Forces Flashcards
What is a contact force?
When two objects have to be touching for a force to act.
What is a non-contact force?
When the objects do not have to be touching for the force to act.
What is a vector quantity?
Quantities that have a magnitude and a direction.
What is a scalar quantity?
Quantities that only have magnitude and no direction.
Give 3 examples of vector quantities.
Any 3 from: force, velocity, displacement, acceleration, momentum.
Give 3 examples of scalar quantities.
Any 3 from: 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.
Give 3 examples of contact forces.
Any 3 from: friction, air resistance, tension in ropes, normal contact force.
Give 3 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 is the equation linking, mass, weight and gravitational field strength?
Weight (N) = Mass (kg) x Gravitational field strength (N/kg)
What is a resultant force?
The overall force on a point or object.
What type of diagrams show all the forces acting on an object?
Free Body Diagrams.
What is work done?
When a force moves on an object through a distance, energy is transferred and work is done on the object.
What is the equation linking force, work done and distance?
Work done (J) = Force (N) x Distance (m)
What type of drawings can you use to find resultant forces?
Scale drawings.
How do you draw a scale drawing for a resultant force?
- Draw all the forces acting on an object to scale.
- Draw a straight line from the start of the first force to the end of the last force (the resultant force).
- Measure the length of the resultant force on the diagram to find the magnitude and the angle to find the direction.
Complete the sentence: An object is in e____________ if the forces on it are b___________.
Equilibrium, Balanced
What does it mean for an object to be in equilibrium?
When all of the forces acting on an object combine to give a resultant force of zero.
What does it mean for an object to be elastically deformed?
It can go back to its original shape and length after the force has been removed, e.g. a spring.
What does it mean for an object to be inelastically deformed?
If it cannot return to its original shape and length after the force has been removed.
Complete the sentence: Extension is d__________ p________ to force.
Directly proportional.
What is the equation linking extension, force and spring constant.
Force (N) = Spring constant (N/m) x Extension (m)
What does the spring constant depend on?
The material you are stretching. A stiffer spring has a great spring constant.
Complete the sentence: There is a l______ to the amount of force you can apply to an object for the extension to keep on i______________ p______________.
limit, increasing proportionally.
What is the limit of proportionality in force and extension?
There is a maximum force above which the graph curves, showing that extension is no longer proportional to force.
What is the unit for spring constant?
N/m
Give the method for the required practical, investigating the link between force and extension.
- Measure the natural length of the spring (with no mass), using a millimetre ruler clamped to the stand.
- Add a mass to the spring and allow the spring to come to rest. Record the mass and measure the new length of the spring. The extension is the change in length.
- Repeat this process until you have enough measurements.
How can you make the required practical on force and extension more accurate?
- Make sure you take the reading at eye level and add a marker to the bottom of the spring to make the reading more accurate.
- Repeat and calculate a mean.
What is the equation linking spring constant, elastic potential energy and extension?
Elastic potential energy (J) = 0.5 x spring constant (N/m) x extension^2 (m)
How can you calculate the energy stored in a springs elastic potential energy store?
Ee = 1/2ke^2
What is a moment?
The turning effect of a force.
How can you calculate the moment of a force?
Moment (Nm) = Force (N) x Distance (m)
Explain how a moment works using a spanner and nut as an example.
- The force on the spanner causes a turning effect or moment on the nut (which acts as a pivot). A larger force or longer distance (spanner) would mean a larger moment.
- To get the maximum moment, you need to push at right angles to the spanner. Pushing at any other angle means a smaller distance and so a smaller moment.
What does it mean if the total anticlockwise moment equals the total clockwise moment about a pivot?
The object is balanced and wont turn.
What do levers do?
They make it easier to do work. They increase the distance from the pivot at which the force is applied meaning less force is needed to get the same moment.
What do gears do?
Transmit rotational effects.
What are gears?
Circular discs with ‘teeth’ around their edges. The teeth interlock so that turning one causes another to turn in the opposite direction. They transmit the rotational effect of a force from one place to another.
What is pressure?
The force per unit area.
What are fluids?
Substances that can flow because their particles are able to move around.
What does the pressure of a fluid mean?
The force is exerted normal (at right angles) to any surface in contact with the fluid.
How can you calculate the pressure at the surface of a fluid?
Pressure (Pa) = Force normal to a surface (N) / Area of that surface (m^2)
What does the pressure in a liquid depend on?
The depth and density.
What is density?
The compactness of a substance.
Complete the sentence: The m_____ dense a give liquid is, the m_______ p________ it has in a certain space. This means there are more p________ that are able to c_________ so pressure is h_________.
more, more particles, particles, collide, higher
Complete the sentence: As the depth of the liquid i____________ the number of particles above that point __________.
increases, increases
How can you calculate pressure at a certain depth due to the column of liquid above?
Pressure (Pa) = Depth (m) x Density of the liquid (kg/m^3) x Gravitational field strength (N/kg)
What is upthrust?
A force that determines whether an object will sink or float.
Complete the sentence: Objects in fluids experience ___________.
upthrust
What happens when an object is submerged in a fluid?
The pressure of the fluid exerts a force on it from every direction.
What is an upwards resultant force known as?
upthrust
Complete the sentence: If an object floats its weight is _________ to upthrust.
equal
What is atmospheric pressure?
The pressure exerted by the air around you, it acts on you at all times and varies with your height above sea level.
Complete the sentence: Atmospheric pressure __________ with height.
decreases
Is the atmospheric pressure higher or lower at sea level than at the top of a mountain? Why?
Higher, because as altitude increases the atmosphere gets less dense so there are fewer air molecules able to collide with the surface.
What type of quantity is distance?
Scalar.
What type of quantity is displacement?
Vector.
What is the difference between distance and displacement?
Distance (scalar) is how far an object has moved. Displacement (vector) measures the distance and direction in a straight line from an objects starting point to its finishing point.
What type of quantity is speed?
Scalar.
What type of quantity is velocity?
Vector.
What is the difference between speed and velocity?
Speed is just how fast you’re going with no regard to the direction. Velocity is speed in a given direction (e.g. 30mph north).
What is the equation linking distance travelled, speed and time?
Speed (m/s) = Distance travelled (m) / Time (s)
What is the typical speed for the following: a person walking, running, cycling, a car, a train and a plane.
Person walking -> 1.5m/s
Person running -> 3m/s
Person cycling -> 6m/s
A car -> 25m/s
A train -> 55m/s
A plane -> 250m/s
What is acceleration?
The rate of change of velocity (how quickly you’re speeding up).
What is the equation linking change in velocity, acceleration and time?
Acceleration (m/s^2) = Change in velocity (m/s) / Time (s)
What is meant by uniform acceleration?
A constant acceleration.
What is the value of uniform acceleration due to gravity for objects in free fall?
9.8m/s^2
What is the equation for uniform acceleration?
final velocity^2 (m/s) — initial velocity^2 (m/s) = 2 x acceleration (m/s^2) x distance (m)
What does the gradient of a distance time graph tell you?
The speed.
What do flat sections of a distance time graph tell you?
That the object is stationary (stopped).
What do straight uphill sections on a distance time graph tell you?
That the object is travelling at a steady speed.
What do curves in a distance time graph represent?
Acceleration or deceleration.
What does a steepening curve on a distance time graph represent?
The object is speeding up.
What does a levelling off curve on a distance time graph show?
That the object is slowing down.
What does the gradient of a velocity time graph represent?
The acceleration.
What do flat sections on a velocity time graph represent?
The object is travelling at constant speed.
Complete the sentence for a velocity time graph: The steeper the graph, the ______________ the acceleration or deceleration.
greater
What does the area under any section of a velocity time graph represent?
The distance travelled in that time interval.
What is drag?
The resistance in a fluid, the frictional force acting on a moving object.
Give an example of drag.
Air resistance.
What is meant by an object being streamlined?
Where the object is designed to allow fluid to flow easily across it, reducing drag.
Complete the sentence: Drag increases as speed ____________.
Increases.
What is terminal velocity?
When the acceleration of an object is equal to the resistance acting on it - the resultant force is zero. The object is falling at steady speed.
What does terminal velocity depend on?
Shape and area.
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 just carry on moving at the same velocity (same speed and direction).
What is Newton’s Second Law?
The larger the resultant force acting on an object, the more the object accelerates - the force and the acceleration are directly proportional. F = ma
What is the equation linking mass, acceleration and force?
Force (N) = Acceleration (m/s^2) x Mass (kg)
What is inertia?
The tendency for motion to remain unchanged.
What is Newton’s Third Law?
When two objects interact, the forces they exert on each other are equal and opposite.
What does an objects inertial mass measure?
How difficult it is to change the velocity of an object.
Give the method for the required practical on investigating how mass and force affect acceleration.
Equipment:
- Metre rule
- Toy car
- Weight stack
- Bench pulley
- String
- Two stands, clamps and bosses
- Rubber bands
- Chalk
- Stop watch
Method
1. Draw a series of straight lines, each 20cm apart, perpendicular to the edge of the bench.
2. Attach the car to the string at one end, with the other end running across the bench pulley.
3. Attach the weight stand to the loose end of the string. Hold the weight of the pulley, so it doesn’t pull the car but so that the string is fully extended.
4. Release the weight stand and begin the timer. Stop timing when the car reaches the pulley at the other end of the bench.
To investigate changing force on a constant mass:
5. Add mass to the weight stack and time the car travelling across the bench to the pulley.
6. Repeat, adding more mass to the weight stack each time.
To investigate changing mass on a constant force:
5. Attach mass on top of the toy car using rubber bands. Pull the car back and time the car travelling to the pulley.
6. Repeat by adding more mass on top of the toy car.
What is stopping distance?
The distance it takes a car to stop in an emergency. The sum of two distances.
What is thinking distance?
How far the car travels during the drivers reaction time.
What is braking distance?
The distance taken to stop under the braking force.
What affects the total stopping distance?
Thinking distance:
- reaction time
- tiredness, drugs, alcohol
- distractions
Braking distance:
- weather or road surface
- condition of the tyres
- worn or faulty brakes
Speed - the faster the speed the further a vehicle travels before it comes to a stop.
How can you measure a person’s reaction time?
The ruler drop test.
What is the equation linking, stopping distance, braking distance and thinking distance?
Stopping distance = braking distance + thinking distance
What does braking rely on?
The friction between the brakes and wheels.
Explain what happens between the driver braking and the car stopping.
- When the brake pedal is pushed, the brake pads are pressed onto the wheels. This contact causes friction.
- The wheels transfer kinetic energy to thermal energy of the brakes.
- The faster a vehicle is going, the more kinetic energy it has so the longer it takes for the energy to be transferred to thermal.
- A larger braking force means a larger deceleration.
Why does speed affect braking distance more than thinking distance?
As the car speeds up, the thinking distance increases at the same rate. This is because thinking time stays pretty constant.
What is momentum?
How much ‘oomph’ an object has.
How can you calculate momentum?
Momentum (kg m/s) = mass (kg) x velocity (m/s)
Complete the sentence: In a closed system, the total momentum before an event is __________ after the event.
The same as/ equal to.
What is the conservation of momentum?
In a closed system, the momentum before the event is equal to the momentum after.
Complete the sentence: When a force acts on an object, it causes the object to change momentum. The bigger the force, the _________ the change in momentum.
faster
What is the equation linking force, change in time and change in momentum?
Force (N) = Change in momentum (kg m/s) / Change in time (s)
Give 3 examples of safety features in cars to reduce the rate of change in momentum and force.
Crumple zones - increases time taken for the car to stop.
Seat belts - stretch slightly increasing the time taken for the wearer to stop
Air bags - inflate, compressing air inside slows down how fast you hit the dashboard.
