P5 Forces

what is the difference between vector and scalar quantities?

• scalar - only have magnitude (speed, mass, time, distance etc)

• vector - have both direction and magnitude (velocity, weight, displacement etc)

what are the contact and non-contact forces?

• contact - friction, air resistance, tension, normal contact force etc

• non contact - magnetic, gravitational and electrostatic forces etc

what is a resultant force and how do you find it?

the overall force on a point or object

• if you have a number of forces acting at a single point, you can replace them with a single force (as long as it has the same effect)

• if the forces all act along the same line, the overall force can be found by adding those going in the same direction and subtract any going in the opposite direction

what happens if a resultant force moves an object?

energy is transferred and work is done

1. to make something move, a force must be applied - this requires a source of energy

2. the force does work to move the object

3. whether energy is transferred usefully or is wasted you can still say work is done

4. w = fs → one joule of work is done when a force of one newton causes an object to move one metre (1J = 1Nm)

how can you calculate resultant forces using scale drawings?

1. make a scale drawing with a sensible scale (e.g. 1cm = 1N)

2. draw the resultant from the tail of the first arrow to the top of the last arrow

3. measure the length and use the scale to find the force in N

4. use a protractor to measure the direction as a bearing (37 = 037)

what happens when you stretch/compress/bend an object?

• more than one force is needed to compress, bend or stretch an object

• an object has been elastically deformed if it returns to its original shape one then force has been removed (inelastic vice versa)

• work has been done and causes energy to be transferred to the elastic potential store of the object (if elastically deformed, ALL energy)

how is extension proportional to force?

extension is directly proportional to force:

f = ke

however this stops working when the force is great enough - limit of proportionality

REQUIRED PRACTICAL - spring extension

1. measure the natural length of the spring with a ruler clamped to the stand

2. add a mass to the spring and allow it to come to rest and measure the new length - extension is the change in length

3. repeat the process until you have 6+ measurements

4. plot a force extension graph of your results

how do you work out energy stored for linear relationships?

a long as it is not past it’s limit of proportionality, work done can be found using:

elastic potential energy = ½ke²

for elastic deformation, this formula can be used to calculate the energy stored, or energy transferred to the spring as it is deformed

what is a moment?

the turning effect of a force

M = Fd

a larger force or a longer distance would mean a larger moment - to get the maximum moment, you need to push perpendicular to the spanner

what happens when an object is balanced?

the total anti-clockwise moment = total clockwise moment

what do levers do?

• levers increase the distance from the pivot at which the force is applied

• since M = Fd this means less force is needed to get the same moment

• levers make it easier to do work

what do gears do?

• teeth interlock, turning one causes the other to turn in the other direction

• they are used to transmit rotational effects

• different sized gears can be used to change the moment of a force - a force transmitted to a larger gear will cause a bigger moment, as the distance to the pivot is greater

• larger gear → slower, larger moment

• smaller gear → faster, smaller moment

what causes pressure in fluids?

• fluids are substances that can flow as their particles can move around

• as these particles move around, they collide with surfaces and other particles

• they expert a force on the object they collide with - pressure is force per unit area, so they exert a pressure

• p = F/A

what does pressure in a liquid depend on?

density:

• the more dense a given liquid is, the more particles it has in a certain space - more particles that are able to collide so pressure is higher

depth:

• as depth increases, weight of number of particles above increases - adds to pressure

what is upthrust?

• when an object is submerged in fluid, the pressure of the fluid exerts a force in every direction

• pressure increases with depth so force on the bottom is greater than force on the top

• this causes a resultant force upwards, called upthrust

• upthrust = weight of fluid that has been displaced

how does an object’s weight relate to upthrust?

• if upthrust = weight, forces balance and object floats

• if upthrust < weight, object sinks

• am object that is less dense than the fluid is placed weighs less than the equivalent volume of fluid - displaced volume of fluid equal to its weight - object floats

how does atmospheric pressure change with height?

1. atmospheric pressure is created on a surface by air molecules colliding with the surface

2. as the altitude increases, atmospheric pressure decreases

3. this is because as the altitude increases, the atmosphere gets less dense, so there are fewer molecules that are able to collide with the surface

4. there are also fewer air molecules above a surface as the height increases - this means that the weight of the air above it, which contributed to pressure, decreases with altitude

what are some typical everyday speeds?

• a person walking - 1.5 m/s

• a person running - 3 m/s

• a person cycling - 6 m/s

• a car - 25 m/s

• a train - 30 m/s

• a plane - 250 m/s

what is uniform acceleration?

constant acceleration

• acceleration due to gravity is uniform for objects in free fall - its roughly equal to 9.8 m/s²

distance-time graphs

• gradient = speed (the steeper the gradient, the faster it’s going)

• flat sections are when the object is stationary

• straight uphill sections means it’s travelling at a steady speed, but curves represent acceleration or deceleration

• steepening curve = acceleration, levelling off curve = deceleration

• if an object is changing speed, you can find its speed at a point by finding the gradient of the tangent at that point

velocity-time graphs

• gradient = acceleration, since acceleration is change in velocity / time

• flat sections represent travelling at a steady speed

• the steeper the graph, the greater the acceleration or deceleration (uphill = a, downhill = d)

• a curve means changing acceleration

• the area under the graph is equal to the distance travelled (or by counting squares and multiplying by value of the squares)

what is friction/drag?

• friction - acts in opposite direction to movement, to travel at a steady speed, the driving force needs to balance the frictional force

• drag - the resistance you get in a fluid - to reduce drag the object should be streamlined, so the fluid can flow over it easier - frictional forces from fluids always increases with speed (e.g. a car has more friction to work against at 70mph than 30)

what is 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, which gradually reduces the acceleration until eventually the frictional and accelerating forces are equal (the resultant force is zero) - it will have reached its terminal velocity and will fall at a steady speed

• terminal velocity depends on shape and area as that will change the drag/air resistance (the terminal velocity is determined by its drag in comparison to its weight)

what is Newton’s First Law?

if the resultant force on a stationary object is zero, the object will remain stationary. if the resultant force on a moving object is zero, it will carry on moving at the same velocity

• the driving forces and resistive forces must be balanced, a non zero resultant force will produce acceleration in some way (slowing, speeding up, changing direction etc) - on a free body diagram the arrows will be unequal

what is Newton’s Second Law?

acceleration is proportional to the resultant force

• the larger the resultant force acting on an object, the more the object accelerates

• acceleration is also proportional to the mass - an object with a larger mass will accelerate less than one with a smaller mass (with same r force)

• F = ma

what is inertia?

the tendency for an object to remain unchanged

• until acted on by a resultant force, objects at rest or moving at a constant speed will stay moving at that speed - this tendency to continue in the same state of motion is inertia

• an objects inertial mass measures how difficult it is to change the velocity of an object

• inertial mass can be found by rearranging F = ma (so inertial mass is just the ratio of force over acceleration)

what is Newton’s Third Law?

when two objects interact, the forces they exert on each other are equal and opposite

• an example of this in equilibrium is someone pushing against a wall - as they push the wall, there is a normal contact force acting back on them, these two forces are the same size

• another example is an ice skater pushing against another - when A pushes on B, they feel an opposite and equal force from B’s hand - both skaters feel the same force, so will accelerate away from each other (though their acceleration will differ by mass)

how do you investigate how changing force affects acceleration?

1. position an air track on a bench with a bench pulley on one end and two light gates above the track. cut an interrupt card to a known length and attach it to an air track glider

2. connect the glider to a hanging mass by a string the length of the air track passing over the pulley - make sure the air track is level and that the card will pass through the gates before the mass hits the floor

3. set the data logging software to calculate acceleration

4. add 5 × 20g slotted masses (0.98N of force) to the end of the string

5. release the glider then record the weight and acceleration

6. repeat steps 4-5 two more times to get a mean for acceleration

7. repeat steps 4-6, removing one of the slotted masses each time (0.78, 0.59, 0.39, 0.20 N)

how do we investigate how changing mass effects acceleration? (produced by a constant force)

the same apparatus can be used as the force investigation, however use 100g (0.98N) for each run, but add increasing masses to the glider

what is momentum?

mass x velocity - a property that all moving objects have

• the greater an object’s mass or velocity, the greater its momentum is

• momentum is a vector quantity - has both size and direction

• measured in kg m/s

what is conservation of momentum?

in a closed system, the total momentum before an event (e.g. a collision) is the same as after the event. ex:

• a moving car hits a parked car, the cars lock together and continue moving in the direction the original car was, but at a lower velocity

• before - momentum was equal to mass of moving car x its velocity

• after - the mass has increased, but the momentum stays the same, so an increase in mass causes a decrease in velocity

• conservation of momentum can be used to calculate unknown velocities or masses

what causes a change in momentum?

forces:

• the force causing the change is equal to the rate of change of momentum - a larger force means a faster change of momentum

• likewise, if someone’s momentum changes very quickly (like in a car crash) the forces on the body will be very large, likely causing more injury

• cars are designed to slow people down over a longer time when they have a crash - the longer it takes for a change in momentum, the lower the force, so less injurywhat (e.g. seat belts or airbags)

what affects stopping distance?

stopping distance = thinking distance + braking distance

thinking distance is affected by:

• speed - the faster you’re going, more distance will be travelled in the time it takes you to react

• reaction time - longer time, longer thinking distance

braking distance is affected by:

• speed

• weather or road surface

• condition of tyres or breaks