forces and motion Flashcards
what is the equation for av speed?
speed = d/t
what is speed? what is velocity? how are they different?
speed is the rate of change of distance over time or Distance travelled per unit time, while velocity is the rate of change of displacement over time or Speed in a given direction.
speed is a scalar quantity whereas velocity is a vector quantity with magnitude and direction
what is scalar?
This is because it only contains a magnitude (without a direction)
what is vector?
a vector quantity has magnitude and direction
what is the difference between acceleration, and speed or velocity
acceleration id not the same as them. Acceleration is how quickly velocity is changing
what can a change in velocity be?
change in speed or a change in direction or both
what is the unit for acceleration?
m/s^2
what is the unit for velocity?
m/s
what is the formula for acceleration?
acceleration =change in velocity/ time taken
or
(v-u) / a x t
what does a negative value mean for acceleration
something is slowing down/ decelerating
what is the equation for final velocity?
v^2 = u^2 + 2as
(u=initital velocity)
(s= distance travelled while accelerating)
what does a distance-time graph tell you?
how far something has travelled
what does it mean if a section is flat on a d-t graph?
no movement
what does the gradient on a d-t graph show?
the speed of an object
what do curves, steeper lines, levelling off curve mean in a d-t graph?
curve - acceleration
steep lines- going faster
a curve getting steeper - it is speeding up
levelling off curve- slowing down (decreasing gradient)
what doe the gradient of a velocity time graph mean?
acceleration
what does a flat, steep, uphill/downhill, curve on a v-t graph mean?
flat - represents steady speed
steep - greater acceleration
uphill - acceleration
downhill - deceleration
a curve means changing acceleration e.g., curve getting steeper means greater acceleration and vice versa
what does the area under a v-t graph show?
any part under the graph is equal to the distance travelled in the time interval
what does gravity attract?
all masses
what are the 3 important effects of gravity?
- on the surface of a planet, it makes all things accelerate towards the ground (all with the same acceleration, g, which is about 10m/s^2 on earth)
- it gives everything a weight
- it keeps planets, moons, and satellites, in their orbits. the orbit is a balance between the forward motion of the object and the force of gravity pulling it inwards
what is weight caused by?
the pull of gravity. the wight of an object is often just the force of gravity pulling it towards the centre of the earth
what is special about mass
it is the same anywhere in the universe for any given object
why will an object weigh less on the moon than it does on earth?
because the force of gravity is pulling on it less
what is the unit for weight?
N newtons
how is weight measured?
using a spring balance or newton meter
is mass a force? is weight a force?
mass - no
weight - yes
what is mass measured in? how?
kg kilograms, with a mass balance (old-fashioned balancing scales)
what is the formula linking, weight, mass, gravity?
w = mg
weight = mass x gravitational field strength
what is gravitational force?
Gravitational force is an attractive force. Gravitational force can pull planets together in the Solar System, and can also make objects stay on the surface of the Earth. Gravitational force helps us to define the weight of an object, rather than just the mass.
what are electrostatic forces?
Electrostatic forces occur between charges. Similar to magnetic forces, electrostatic forces between opposite charges will lead to attraction, whilst forces between similar charges will lead to repulsion.
what are magnetic forces?
Magnetic forces occur due to electromagnetic forces. When a charged particle moves in a magnetic field, it will experience a force called a magnetic force. This is related to the motor effect.
name 3 examples of non contact forces.
magnetic, electromagnetic, graviational
what is friction?
Friction occurs when two surfaces move across each other. A frictional force will oppose motion, preventing two surfaces from sliding over each other.
what is air resistance?
Air resistance is a type of frictional force. Air opposes the motion of an object, leading to air resistance. Some call it the ‘friction of air’. The faster the object is moving, the more air resistance it will experience.
what is tension?
Tension occurs when objects are stretched. Tension usually happens in ropes, cables, strings and chains. When an object is suspended from a string, there are two forces involved. One is a force acting downwards due to the object. The other is a tension force acting upwards in the string to keep everything in place, as shown in Fig 2.
name some contact forces.
tension, friction, air resistance
what is a reaction force?
the force that a surface exerts on an object resting on it. The reaction force acts at right angles to the surface. For example, when a football is resting on the grass, the grass pushes up on the football.
what is thrust?
a thrust force is the force that causes an object to move, such as the force from a rocket engine.
what is drag?
a frictional force that occurs when an object moves through a fluid, such as water or air:
Drag force is a resistance force that acts in the opposite direction to the object’s motion through the fluid. The particles in the fluid collide with the object, slowing it down. For example, when you throw a pebble into water, the water molecules flow against the pebble, causing it to slow down.
what is lift?
lift is the force that acts perpendicular to the direction of a fluid’s flow around an object. It’s usually directed upwards to counteract gravity, but it can act in any direction.
what happens if there is no force propelling an object?
it will come to a stop due to friction which opposes movement
what do you need to travel at a steady speed?
you need a driving force to counteract the friction
what are the three main ways in which friction works?
- friction between solid surfaces which are gripping
- friction between solid surfaces which are sliding past each other
1 & 2 can be reduced by using a lubricant like oil or grease between the surfaces. Friction between solids can often cause wear of the two surfaces in contact - resistance or drag from fluid (liquids or gases like air)
the most important factor to reduce drag in fluids is by keeping the shape of the object streamlined, like sports cars. In a fluid, friction increases as speed increases
lorries and vans have deflectors on them to make them more streamlined whereas cars with roof boxes are slowed down
what is the unit for distance?
m metre
what is the unit for time?
s seconds
how do vector quantities differ from scalar quantities?
scalar quantities only have magnitude, while vector quantities have both magnitude and direction
what type of quantity is force?
vector
name some examples of scalar quantities
Temperature: For example, 10 degrees Celsius (°C)
Mass: For example, 5 kilograms (kg)
Energy: For example, 2,000 joules (J)
Distance: For example, 19 meters (m)
Speed: For example, 8 meters per second (m/s)
Density: For example, 1,500 kilograms per meter cubed (kg/m3)
name some examples of vector quantities
Displacement: The length and direction of a straight line from a starting point to an ending point
Force: For example, 20 newtons to the left
Weight: For example, 600 newtons downwards
Velocity: For example, 11 meters per second upwards
Acceleration: For example, 9.8 meters per second squared downwards
Momentum: For example, 250 kilogram meters per second south west
what is the equation for force (mass and acceleration)?
force = mass x acceleration (f=am)
what are the three laws of motion?
1 balanced forces mean no change in velocity (does not mean there needs to be a constant overall force. to keep going at a steady speed there must be a resultant force of 0)
2. a resultant force means acceleration (if there is an unbalanced force, then the object will accelerate in that direction - this doesn’t mean that is something is moving there is an overall resultant force because you get steady speed form balanced forces)
3. reaction forces - if object A exerts a force on object B then object B exerts an equal and opposite force on object A (e.g., pushing a shopping strolley)
what are the 5 different forms of acceleration?
starting, stopping, speeding up, slowing down, changing direction
if two skaters, of two diff weight push against each other what happens?
both skaters feel the same sized force in opposite directions, and so can accelerate away from each other. the lighter skater will be accelerated more though because she has a smaller mass and F=ma
what does Hooke’s law say?
extension is proportional to force.
what is the equation for stopping distance?
stopping distance = thinking distance + braking distance
what 4 main factor affect braking distance?
how fast you are going - the faster you go, the further it takes to stop
the mass of the vehicle - with the same breaks, the larger the mass of a vehicle, the longer is takes to stop. a car wont stop as quickly if it is full of people and luggage
how good your brakes are - they must be regularly maintained and checked
how good the grip it - this depends on 3 things: road surface, weather conditions, tyres
what 2 main factors affect thinking distance?
- how fast you’re going - the faster you go, the further you go
- reaction time - affected by tiredness, frugs, alcohol, old age, sometimes inexperience, among other things
what is thinking distance?
the distance the car travels in the time between the driver noticing the hazard, and applying the brakes
what is braking distance?
the distance the car travels during its deceleration whilst the brakes are being applied
how can you set up a practical to investigate Hooke’s law?
- Set up the apparatus (weighted stand, clamp, spring, hanging mass, extra masses, ruler)
- Measure the length of the spring (e.g. with an accurate mm ruler) when no load is applied. Ensure the ruler is vertical (e.g. with a set square) and measure the spring at eye level. (This is the spring’s natural length.)
3.Add one mass at a time and allow the spring to come to rest, then measure the new length of the spring. The extension is the change in length from the original length. Adding a marker to the top and bottom of the spring might make measuring lengths easier. Repeat this process until you have enough measurements (no fewer than 6). - Once you’re done, repeat the experiment and calculate an average value for the length of the spring for each applied weight.
- Plot your results on a graph - show force (i.e. the total weight) on the vertical axis and the total extension on the horizontal axis. You should find that the same increase in the weight on the end of the spring always leads to the same increase in extension — this is Hooke’s law in action.
Repeat the experiment using a metal wire or a rubber band instead of the spring.
- Plot your results on a graph - show force (i.e. the total weight) on the vertical axis and the total extension on the horizontal axis. You should find that the same increase in the weight on the end of the spring always leads to the same increase in extension — this is Hooke’s law in action.
what does Hooke’s law say?
extension is proportional to force
when does Hooke’s law stop working?
when the force is great enough
- this can be seen on a force-extension graph
- when the force is great enough it begins to curve
- when it starts to level (this is called the elastic limit), the material will be permanently stretched so when all the forces are removed, the material will be longer than at the start
what is elastic behavior?
when a material returns to its original shape once the forces are removed (which have caused deformation). Metals display elastic behavior as long as hooke’s law is obeyed
what graph shows Hooke’s law?
the initial linear region of a force-extension graph is associated with Hooke’s law
what is the equation for a moment?
moment = force x perpendicular distance from the pivot
name a practical where you can investigate the motion of everyday objects like toy cars.
- Set up your apparatus (toy car, light gate, start line, ramp, another two light gates (so one is at the beginning, middle, and end), and a runway), holding the car still just before the first light gate.
- Mark a line on the ramp — this is to make sure the car starts from the same point each time.
- Measure the distance between each light gate — you’ll need this to find the car’s average speed.
- Let go of the car just before the light gate so that it starts to roll down the slope.
- The light gates should be connected to a computer. When the car passes through each light gate, a beam of light is broken and a time is recorded by data-logging software. Repeat this experiment several times and get an
average time it takes for the car to reach each light gate. - Using these times and the distances between light gates you can find the average speed of the car on the ramp and the average speed of the car on the runway - just divide the distance between the light gates by the average time taken for the car to travel between gates
what factors could you change when investigating the motion of a toy car on a ramp?
- You could try seeing if the mass of the car affects its average speed — just load weights onto it (but make sure you don’t overload it so that the wheel axles grind).
- To see how friction affects the motion of the car you could try placing different materials on the ramp. If you do this, make sure they’re laid flat and they don’t change the angle of the ramp in any way.
- You could investigate the acceleration of the car due to gravity by starting it off higher up the ramp and seeing how this affects its average speed between the gates.
- You could change the angle of the ramp to see how that affects the car’s speed down the slope.
- You could even try it with different cars - see how the
You’d expect the more streamlined
size, shape and weight of the car affects how fast it
ones to go quicker
How do you plot a velocity-time graph? What is it? How do you find the acceleration or distance travelled using it?
Velocity (m/s)- y axis
time (s) – x axis
to find acceleration do a= change in v/t
the area under a V-T graph is distance travelled
The gradient of a V-T graph is the acceleration
What is the difference between contact and non contact forces? Which forces are non contact?
Contact – your hand touches something to exert a force (speed?)
non-contact – gravitational, magnetic, electric forces
Name as many types of forces as you can
Gravitational (or weight) - the force between any two objects with mass (like the Earth and the Moon)
Electrostatic - the force between any two objects with charge (like a proton and an electron)
Thrust - the force pushing a vehicle (like the push from rocket engines on the shuttle)
Upthrust - the upward force on any object in a fluid (like a boat on the surface of a river)
Air resistance (or drag) - the force of friction between objects falling through the air (like a skydiver in freefall)
Compression - forces that squeeze an object (like squeezing a spring)
Tension - forces that stretch an object (like two teams in a tug-of-war)
Reaction force - the force between any two objects in contact (like the upwards force from a table on a book)
What are the effects of forces between bodies (e.g., changes in speed, shape, direction)?
Forces can affect bodies in a variety of ways:
Changes in speed: forces can cause bodies to speed up or slow down
Changes in direction: forces can cause bodies to change their direction of travel
Changes in shape: forces can cause bodies to stretch, compress, or deform
what are the forces acting upon a falling object (someone jumping out of an aeroplane)
- Accelerates as she leaves the aeroplane
- Acceleration decreases due to air resistance (force acting against her weight)
- the air resistance becomes the same as her wight and she is moving at a constant speed because the resultant force acting on her is 0 (terminal velocity- constant velocity)
- Opens her parachute and she decelerates rapidly because of the large air resistance
- She falls at a constant speed until she hits the ground because the air resistance it now the same size as her weight
the large the area, the grater the air resistance and vise versa
What is elastic deformation? What is elastic behavior?
Deformation - When large forces are applied to a spring and it doesn’t return to its initial shape.
Behavior – the ability of a material to recover to its original shape after the forces causing deformation have been removed
what is the equation for clockwise and anticlockwise forces
ACWM = CWM
(force x distance = forcx distance)
where does the weight of a body act?
through its centre of gravity
what is the centre of gravity?
where the object can be considered as acting from
what is a moment?
the turning effect of a force
what is the unit for a moment?
Newton metres - Nm
what is rotational equilibrium?
the clockwise and anti clockwise moments are balanced. the resultant moment on the object is 0. ACWM = CWM
what is translocational equilibrium?
the forces are balanced and the resultant force on an object is 0
when a train slows down, what happens to the kinetic energy of the train?
it is transferred to the surroundings
what happens to the poeple in a lift when they go upwards?
they gain gravitational potential energy
because the passengers have moved to a higher point/upwards; work is done to move the passengers; passengers are further from the centre of the earth;
GPE = MGH
The tunnel is designed so that the trains go up a slope as they enter the station
and go down a slope as they leave.
The driver uses brakes to stop the train in the station and a motor to make the
train move away.
Explain how the sloping parts of the tunnel affect the amount of work that needs
to be done on the train by the brakes and by the motor
When entering station-
MP1. KE - GPE;
MP2. Less work done by the brakes (to stop the train);
MP3. Less (braking) force needed (to
stop) ;
MP4. train stops more quickly OR
brakes are needed for less time
(to stop);
When leaving station-
MP5. GPE KE;
MP6. Less work done by the motor (to
accelerate);
MP7. Less force needed (to accelerate
the train);
train accelerates more quickly OR
force needed for a shorter time
(to reach a given speed);
Explain how the upward acceleration of the balloon changes during the first few
seconds of its flight.
MP1. speed increases;
MP2. so drag {starts to act /
increases};
MP3. downward forces increase;
MP4. (hence) acceleration decreases
While the balloon is still accelerating, the pilot controls the balloon by pouring
some sand from the bags.
Explain how this affects the upward acceleration of the balloon.
acceleration increases;
with any one from:
weight decreases / downward
force reduces;
unbalanced force increases;
mass decreases;
Use ideas about forces to explain how a falling object can reach a terminal velocity
MP1. Object has weight or there is a downward
force (due to gravity on the object);
MP2. So it accelerates (downwards);
MP3. there is (a force of) drag (upwards or to
oppose movement);
. drag increases as speed increases;
eventually drag = weight ;
(hence) resultant force is zero;
(hence) object travels at constant speed;
