Topic 5 - Forces Flashcards
What quantity is a force?
Vector.
What are vectors?
Quantities with magnitude and direction.
What are examples of vectors?
Force, velocity, displacement, acceleration, momentum…
What are scalars?
Quantities with magnitude but no direction.
What are examples of scalar quantities?
Speed, distance, mass, temperature, time…
How are vectors usually represented?
By an arrow - the length shows the magnitude and the direction shows the direction of the quantity.
What is a force?
A push or pull on an object that is caused by it interacting with something. They are either contact or non-contact.
What is a contact force?
When two objects have to be touching for a force to act.
What are examples of contact forces?
Friction, air resistance, tension, normal contact force..
What is a non-contact force?
The objects do not need to be touching for the force to act.
What are examples of non-contact forces?
Magnetic force, gravitational force, electrostatic force…
What is an interaction pair?
A pair of forces that are equal and opposite and act on two interacting objects.
What are the two important effects of gravity?
On the surface of a planet, it makes all things fall towards the ground. It gives everything weight.
What is mass?
The amount of ‘stuff’ in an object. Will have the same value anywhere.
What is weight?
The force acting on an object due to gravity (the pull of the gravitational force on the object). Measured in newtons.
Describe gravitational field strength.
It varies with location. It is stronger the closer you are to the mass causing the field and stronger for larger masses.
Where is the centre of mass for a uniform object?
At the centre of the object.
How is weight measured?
Using a calibrated spring balance or newton meter.
How is mass measured?
In kilograms with a mass balance.
How can you calculate the weight of an object?
Weight (N) = Mass (kg) x gravitational field strength (N/kg).
What is the gravitational field strength on earth and moon?
Earth = 9.8 N/kg, Moon = 1.6 N/kg.
What happens when you increase the mass of an object?
Increases the weight. Weight and mass are directly proportional.
What is a free body diagram?
A diagram which shows all the forces acting on an object.
What is a resultant force?
When two or more forces act on an object, the resultant force can be found by adding up the individual forces.
What happens when a force moves an object through a distance?
Energy is transferred an work is done on the object.
What happens when a force does work to move an object?
Energy is transferred from one sotre to another.
How can you calculate work done?
W=Fs, Work done (J) = Force (N) x Distance (m).
How do you convert joules into newton metres?
1 J = 1 Nm.
How do you draw the resultant force?
The line connecting the first force to the end of the last force.
When is an object in equilibrium?
If all the forces acting on an object combine to give a resultant force of zero.
What can happen when you apply a force to an object?
It streches, compresses or bends.
What are objects that have been elastically deformed called?
Elastic objects. All its energy has been transferred to their elastic potential store.
What happens if an object has been inelastically deformed?
It doesn’t return to its original shape and length after the force has been removed.
What happens when a spring is supported at the top and then a weight is attached to the bottom?
It stretches.
What is the extension of a spring directly proportional to?
The load or force applied.
What is an equation that links extension force and spring constant?
F=ke, Force (N) = Extension (m) x Spring constant (N/m).
Describe spring constant.
Depends on the material that you are stretching. A stiffer spring has a greater spring constant.
How do you calculate compression?
Use F=ke, where e is the difference between the natural and compressed lengths.
What is the limit of proportionality?
When the extension is no longer proportional to the force.
How can you investigate the link between force and extension?
- Measure the natural length of the spring when no load is applied with a millimeter ruler clamped to the stand. Take the reading at eye level.
- Add a mass to the spring and allow it to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length.
- Repeat this until you have enough measurements.
- Plot a force extension graph. It will only start to curve if it exceeds the limit of proportionality.
How can you find the work done in stretching or compressing a spring?
Ee=1/2ke2.
How do you find the energy in the elastic potential energy store of a stretched spring?
The area under the force extension graph.
What is a moment?
The turning effect of a force.
How do you calculate the size of the moment of the force?
M=Fd, Moment of force (Nm) = Force (N) x Distance (m).
Why do you need to push at right angles to get the maximum moment (spanner)?
Larger distance so larger moment.
How do you have a larger moment?
A larger force or a longer distance.
What increases the distance from the pivot at which the force is applied?
Lever.`
What are gears and how do they work?
Circular discs with ‘teeth’ around the edges. Their teeth interlock so that turning one causes another to turn, in the opposite direction. They are used to transmit the rotational effect of a force from one place to another.
What are fluids?
Substances that can ‘flow’ becuase their particles are able to move around. As these particles move around, they collide with surfaces and other particles.
What is pressure?
Force per unit area.
What is the pressure of a fluid?
A force is exerted normal (at right angles) to any surface in contact with the fluid.
How do you calculate pressure at the surface of a fluid?
p=F/A, Pressure (Pa) = Force normal to a surface (N) x Area of that surface (m2).
What is density?
A measure of ‘compactness’ of a substance, ie. how close the particles are to each other.
What does it mean if something is more dense?
It has more particles in a certain space. This means that more particles are able to collide so the pressure is higher.
What happens as the depth of a liquid increases?
The number of particles above that point increases. The weight of these particles adds to the pressure felt at that point, so liquid pressure increases with depth.
How can you calculate pressure at a certain depth due to the column of liquid above?
p=hρg, Pressure (Pa) = Height of the column of liquid (the depth) (m) x density of the liquid (kg/m3) x gravitational field strength (N/kg).
What happens when an object is submerged in a fluid?
The pressure of the fluid exerts a force on it from every direction.
What happens to pressure as the depth increases?
Pressure increases. The force exerted on the bottom of the object is larger than the force acting on the top of the object.
How is upthrust caused?
As pressure increases with depth, the force exerted on othe bottom of an object is larger than the top causing a resultant force (upthrust).
What is upthrust equal to?
The weight of fluid thart has been displaced by the object.
Why do objects float?
The upthrust is equal to the objects weight. Depends on their density.
When do objects sink?
When the objects weight is more than the upthrust.
Why do less dense objects float?
Less dense than the fluid it is place in weights less than the equivilent volume of fluid. So it displaces a volume of fluid that is equal to its weight before it is completely submerged. At this point, the weight is equal to the upthrust, so the object floats.
Why do denser objects sink?
It is unable to displace enough fluid to equal its weight. Weight is always larger than upthrust.
What is the atmosphere?
A layer of air that surrounds Earth. It is thin compared to the size of earth.
How is atmospheric pressure created?
On a surface by air molecules colliding with the surface.
What happens to atmospheric pressure as altitude increases? why?
Atmospheric pressure decreases. As the altitude increases, the atmosphere gets less dense, so there are fewer air molecules colliding with the surface.
What is the difference between speed and velocity?
Speed is a scalar, how fast you are going with no regards to direction. Velocity is a vector, speed in a given direction.
What is an example of an object with a constant speed but contantly changing velocity.
Car going round a roundabout.
How can you measure the speed of an object that’s moving with a constant speed?
s=vt, distance travelled (m) = speed (m/s) x time (s).
s=vt: why is this formula only an average (mean) speed?
Objects rarely travel at a constant speed, speed is constantly changing.
What is the typical speed for a person walking?
1.5 m/s.
What is the typical speed for a person running?
3 m/s.
What is the typical speed for a person cycling?
6 m/s.
What is the typical speed for a car?
25 m/s.
What is the typical speed for a train?
55 m/s.
What is the typical speed for a plane?
250 m/s.
What things can effect speed?
Fitness, age, distance travelled, terrain.
What can wind speed be affected by?
Temperature, atmospheric pressure, large buildings/ structures.
What is acceleration?
The change in velocity in a certain amount of time.
How can you calculate average velocity?
a= Δv/t, acceleration (m/s2) = change in velocity (m/s)/ time (s).
What is deceleration?
Negative acceleration (when something slows down).
What is another name for constant acceleration?
Uniform acceleration.
What is the equation for uniform accelerations?
v²-u²=2as, Final velocity - initial velocity = 2 x acelleration x distance.
What does the gradient show on a distance time graph?
Speed, steeper = faster, speed = distance/ time.
What do flat sections show on a distance time graph?
Stationary object, stopped.
What does a straight uphill section show on a distance time graph?
Travelling at a steady speed.
What do curves represent on a distance time graph?
Acceleration or deceleration.
What does a steepening curve show on a distance time graph?
Its speeding up.
What does a levelling off curve show on a distance time graph?
Its slowing down.
How can you find the speed when an object is changing speed?
Find its speed at a point by finding the gradient of the tangent to the curve at the point.
What does the gradient represent on a velocity time graph?
Acceleration (velocity/time).
What do the flat sections represent on a velocity time graph?
Steady speed.
What does it mean if the velocity time graph is steeper?
Greater acceleration or deceleration.
What do uphill sections on a velocity time graph show?
Acceleration.
What do downhill sections on a velocity time graph show?
Deceleration.
What do a curve show on a velocity time graph?
Changing acceleration.
What do the area under a velocity time graph show?
The distance travelled in that time interval.
Why will an object always slow down and stop if it has no force propelling it along?
Friction.
What direction does friction act in?
Opposite to direction of movement.
What does the object need to travel at a steady speed?
Driving force needs to balance the frictional forces.
When do you get friction?
Between two surfaces in contact, or when an object passes through a fluid (drag).
What is drag?
The resistance you get in a fluid.
Give an example of a type of drag?
Air resistance.
What is the most important factor in reducing drag?
Keeping the shape of the object streamlined - object is designed to allow fluid to flow easily across it, reducing drag.
What works in the opposite way of streamline objects?
Parachutes.
Frictional forces from fluids ________ with speed?
Increases.
Describe how objects falling through fluids reach a terminal velocity?
When a falling object first sets off the force of gravity is more than the frictional force slowing it down, so it accelerates. As the speed increases, the friction builds up, gradually reducing the acceleration until eventually the frictional forces is equal to the accelerating force (resultant force is zero). It will have reached its maximum speed/ terminal velocity and will fall at a steady speed.
What accelerating force acts on all falling objects?
Gravity.
What is teminal velocity determined by?
Drag in comparism to weight.
What is Newton’s First Law?
A resultant force is needed to make something start moving, speed up or slow down: If the resultant force on a stationary object is zero, the object will remain stationary. It the resultant force on a moving object is zero, it will carry on moving at the same velocity.
What will happen to an object if the resultant force isn’t zero? What froms can this acceleration take?
It will produce acceleration or deceration in the direction of the force.
- Starting
- Stopping
- Speeding up
- Slowing down
- Changing direction
What will a larger resultant force acting on an object do?
The larger the resultant force, the more the object accelerates - directly proportional.
Acceleration is _________ to the mass of the object?
Inversely proportional. An object with a larger mass will accelerate less than one with a smaller mass (for a fixed resultant force).
What formula describes Newton’s Second Law?
F=ma, Resultant force (N) = Mass (kg) x Acceleration (m/s²).
What is Newton’s second law?
A body accelerates when acted upon by a net external force. The acceleration is proportional to the net force and is in the direction that the net force acts.
What is inertia?
The tendency to continue in the same state of motion.
What is an object’s inertial mass?
Measures how difficult it is to change the velocity of an object.
How can inertial mass be found?
Rearranging newtons second law - m=f/a.
What is Newton’s Third Law?
When two objects interact, the forces they exert on each other are equal and opposite.
How can you investigate how mass and force affect acceleration?
- Set up the trolley so it holds a piece of card with a gap in the middle that will interrupt the signal on the light gate twice. Input the length of each bit of card into the software so it can measure the velocity. Use this to work out the acceleration of the trolly.
- Connect the trolly to a piece of string that goes over a pully and is connected on the other side to a hook.
- Attach masses to the hook to provide the accelerating force, equal to the mass of the hook x acceleration due to gravity.
- Mark a starting line on the table the trolley is on so it travels at the same distance to the light gate. Place the trolley on the start line and release the hook.
- Record the acceleration measured by the light gate. Repeat this to get an average acceleration.
- To investigate the effect of mass, add masses and record the average acceleration for each mass.
- To investigate the effect of force you need to keep the total mass the same but change the mass of the hook. Record the average acceleration for each force.
Investigating how mass and force affect acceleration: How can you use f=ma to describe the results?
F= weight of hanging massses, m= mass of the whole system, a= acceleration of system.
What is an emergency stop?
Where maximum force is applied by the brakes in order to stop the car in the shortest possible distance.
How can you calculate stopping distance?
Stopping distance= Thinking distance + Braking distance.
What is thinking distance?
How far the car travels during the driver’s reaction time.
What is braking distance?
The distance taken to stop under the braking force.
What are typical car braking distance?
14m at 30mph, 55m at 60mph and 75m at 70mph.
What can thinking distance be effected by?
Speed - faster your going the further you’ll travel during the time you take to react.
Reaction time - longer your reaction time, longer your thinking distance.
What can braking distance be effected by?
Speed - for a given braking force, the faster the vehicle travels, the longer it takes to stop.
Weather/ road surface - if it’s wet or icy, or leaves or oil on the road, there is less grip (so less friction) between the tires and the roaf, which causes tyres to skid.
Tyre condition - if they are bald (no tread) they can’t get rid of water in wet conditions, leading to them skidding.
Good brakes - If they are worn or faulty they won’t be able to apply as much force as well maintained brakes, which could be dangerous when you need to break hard.
What else affects stopping distance?
Speed limits.
Why is a greater braking force needed to stop a car within a certain distance?
When the brake pedal is pushed, this causes brake pads to be pressed onto the wheels. This contact causes friction, which causes work to be done. The work done between the pedal and wheels transfers energy from the kinetic energy stores of the wheels to the thermal energy stores of the brakes. The brakes increase in temperature. The faster a vehicle is going, the more energy it has in its kinetic store, so the more work need to be done to stop it.
Why can very large decelerations be dangerous?
May cause brakes to overheat and could cause vehicles to skid.
What can reaction time be effected by?
Tiredness, drugs, alcohol, distractions.
Describe how to measure reaction time with the ruler drop test.
- Sit with your arm resting on the edge of a table. Get someone to hold a ruler so it hangs between your thumb and forefinger, lined with zero.
- The person holding the ruler should drop it without warning and the other person should catch the ruler as quickly as possible.
- The point on the ruler that is caught is the time it took to react. A longer distance means a longer reaction time.
Measuring reaction time with the ruler drop test: How can you increase accuracy?
Add a blob of modelling clay to stop the ruler from waving about.
Measuring reaction time with the ruler drop test: How can you make sure it is a fair test?
Use the same ruler, have the same person dropping it.
Measuring reaction time with the ruler drop test: What other factors effecting reaction time could you investigate?
Distractions, eg. music or someone talking to you. Make sure to calculate a mean and compare to without distractions.
Describe how thinking distance increases with speed.
As the car speeds up, the thinking distance increases at the same rate as speed. The graph is linear. Thinking time stays constant, but a higher speed means more distance is covered.
Describe how braking distance increases with speed.
Increase faster the more you speed up. The work done to stop the car is equal to the energy in the cars kinetic store (1/2mv2). So as the speed doubles, the kinetic energy increases four-fold, and so the work done to stop the car increases four-fold. Since W=Fs, and the braking force is constant, the braking distance increases four-fold.
What is momentum?
A property of all moving objects. The greater the mass or velocity, the more momentum it has. It is a vector quantity.
How can you calculate momentum?
p=mv, momentum (kg m/s) = mass (kg) x velocity (m/s).
What is consveration of momentum?
In a closed system, the total momentum before and event is the same as after the event.
When is there a change in momentum?
When a non-zero resultant force acts on a moving object causing the velocity to change.
What is an equation which involves change of momentum?
F= (mΔv)/Δt,
Why are cars designed to slow people down over a longer time when they have a crash?
If someones momentum changes quickly, the forces on the body will be very large and more likely to cause injury. The longer it takes for a change in momentum, the smaller the rate of change of momentum, and so the smaller the force.
What safety features do cars have?
- Crumple zones - crumple on impact, increasing time taken for the car to stop.
- Seat belts strech slighty, increasing time taken for the wearer to stop.
- Airbags inflate before you hit the dashboard of the car. The compressing air inside it slows you down more gradually than if you had just hit the hard dashboard.
What safety feature does a bike helmet have?
Contain a crushable layer of foam which helps to lengthen the time taken for your head to stop in a crash, reducing the impact on your brain.
Why do crash mats and cushioned playground flooring reduce momentum?
They increase the time taken for you to stop if you fall on them becuase they are made from soft, compressible materials.
