
What is Newtons Second Law of Motion?
What equation is linked to it?
The acceleration of an object is proportional to the resultant force acting on it and inversely proportional to the object’s mass
- An object will accelerate (change its velocity) in response to a resultant force
- The bigger this resultant force, the larger the acceleration
- For a given force, the greater the object’s mass, the smaller the acceleration experienced
Calculating Force & Acceleration
- Newton’s second law can be expressed as an equation:
F = ma
- Where:
- F = resultant force on the object in Newtons (N)
- m = mass of the object in kilograms (kg)
- a = acceleration of the object in metres per second squared (m/s2)
What would happen if you were too move two objects a & b with the same mass.
A with a resultant force of 20N to the right while B with a resultant force of 10N
What is the Equation for calculating Force?
What is the Equation for calculating Force?
What is Newton’s Third Law of Motion?
- Newton’s third law of motion states:
Whenever two objects interact, the forces they exert on each other are equal and opposite
- Newton’s third law explains the following important principles about forces:
- All forces arise in pairs - if object A exerts a force on object B, then object B exerts an equal and opposite force on object A
- Force pairs are of the same type - for example, if object A exerts a gravitational force on object B, then object B exerts an equal and opposite gravitational force on object A
- Newton’s third law explains the forces that enable someone to walk
- The image below shows an example of a pair of equal and opposite forces acting on two objects (the ground and a foot):
- Examples
When a man uses a paddle to push on the water. At the same time, the water pushes back on the paddle. It is the same is magnitude but opposite in direction.
What forces act on a skydiver as he jumps out of the building?
- As soon as the sky diver jumps out of the plane, the only force acting is weight. THis is due to gravity and this force will not change. So the skydiver experiences a downward resultant force so they accelerate towards the ground.
- As they fall they experience friction with molecules called air resistance and it acts upwards. However, the weight is still greater then the air resistance so the skydiver sill accelerates towards the ground.
- As the skydiver’s velocity increases, the air resistance also increases.
- At a certain point, the forces balance and there is now no resultant force. As a result, the velocity stays constant(TERMINAL VELOCITY).
- The velocity is very high and they would die if they hit the ground so at this point they deploy the parachute which increases surface area and causes the air resistance to increase.
- At this Point the air resistance is greater than the weight so there is a resultant force acting upwards. This causes the skydiver to decelerate.
- Because the velocity decreases, the air resistance decreases. The air resistance will then balance and the resultant force is 0. They are now at a lower and safer terminal velocity which allows them to land safely.
What is thinking Distance?
What is the braking Distance?
What is the stopping distance?
It can be divided into two parts:
- Thinking distance is the distance travelled by the car during the driver’s reaction time.
- Braking Distance is the distance the car travels from the when the driver applies the brakes to when the car stops.
Stopping Distance = Braking + Thinking
The greater the speed, the greater the Braking distance.
Reaction time can vary from 0.2 to 0.9 s.
You can test for someone’s reaction time using the ruler drop test.
What happens if we double the velocity of the car?
What is the formula for kinetic energy?
What happens to the energy store when a car brakes?
What is Momentum?
What is the momentum of a stationary object?
What is the equation for momentum?
When does the momentum of an object change?
All moving objects have momentum. But if an object is not moving then its momentum is zero.
- A moving object has momentum which is defined by the equation:
p = mv
- Where:
- p = momentum in kilogram metre per second (kg m/s)
- m = mass in kilograms (kg)
- v = velocity in metres per second (m/s)
- Momentum keeps an object moving in the same direction, making it difficult to change the direction of an object with a large momentum
- Since velocity is a vector this means that the momentum of an object also depends on its direction of travel
- This means that momentum can be either positive or negative
- If an object travelling to the right has positive momentum, an object travelling in the opposite direction (to the left) will have negative momentum
- Therefore, the momentum of an object will change if:
- The object accelerates or decelerates
- Changes direction
- Its mass changes
What is the conservation of momentum?
- The principle of conservation of momentum states that:
In a closed system, the total momentum before an event is equal to the total momentum after the event
- Since momentum is a vector quantity, a system of objects moving in opposite directions (e.g. towards each other) at the same speed will have an overall momentum of 0 since they will cancel out
- Momentum is always conserved over time
- The diagram below shows two masses m with velocity u and M at rest (ie. zero velocity)
- Before the collision:
- The momentum is only of mass m which is moving
- If the right is taken as the positive direction, the total momentum of the system is m × u
- After the collision:
- Mass M also now has momentum
- The velocity of m is now -v (since it is now travelling to the left) and the velocity of M is V
- The total momentum is now the momentum of M + momentum of m
- This is (M × V) + (m × -v) or (M×V) – (m×v)
Examples of conservation of momentum
Conservation of momentum explains why a gun or cannon recoils backwards when it is fired. When a cannon is fired, the cannon ball gains forward momentum and the cannon gains backward momentum. Before the cannon is fired (the ‘event’), the total momentum is zero. This is because neither object is moving.
The total momentum of the cannon and the cannon ball after being fired is also zero, with the cannon and cannon ball moving in opposite directions.
What can rapid changes in momentum do?
How do car manufacturers use crash tests?
What is the Equation for force and change in momentum?
Rapid changes of momentum place huge forces on the passenger and this can be lethal.
Since force is equal to the rate of change in momentum, the force of an impact in a vehicle collision can be decreased by increasing the contact time over which the collision occurs over which their momentum is reduced creating a smaller impact force and a lower chance of injury
What does mass multiplied by change in velocity mean?
How do you calculate inertial mass?
- Inertial mass is the property of an object which describes how difficult it is to change its velocity
- It is defined as the ratio between the force applied to it and the acceleration it experiences:
- Where:
- m = inertial mass in kilograms (kg)
- F = force in newtons (N)
- a = acceleration in metres per second squared (m/s2)
- This equation shows that for a given force, inertial mass is inversely proportional to acceleration
- Larger inertial masses will experience small accelerations
- Smaller inertial masses will experience large accelerations
What is Acceleration?
Defining Acceleration
- Acceleration is defined as the rate of change of velocity
- In other words, it describes how much an object’s velocity changes every second
- The equation below is used to calculate the average acceleration of an object:
a = Δv/t
- Where:
- a = acceleration in metres per second squared (m/s2)
- Δv = change in velocity in metres per second (m/s)
- t = time taken in seconds (s)
Speeding Up and Slowing Down
How do you represent terminal velocity?
Why do Objects sink or float?
Floating & Sinking
- Whether an object sinks or floats depends on the upthrust:
- If the upthrust on an object is equal to (or greater than) the object’s weight, then the object will float
- If the upthrust is smaller than the weight then the object will sink
- The outcome also depends on the object’s density:
- If it has a density less than the density of the fluid it is immersed in, the object will float
- If it has a density more than the density of the fluid it is immersed in, the object will sink
- This is because if the density of the object is greater than the density of the fluid, the object can never displace enough fluid to create an upthrust that will hold its weight up (and therefore sinks)
- A polystyrene block will float in water
- This is because polystyrene has a density of 0.05 g/cm3 which is much less than the density of water (1.0 g/cm3)
- A wooden block will be partially submerged but will still float
- This is because the density of a wooden block (0.9 g/cm3) is slightly less than the density of water
- An iron block will sink
- This is because iron has a density (7.9 g/cm3) that is much higherthan water
what happens when you bend an object?
- An example of bending is a diving board bending when a swimmer stands at the far end
- The two forces are:
- The weight of the swimmer
- The reaction force from the block to the dividing board
- These two forces act towards each other, but at different points on the object
- Bending can also be caused by two forces at an angle to each other
what is Hooke’s law?
What equation does Hooke’s law define?
- The relationship between the extension of an elastic object and the applied force is defined by Hooke’s Law
- Hooke’s Law states that:
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
- Directly proportional means that as more force is applied, the greater the extension (and vice versa)
- The limit of proportionality is where if more force is added, the object may extend but will not return to its original shape when the force is removed (it will be inelastically deformed)
A tennis ball is thrown at an angle of 45° to horizontal at a speed of 5 m/s. A second tennis ball is thrown in the same direction at a speed of 10 m/s.Draw the velocity vectors of the balls.
