3 Forces and Motion Flashcards
Instantaneous speed
The speed of the car over a very short interval of time
Found by drawing tangent to distance-time graph at that time
Average velocity graph
Change in displacement/ time taken
Average acceleration equation
Change in velocity/ change in time
Acceleration from graph
Gradient of velocity-time graph
Stopping distance
Thinking distance + braking distance
Total distance travelled from when the driver first sees a reason to stop, to when the vehicle stops
Thinking distance
Distance travelled between the moment when you first see a reason to stop, to the moment when you use the brake
Braking distance
The distance travelled from the time the brake is applied until the vehicle stops
Thinking distance equation
Speed x reaction time
Free fall
When an object is accelerating under gravity, with no other force acting on it
Determining g in lab experiments
Electromagnet and trapdoor
Light gates
Taking photos of dropped ball next to metre rule
The moment of a force
The turning effect of a force about some axis or point
Moment= force x perpendicular distance of the line of action of force from the axis or point of rotation
Perpendicular distance
Use the perpendicular distance in calculations involving moments, not just the distance from force to pivot
The principle of moments
For a body in rotational equilibrium, the sum of the anti-clockwise moments about any point is equal to the sum of the clockwise moments about the same point
Torque of a couple
The moment of a couple is known as a torque. The torque of a couple is defined as
torque of the couple = one of the forces X perpendicular separation between the forces
=Fd
Density
Mass per unit volume
Kgm-3
Pressure
Normal force exerted per unit cross sectional area
Nm-2 or Pa
Pressure in liquids
p=hpg
Height x density x acceleration of free fall
Pressure at base is equal to the weight of column divided by cross sectional area
Archimedes’ principle
The upthrust exerted on a body immersed in a fluid is equal to the weight of the fluid that the body displaces
When will an object sink
If the upthrust is less than the weight of the object
Floating objects
The upthrust equals the eight of the objects
Work done
Energy transferred
Nm or J
Work done at angle to motion
Work done W= (F cos(theta)) * x
W=Fxcos()
Energy
The capacity for doing work
Scalar
Joules
Forms of energy
Kinetic energy Gravitational potential Chemical Elastic potential Electrical potential Nuclear Radiant/electromagnetic Sound Thermal
The principle of conservation of energy
The total energy of a closed system remains constant: energy can never be created or destroyed, but it can be transferred from one form to another
Kinetic energy
Energy associated with an object as a result of its motion
Kinetic energy = 0.5 X mass X velocity2
Gravitational potential energy
The capacity for doing work as a result of an object’s position in a gravitational field
Gravitational potential energy = mass X gravitational field strength X height
Power
The rate of work done
Js-1 or W
Tensile forces
Forces that produce extension
Compressive forces
Forces that shorten an object
Hooke’s law
The extension of the spring is directly proportional to the force applied. This is true as long as the elastic limit of the spring is not exceeded
Elastic deformation
Means that the spring will return to its original lights when the force is removed
Plastic deformation
Permanent structural changes to the spring occur and it does not return to its original length when the force is removed
Force constant K
For a spring obeying hooke’s law, the applied force F is directly proportional to the extension x.Therefore F =kx
What shows work done on a force – extension graph
Area under the graph
Loading and unloading metal wire
Follows Hooke’s law until the elastic limit of the wire
Beyond the elastic limit it is parallel to the loading graph but not identical to it
The wire is permanently extended after the force is removed- it’s longer than it was at the start
Loading and unloading rubber
Don’t obey Hooke’s law
Rubber band will return to its original length after the force is removed- elastic deformation
The loading and unloading graphs are both curved and are different
Hysteresis loop
Unloading and loading polythene
Doesn’t obey Hooke’s law
Suffer plastic deformation under little force
Tensile stress
The force applied per unit cross sectional area of the wire
Sigma is symbol for tensile stress
Tensile strain
The fractional change in the original length of the wire Curvy E (epsilon) is tensile strain
Ultimate tensile strength
The maximum stress that a material can withstand when being stretch before it breaks
Beyond this point the material may become longer thinner at its weakest point, a process called necking
Breaking point
Where the material eventually snaps from the stress it’s put under
Breaking strength
The stress value at the point of fracture
Strong material
One with a high ultimate tensile strength
Young modulus
Tensile stress/ tensile strain
The ratio of stress to strain for a particular material
Gradient of stress-strain graph
Nm-2 or Pa
Stress-strain graph for brittle materials
Straight line. 2 arrows pointing into each other
Ultimate tensile strength is same as breaking strength
Polymeric materials
Materials that consist of long molecular chains
Polythene and rubber
Stress-strain graph for rubber
Marble swirl shape?????
Clockwise arrows
Stress-strain graph for polythene
Straight line then levels off
Newton’s first law of motion
An object will remain at rest or continue to move with constant velocity unless acted upon by a resultant force
Newton’s third law of motion
When two objects interact, they exert equal and opposite forces on each other
Types of interaction
All interactions can be explained in terms of four fundamental forces- gravitational, electromagnetic, strong nuclear and weak nuclear.
The two nuclear forces have a very short range and so very little impact on things we observe in daily life.
When you push your hands together, the contact force you feel is due to the electrostatic repulsive forces between the electron clouds around the atomic nuclei in your hands
Momentum
Momentum= mass x velocity
Kg m s-1
Conservation of momentum principle
For a system of interacting objects, the total momentum in a specified direction remains constant, as long as no external forces act on the system
Total momentum before collision=
Total momentum after collision
Zero momentum example- gun
A gun recoils when a bullet is fired. The total momentum of this system remains the same and is equal to zero. The momentum of the gun and the momentum of the bullet have the same magnitude but act in opposite directions
Two types of collisions
Perfectly elastic
Inelastic
Summary of perfectly elastic collision
Momentum- conserved
Total energy- conserved
Total kinetic energy- conserved
Summary of inelastic collision
Momentum- conserved
Total energy- conserved
Total kinetic energy- not conserved
Newton’s second law of motion
The net (resultant) force acting on an object is directly proportional to the rate of change of its momentum, and is in the same direction Net Force= change in momentum/time Also F=ma
Why is momentum conserved in collisions?
The net force on the objects in a closed system is zero. According to Newton’s second law change in momentum/time=0. The change in momentum of both objects must be zero; therefore, the total momentum of the objects doesn’t change
Momentum is always conserved
Impulse of a Force
Forces accelerating or decelerating an object usually change over time e.g. kicking a ball. This type of motion can be analysed using the idea of impulse
Impulse of a Force= change in momentum
=Ft
Unit of impulse
Ns
Kg m s-1
Area under force-time graphs
Impulse
Area=Ft
