Topic P5 - Forces Flashcards
Describe a vector quantity.
- Vector quantities have magnitude and direction
- Force is a vector quantity as well as; velocity, displacement, acceleration, momentum, etc…
- Vectors are usually represented by an arrow (the length shows the magnitude and direction of the arrow shows the direction of the quantity)
Describe a scalar quantity.
- Scalar quantities only have magnitude and not direction
- Some scalar quantities are; speed, distance, mass, temperature, time, etc…
What is the contrast in non-contact and contact forces?
Non-contact forces;
- the objects do not need to be touching
- eg; magnetic, gravitational, electrostatic…
Contact forces;
- the objects will need to be touching
- eg; friction, air resistance, normal contact force
When two objects interact, there is a force produced on both objects.
Describe gravitational force.
- on the surface of a planet, it makes all things go towards the ground
- it gives everything a weight
Describe weight.
- weight is the force acting on an object due to gravity
- it depends on the strength of the gravitational field at the location of the object
- it is a force that is measured in newtons and is measured by a newtonmeter
How do you calculate the weight?
Weight (N) = Mass (kg) x Gravitational Field Strength (N/kg)
What is a resultant force?
- if you have a number of forces that are acting at a single point, you can replace them with a single force
- the single force is a resultant force
- if a resultant force moves an object, work is done
How do you calculate work done?
Work Done (J) = Force (N) x Distance (m)
What is an equilibrium?
- an object is an equilibrium if the forces on it are balanced
- (if all of the forces combine to make zero)
Describe how to transfer energy by stretching, compressing and bending.
- when you apply a force to an object, it can cause it to stretch, compress and bend.
- to do this you will need more than one force acting on the object, because otherwise it will just move rather than change shape.
- work is done when a force stretches or compresses an object and causes energy to be transferred to the elastic potential energy store of the object.
How do you work out force?
Force (N) = Spring Constant (N/m) x Extension (m)
How do you work out the elastic potential energy?
Elastic Potential Energy (J) = 1/2 x Spring Constant (N/m) x Extension^2 (m)
Ee = 1/2 x k x e^2
Describe distance and displacement
- Distance is scalar because it doesn’t involve direction
- Displacement is a vector
- Displacement measures the distance and direction in a straight line from the object’s starting point to it’s finishing point
Describe speed and velocity.
- Speed and velocity both measure how fast you are going
- speed is scalar
- velocity is vector
- this means that you can have objects travelling at a constant speed with a changing velocity.
- objects rarely travel at a constant speed, so in this case you will find the average speed during that time.
How do you work out the distance travelled?
Distance Travelled (m) = Speed (m/s) x Time (s)
s = v x t
Describe acceleration.
- acceleration is how quickly you’re speeding up
- it is not the same as speed or velocity
- acceleration is the change in velocity in a certain amount of time
- deceleration is just negative acceleration
How do you work out the acceleration?
Acceleration (m/s^2) = Change in Velocity (m/s) / Time (s)
a = (triangle V) / t
What is a distance-time graph?
- if an object moves in a straight line, it’s distance travelled can be plotted on a distance-time graph
- the steeper the graph, the faster it’s going
- flat sections is where the object has stopped
- if the object is changing speed, you can find it’s speed at a point by finding the gradient of the tangent to the curve at that point.
How do you work out the speed?
Speed = Distance / Time
What is a velocity-time graph?
- how an object’s velocity changes as it travels can be plotted on a velocity-time graph
- flat sections represent steady speed rather than that the object has stopped.
- if the graph is curved, you can use a tangent to the curve at a point to find the acceleration at that point.
Describe friction.
- Friction is always there to slow things down
- it always acts in the opposite direction to movement
- to travel at a steady speed, the driving force will need to balance the frictional forces
Describe drag.
- drag is the resistance you get in a gas or liquid. (for example air resistance is a type of drag)
- to reduce drag, you have to keep the shape of the object streamlined
What is terminal velocity?
- as the speed of a falling object increases, the friction builds up and then gradually reduces the acceleration until eventually the frictional force is equal to the accelerating force
- it will have then reached it’s maximum speed or terminal velocity and will fall at a steady speed.
- terminal velocity depends on shape and area
What is Newton’s first law?
- a resultant force is needed to make something start moving, speed up or slow down.
- a non-zero resultant force will always produce acceleration or deceleration in the direction of the force
- this acceleration can take five different forms; starting, stopping, speeding up, slowing down and changing direction.
What is Newton’s second law?
- The larger the resultant force acting on an object, the more it accelerates - the force and acceleration are directly proportional
- acceleration is also inversely proportional to the mass of the object - so an object with a larger mass can accelerate less than one with a smaller mass
- Resultant Force (N) = Mass (kg) x Acceleration (m/s^2)
How do you work out the stopping distance?
Stopping Distance = Thinking Distance + Braking distance
What is stopping distance affected by?
- the thinking distance which is how far the car travels during the driver’s reaction time
- the braking distance which is how long it takes the car to stop when you have your foot down in the brake
What is the thinking distance affected by?
- speed (faster you’re going = further to travel when reacting)
- reaction time (longer reaction time = longer the thinking distance)
What is the braking distance affected by?
- speed
- weather or the road surface
- condition of the tyres
- how good the brakes are
How does braking rely on friction?
- When the brake pedal is pushed, it causes the brake pads to press on the wheels which causes friction
- the faster a vehicle is going, the more energy in it’s kinetic stores of the wheels to the thermal energy of the brakes
How can you measure reaction time?
- the drop test
- you drop a ruler and the other person has to catch it and see how long it took them to react
- you can then calculate how long your reaction time was and repeat the test
How do you work out momentum?
Momentum (kg m/s) = Mass (kg) x Velocity (m/s)
p = m x v
Describe momentum.
- the greater the mass, or greater the velocity, the more momentum it will have.
- it is a vector quantity as it has size and direction
