1) Forces and motion Flashcards
Units: mass, distance, velocity, acceleration, force, time, gravitational field strength
-Mass: kilogram (kg)
-distance: metre (m)
-velocity: metre per second (m/s)
-acceleration: metre per second squared (m/s2)
-Force: newton (N)
-time: second (s)
-gravitational field strength: newton/kilogram (N/kg)
Distance time graph - straight, steep, shallow, flat, curve
-Straight line: constant speed
-Steep slope: high speed
-shallow slope: low speed
-flat, horizontal line: stationary
-curve: changing speed
Calculate speed from a distance-time graph
speed = Gradient = rise/run
Speed
The distance it travels every second
-scalar quantity
average speed = distance moved/ time taken
Practical: investigating motion
- Measure out a height of 1.0 m using the tape measure or metre ruler
- Drop the object from this height
- Use the stop clock to measure how long the object takes to travel this distance
- Record
- Repeat steps 2-3 three times, calculate an average
- Repeat steps 1-4 for heights of 1.2 m, 1.4 m, 1.6 m, and 1.8 m
Acceleration
Rate of change of velocity
-positive: speeding up
-negative: slowing down
acceleration = change in velocity/ time taken
Velocity-time graph - straight, steep, gentle, flat
-straight line: constant acceleration
-steep slope: large acceleration/ deceleration
-gentle slope: small acceleration/ deceleration
-flat line: acceleration is zero - moving at constant velocity
Velocity-time graph - acceleration
acceleration = gradient = rise/run
Velocity-time graph - Displacement
Area beneath the graph
Calculate uniform acceleration
v^2 = u^2+ 2as
(final speed)(m/s)^2 = (initial speed)2 + (2 × acceleration (m/s2) × distance moved (m))
Effect of forces - changes of speed, direction, shape
Speed: cause bodies to speed up, slow down
Direction: cause bodies to change direction of travel
Shape: cause bodies to stretch, compress, deform
Types of forces
-gravitational/ weight
-electrostatic
-thrust
-upthrust
-air resistance/ drag
-compression
-tension reaction force
Gravitational force/ weight
The force between any two objects with mass
-e.g. the earth and the moon
Electrostatic force
The force between any two objects with charge
-e.g. a proton and an electron
Thrust
The force pushing a vehicle
-e.g. push from rocket engines on the shuttle
Upthrust
The upward force on any object in a fluid
-e.g. a boat on the surface of a river
Air resistance/ drag
The force of friction between objects falling through the air
-e.g. a skydiver in freefall
Compression force
Forces that squeeze an object
-e.g. squeezing a spring
Tension force
Forces that stretch an object
-e.g. two teams in a tug of war
Reaction force
Force between two objects in contact
-e.g. upwards force from a table on a book
Scalar quatities
Quantities that only have a magnitude
-distance, speed, mass, energy, volume, density, temperature, power
Vector quantity
Both a magnitude and a direction
-displacement, velocity, weight, force, acceleration, momentum
Free body diagram
Models the forces acting on an object
-The length of the arrow represents the magnitude of the force
-The direction of the arrow indicates the direction of the force
Resultant force
The leftover force when all the other forces have been added up/ taken away
-can be calculated by adding or subtracting all of the forces acting on the object
Friction
The force which opposes the motion of an object
-always act in the opposite direction to the object’s motion
-emerges when two or more surfaces rub against each other
Balanced force
Forces have combined in such a way that they cancel each other out and no resultant force acts on the body
Unbalanced force
The forces have combined in such a way that they do not cancel out completely and there is a resultant force on the object
Calculate force
force (N) = mass (kg) x acceleration (m/s2)
Weight definition
The force acting on an object due to gravitational attraction
-due to weight:
-objects stay firmly on the ground
-objects will always fall to the ground
-satellites are kept in orbit
Calculate weight
Weight (N) = mass (kg) x gravitational field strength (N/kg or ms-2)
Thinking distance
The distance travelled in the time it takes the driver to react (reaction time) (m)
-time takes to see and hit the brakes
Braking distance
The distance your car will travel once you hit the brakes before it comes to a complete stop (m)
Stopping distance
The sum of the thinking distance and braking distance (m)
Factors affecting stopping distance
Vehicle speed - greater the speed, greater the braking distance will be
-vehicle mass - heavy, takes longer to stop
-road conditions - wet or icy roads, harder to decelerate
-driver reaction time - being tired, intoxicated can increase reaction time
Force on falling objects
- Initially, upwards air resistance is very small because the object isn’t falling very quickly
- As object speeds up, air resistance increases
- Eventually growing large enough to balance the downwards weight force
- Therefore, object’s acceleration is zero, travel at constant speed
- Object is at terminal velocity
Investigating how extension varies with applied force - springs & rubber bands
- Add the 100 g mass hanger onto the spring / rubber band
- Record the mass (in kg) and position (in cm) from the ruler now that the spring / rubber band has extended
- Add another 100 g to the mass hanger
- Record the new mass and position from the ruler now that the spring / rubber band has extended further
- Repeat this process until all masses have been added
- Remove the masses and repeat the experiment again, an average length (for each mass attached) is calculated
Hooke’s law
The extension of an elastic object is directly proportional to the force applied, up to the limit of proportionality
Force-extension graph
-Hooke’s law - straight line on the graph
-any material beyond its limit of proportionality will have a non-linear relationship between force and extension
Elastic behaviour
The ability of a material to recover its original shape
after the forces causing deformation have been removed
-elastic deformation
-inelastic deformation
Calculate momentum
momentum (kg m/s) = mass (kg) x velocity (m/s)
p = mv
Momentum
Keeps an object moving in the same direction
-difficult to change direction of an object with a large momentum
-a vector quantity: object travelling right is positive, left is negative
Conservation of momentum
The total momentum before a collision = The total momentum after a collision
Force and momentum
Force = rate of change in momentum/ time taken
Newton’s third law
Whenever two bodies interact, the forces they exert on each other are equal and opposite
-force pairs are equal and the same type
Momentum and safety features
As force is equal to the rate of change in momentum, the force of an impact in a collision can be decreased by increasing contact time over which the collision occurs
e.g.
-crumple zones
-seat belts
-airbags
Momentum - seat belts
-stop passenger from colliding with interior of a vehicle by keeping them in their seats
-designed to stretch slightly
-increases time for passenger’s momentum to reach zero
-reduce force on them in a collision
Momentum - airbags
-deployed when a collision occurs
-acts as a soft cushion to prevent injury on the passenger when they are thrown forward upon impact
Momentum - crumple zones
-exterior
-designed to crush or crumple in a controlled way in a collision
-increase time which vehicle comes to rest
-lower impact force on passengers
Moment
Turning effect of a force about a pivot
moment (Nm) = force (N) x perpendicular distance of the force to the pivot (m)
The Principle of moments
If an object is balanced, the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot
