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Topic 5 - forces
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- 1What are vector quantities?Vector quantities have magnitude and direction e.g. force, velocity, displacement, acceleration, momentum. Usually represented as an arrow, length represents magnitude, direction of the arrow shows the direction.
- 2What are scalar quantities?Scalar quantities are quantities that only have magnitude and no direction e.g. speed, distance, mass, temperature, time.
- 3What is a non/contact force?Contact force - two objects have to be touching for a force to act e.g. friction, air resistance, tension in ropes.
Non-contact force - the objects do not need to be touching for the force to act e.g. magnetic force, gravitational force, electrostatic force. - 4What are the two important effects of gravity?Gravity is the force of attraction between masses.
1. on the surface of the planet it makes everything fall to the ground.
2. it gives everything weight. - 5What is the difference between weight and mass?Mass (kg) is just the 'amount of stuff' in an object, this stays the same anywhere in the universe.
Weight (N) is the force acting on an object due to gravity, depends on the gravitational field strength of the object's location. - 6What is the equation linking mass, weight and gravitational field strength?Weight (N) = Mass (kg) * Gravitational field stregnth (N/kg)
Mass is directly proportional to weight. - 7What is the resultant force on a point or object?A resultant force is the overall force on a point or object. When calculating you work out the verticle and horizontal forces then state the size and direction of the resultant force.
- 8What happens when a resultant force moves an object?When a force moves an object through a distance, energy is transferred and worj is done.
- 9What is the equation linking work done, force and distance (moved along the line of action of the force)?Work done (j) = force (N) * distance(moved along the line of action of the force). One joule of work is done when 1N cuases an object to move 1 metre. 1J=1Nm.
- 10How would you use scale drawings to find resultant forces?Draw all the forces on an object tip to tail, to scale. Draw a line from the sart of the first force to the end of the last one, this line os the resultant force. Measure the legnth of this line to find the magnitude and the angle to find the direction of the force.
- 11How can you tell if an objects in equilibrium?If all the forces on an object combine to a resultant force of zero then the object is in equilibrium. Tip to tail diagram finishes where it starts.
- 12How can you split a force into components?Draw the force arrow to scale as a bearing and connect the top and bottom to make a right angle triangle. Measure the two sides and they will be the vertical and horizontal components.
- 13What is elastic/in elastic deformation?Elastic deformation - Object goes back to original shape after the force has been removed. Objects that can easily be elastically deformed are called elastic objects e.g. a spring.
Inelastic deformation - Object doesn't go back to its original shape after the force has been removed. - 14What is the relationship between extension force on a spring?The extension of a trenched spring is directly proportional to the force applied. Also works for compression (difference between natural and compressed lengths). Only works below the limit of proportionality. F=k*e
- 15What is the limit of proportionality?The point where force is no longer proportional to extension. The graph curves here.
- 16What is the method for investigating the link between force and extension?1. measure the natural length of the spring (no load) with a mm ruler clamped to a stand. Read at eye level and place a marker.
2. add a mass to the spring and allow spring to rest before recording the new mass and extension (change in length).
3. repeat experiment until you have enough measurements (min 6).
4. plot your results on a force-extension graph to see the relationship. - 17How do you work out the energy stored for linear relationships?For a spring not stretched past its limit of proportionality; E=1/2ke^2.
Energy in the elastic potential energy store of a stretched spring is equal to the area under a force-extension graph up to the limit of proportionality. - 18What is a moment?A moment is a turning effect of a force(s).
M(Nm)=F(N)*d(m)
d is the perpendicular distance from the pivot to the line of action of the force. - 19Describe how a spanner creates a bigger moment?The force on the spanner causes a turning effect (moment) on the nut (pivot). A larger force or a longer distance (spanner) would mean a larger moment. To get the biggest moment you need to push at right angles to the spanner as this creates the greatest distance making the maximum moment. If the total anticlockwise moment=clockwise moment then the object will be balanced and wont turn.
- 20How do levers effect moments?Levers increase the distance from pivot at which the force is applied. M=F*d so this means less force is needed for the same moment so its easier to do work.
- 21What do gears do?Gears transmit rotational effects. Larger gear transmits bigger moment as distance from pivot is greater. The larger gear will turn slwer than the smaller gear.
- 22What is a fluid?A substance that can flow (liquid/gas).
- 23Explain fluid pressure?As particles n a fluid can move around, they collide with each other and surfaces. They exert a force on the objects they collide with pressure is force/area so fluids exert a pressure.
- 24What is the equation that links pressure, force and area?Pressure (Pa)=F(N)/A(m^2)
- 25What does pressure in a liquid depend on?The pressure in a liquid depends on depth and density.
The more dense a liquid is the more particles it has in a given space. So more particles collide and the pressure is higher.
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 this point, so liquid pressure increases with depth. - 26What is the equation linking pressure, height of column of liquid, density of liquid and gravitational field strength?p (Pa) = hpg
- 27Explain upthrust?When an object is submerged in a fluid the pressure of the fluid exerts a force on it from every direction. Pressure increases with depth so the force on the bottom of the object is larger than the force on top of the object. This causes an upwards resultant force known as upthrust. The upthrust is equal to the weight of fluid that is being displaced by the object.
- 28Why will an object float?If upthrust is equal to the object's weight then the forces balance and the object floats. If the weight is greater than the upthrust then the object will sink. An object that is less dense than the fluid it is in weighs less than the equivalent volume of fluid so it displaces a volume of liquid that is equal to its weight before its completely submerged. At this point it floats. If an object is denser than the fluid it is placed in it is unable to displace enough fluid to equal its weight, so its weight is always greater than upthrust so it sinks.
- 29What is the atmosphere?The atmosphere is a thin layer of air that surrounds the earth. Atmospheric pressure is created on a surface by air molecules colliding with the surface.
- 30How is atmospheric pressure effected by altitude?As altitude increases, atmospheric pressure decreases as the atmosphere gets less dense so there are fewer air molecules that collide with a surface. There are also fewer air molecules above a surface as the height increases. This means the weight of air above that point decreases which reduces the pressure felt at that point.
- 31What is momentum?Momentum is how much 'oomph' an object has. It is a vector quantity. The greater the mass of an object the greater its velocity, the more momentum it has.
- 32What is the equation linking momentum, mass and velocity?momentum(kgm/s)=mass(kg)*velocity(m/s)
- 33What is the conservation of momentum?In a closed system, the total momentum before an event (e.g. a collision) is the same as after the event.
- 34Explain in terms of momentum what happens in an explosion?In an explosion momentum before is zero, then after the explosion the pieces fly off in different directions, so that the total momentum cancels out to zero.
- 35How can forces cause a change in momentum?When a resultant force acts on an moving object (or an object that can move), it causes its velocity to change, causing a change in momentum.
- 36Show the derivation of the formula that links force, mass, change in velocity and change in time?We know f=ma and that a=change in velocity/time.
So F = mass*change in momentum/change in time. The force causing the change in momentum is equal to the rate of change of momentum. - 37Name three safety features and how they reduce the force produced by a change in momentum?Car crumple zones - crumple on impact increasing the time taken for the car to stop.
Helmets - contain crushable layer of foam lengthens the time taken for your head to stop.
Crash mats - cushoined flooring increases time taken for you to stop. - 38What is distance and displacement?Distance is just how far an object has moved (scalar).
Displacement measures distance and direction in a straight line from the object's start point to finish point (vector). - 39What is speed and velocity?Speed is just how fast you are going.
Velocity is speed in a given direction e.g. 30mph north. - 40Explain how you can have objects travelling at a constant speed but a changing velocity?This happens when the object is changing direction whilst staying at the same speed e.g a car going round a roundabout.
- 41Name some typical everyday speeds?walking - 1.5m/s
running - 3m/s
cycling - 6m/s
car - 25m/s
train - 55m/s
plane - 250m/s - 42What do the different features on a velocity-time graph show?Gradient=acceleration
flat sections=steady speed
steeper line=greater acceleration/decelleration
curve line=changing acceleration
area under any section of the graph=distance travelled in that time. (count squares and multiply by one square.) - 43What is friction?If there is no force propelling an object then it will always slow down and stop because of friction (unless in space). Friction always acts in the opposite direction to movement. To travel at a steady speed you need to balance the frictional forces. You get friction when two surfaces are in contact and when an object passes through a fluid (drag).
- 44What is the main way of reducing drag?Drag is the resistance you get in a fluid (gas/liquid). Air resistance is a type of drag. The main way in reducing drag is streamlining to allow the fluid to easily flow across it. Parachutes work in the opposite way (lots of drag).
- 45Explain how drag increases with speed?Frictional forces from fluids always increase with speed.
A car has much more friction to work against when travelling at 70mph then at 30mph becuase the engine has to work much harder just to maintain a steady speed. - 46Explain terminal velocity in a fluid?When falling objects first set off, the force of gravity is greater than the frictional force slowing them down, so they accelerate.
As speed increases so does friction.
This gradually reduces the acceleration until the frictional force equals the accelerating force (0 resultant force).
It will have reached its maximum speed/terminal velocity and continue to travel at a steady speed. - 47Describe terminal velocity on a skydiver?When the skydiver first steps out of the plane she is not yet moving meaning there is no air resistance so her weight acts downwards (accelerating).
When she is falling, she is still accelerating but there is an air resistance force upwards but here weight>resistance.
As her speed increases so does air resistance until they are equal, so the resultant force is 0 and she travels at a constant velocity, W=R.
When they open the parachute air resistance increases dramatically so now W - 48What 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'll just carry on moving at the same velocity (speed and direction).
- 49What is Newton's second law?Acceleration is directly proportional to the resultant force.
And mass is inversely proportional to acceleration so F=ma. - 50What is Newton's third law?When two objects interact, the forces they exert on each other are equal and opposite.
- 51Give two examples of when Newton's third law is visible?1. when skater A pushes on skater B, she feels an equal and opposite force from skater B's hand (the normal contact force). Both skaters feel the same sized force, in opposite directions, and so accelerate away from each other, but skater A will be accelerated more than B because she has a smaller mass, a=F/m.
2. third law in an equilibrium situation: a man pushing against a wall, as the man pushes the wall, there is a normal contact force acting back on him. These two forces are equal and opposite. - 52Explain how a book resting on a table is in equilibrium?The weight of the book is equal to the normal contact force . The weight of the book pulls it down, and the normal reaction force from the table pushes it up. It's not Newton's Third Law, these forces are different types and they're both acting on the book. The pairs of forces due to Newton's Third Law in this case are:
1. the weight of the book is pulled down by gravity from Earth and the book also pulls back up on the Earth.
2. the normal contact force from the table pushing up on the book and the normal contact force from the book pushing down on the table. - 53What is inertia?Inertia is the tendency for motion to remain unchanged. Until an object is acted upon by a resultant force, it stays at rest or will continue at a steady speed (first law). The inertial mass of an object measures how difficult it is to change the velocity of an object. Inertial mass is the ratio of force and acceleration m=F/a.
- 54Briefly explain the investigation how mass and force affect acceleration?1. set up the trolley holding the piece of card with a gap in the middle to interrupt the light gate twice (measure the length to put into data logger so it can measure velocity).
2. connect trolley to string that goes over pulley. On the other end attach a hook for the masses.
3. the weight of the hook and any masses will provide the accelerating force, equal to the mass of the hook (m) * acceleration due to gravity.
4. the wieght of the hook ad the masses accelerates both the trolley and the masses so we are investigating the acceleration of the system.
5. Mark starting line so travels same distance through the light gate.
6. place the trolley on the starting line and hold it in place, hang the masses so the string is taught then release.
7. record the acceleration from the data logger and repeat for an average.
8. to investigate how mass affects it then add masses to the trolley, to investigate how force affects it then add more weights to the hook. - 55Explain the results of the practical using newtons second law?F(hanging masses)=m(mass of the whole system)a(acceleration of the system). By adding mass to the trolley, mass of whole system increases but force stays the same so there is a decrease in acceleration. By transferring masses to the hook, you are increasing the accelerating force, without changing the mass so increasing the force would increase the acceleration of the trolley.
- 56What is the stopping distance?Stopping distance=thinking distance+braking distance
- 57Name two factors that affect your thinking distance?1. speed (faster=longer time to stop=further distance)
2. reaction time(tiredness, drugs, alcohol affect this) - 58Explain the ruler drop test?1. sit with arm resting on table and get someone to hang the ruler in between your thumb and finger. Check that your thumb is lined up with 0cm.
2. the person should drop the ruler at a random time and the person should catch it as fast as they can.
3. the measurement on where the person caught it is the reaction time.
4. you can calculate the time taken by using the equation for uniform acceleration (V^2-u^2=2as) because acceleration due to gravity is constant. Repeat and make an average or add distractions to see the affect. - 59Explain the process of braking in a car?When the brake pedal is pushed, this causes pads to be pressed onto the wheels, causing friction, causing work to be done. The work done between the brakes and the wheels transfers energy from the kinetic stores of the wheels to the thermal stores of the brakes (increase in temp). The faster the vehicle is going the more energy it has in its kinetic energy store so the more work has to be done to stop it. Meaning a greater braking force is needed to make it stop within a certain distance. A larger braking force means larger deceleration which can cause overheating and skidding.
- 60How does speed affect braking distance?As a car speeds up, the thinking distance increases at the same rate (linear graph).
Braking distance increases at an increasing rate as you speed up. The work done to stop the car is equal to the energy in the car's kinetic energy store (1/2mv^2). So as speed doubles, the kinetic energy of the car increase 4 fold (2^2) and so the work done to stop the car increases the same amount. Since W=Fs and the braking force is constant, the braking distance also increases 4 fold.