Chapter 2 - Mechanics Flashcards
What is an instantaneous value?
It’s a value at one particular time
Instantaneous speed is also the same as velocity
How do you work out relative velocities?
By Simple addition or subtraction as appropriate.
Features of a displacement-time graph
The gradient is equal to the velocity
Features of a velocity-time graph
The gradient is equal to the acceleration
The area under the graph is the displacement
Features in a Acceleration-time graph
The area under the graph is the change in velocity
How do you use light gates to measure acceleration?
It’s a device that senses and records the time of when an object cuts through a beam of light. Use the length of the object that breaks the beam, the average speed can be calculated.
How do you use strobe photography to measure acceleration?
It gives out brief flashes of light at fixed time intervals. Use the camera and if it’s the only source of light, pictures will have captured the objects motion.
How do you use ticker timer to measure acceleration?
It has dots on a strip of paper at regular time intervals. (Normally every 1/5 of a second). Attach it to the object and let it fall. The length of the paper and time are then used.
What is the condition needed to use the SUVAT equations?
Constant acceleration
What does the U and the V stand for in the SUVAT equations?
U - initial velocity
V - final velocity
What does it mean if an objects in free-fall?
When we can ignore the effects of air resistance and the object is falling in a uniform gravitational field.
What’s the shape of projectile motion?
Parabola
What are the only forces acting during a projectile motion?
Gravity and friction. In many situations air resistance can be ignored
What are the components of projectile motion?
The vertical and horizontal motion are independent of one another - assuming the gravitational force is constant.
Horizontal component to projectile motion?
There are no forces in this direction, there is no horizontal acceleration. So the velocity in this direction must be constant
Vertical component in projectile motion
There’s a constant vertical force downwards of 9.81/ 10 ms^-2,
so there is constant vertical acceleration.
What’s the method to solve any problem of projectile motion?
1) . Use angle of launch to resolve initial velocity into components
2) . Use vertical component of velocity to determine time of flight
3) . Use the horizontal component and time of flight for the range
4) . Use vector addition to solve for velocity at any point
If there is a given speed, what angle of launch is needed for the greatest range?
45 degrees
What will happen if 2 objects are released together, one from rest and the other one already with a horizontal velocity?
They will both hit the ground together because the forces pulling them down are the same.
What happens when an object moves through a fluid?
There will be frictional fluid resistance e.g terminal velocity
It can also be called viscous drag
What factors effect the drag force on an object?
Relative velocity of the object with respect to the fluid
Shape and size of the object
The fluid used
What happens to the projectile motion of an object when fluid resistance is taken into account?
The vertical and horizontal components of velocity will both be reduced, thus the path the object takes will be smaller.
What is force?
The cause of a deformation or a velocity change
What’s the effect of a resultant force?
A change in velocity, thus acceleration.
If the resultant force is zero then velocity is constant
To describe a force, on must include….
magnitude Direction The object on which it reacts The object exerting the force Nature of the force
What are all the different types of forces?
Gravitational, electrostatic, magnetic, normal reaction, friction, tension, compression, upthrust and lift.
Hooke’s law
Up to the elastic limit, the extension ‘X’ of a spring is proportional to the tension force F.
The constant of proportionality is K - spring constant.
Forces as vectors
Forces can be split up into components
Free-body diagrams
One object ONLY is chosen
All the forces acting on that object are shown and labelled
Newton’s first law
An object continues in a uniform motion in a straight line or at rest unless a resultant external force acts
When does Translational equilibrium occur?
When the resultant force is zero (in any direction)
It doesn’t mean the same as being at rest
What is tension?
When a spring/string is stretched, it has equal and opposite forces on its ends pulling outwards.
It’s the force that the end of the string applies to another object.
What is Newton’s second law?
A resultant force causes an acceleration
What is Newton’s second law in terms of momentum?
The resultant force is proportional to the rate of change of momentum
Equation for resultant force
F = 🔼p / 🔼t
Equation for force and the condition needed?
F = m x a
It only applies if we use SI units
If there are several forces acting on an object then work out the resultant force first and then apply the law.
The method used to find the resultant force when friction is involved.
The resultant force = forward force - friction
Newton’s third law
When two bodies A and B interact, the force that A exerts onto B is equal and opposite to the force that B exerts into A
2 Key points about Newton’s third law:
Equal and opposite forces that act on the same object are not Newton’s third law pairs
The forces must also be the same type
How does size of mass influence velocity gained?
The smaller mass will gain the greater velocity
The mass of some objects are so large that the force on it doesn’t cause any acceleration.
What’s the difference between mass and weight?
Mass is the amount of matter contained in an object
Weight is the gravitational force on an object
What is the equation for gravitational force and what’s the value of g on the moon?
Gravitational force = m x g
g on the moon = 1.6 N kg^-1
What’s the definition of work done and it’s units?
The amount of energy transferred is equal to the work done.
Unit of work done is joules, j
What’s the conservation of energy?
Energy is neither created or destroyed it just changes form
What are the 12 types of energy?
Kinetic, Gravitational potential, Elastic potential, Electrostatic potential, Radiant, Thermal, Nuclear, Solar, Chemical, Electrical, Internal and Light
Equation for K.E
K.E = 1/2 mv^2
Equation for G.P.E
G.P.E = mgh
Equation for elastic potential energy
Elastic potential energy = 1/2 k 🔼x^2
k = Spring constant in NM^-1 🔼x = the extension
Definition of power
The rate at which energy is transferred
Equation for power and its units
Power = energy transferred ( work done) / time taken
SI unit for power = Js^-1 or W (watts)
Equation for efficiency
Efficiency = useful energy out / total energy in
Equation for momentum and its units
Momentum = mass x velocity
Units: Kg ms^-1
It’s a vector
Definition of impulse and its equation
Impulse is the change in momentum
🔼p = F x 🔼t
What is the important feature of a force time graph?
The area under the graph is the total impulse
What’s the convention of momentum?
The total linear momentum of a system of interaction particles remains constant provided there is no resultant external force.
What is an elastic collision?
It’s a collision where no mechanical energy is lost. Relative velocity is the same before and after.
In reality collisions do lose some energy
What’s a totally inelastic collision?
A large amount of mechanical energy is lost but total momentum stays the same. (The objects start moving together)
There’s no relative velocity in this case because they aren’t compared against each other as they’re moving together
What’s an inelastic collision?
Some energy is lost but the objects don’t carry on moving together. Most likely in an real life situation.
