What is instantaneous velocity?
It is the velocity at any given time, equal to the gradient of the tangent on a displacement time graph.
What quantities are used in SUVAT situations, and what are their units?
s = displacement (m) u = initial velocity (ms^-1) v = final velocity (ms^-1) a = constant acceleration (ms^-2) t = time taken (s)
What represents the following on a velocity time graph?
acceleration
displacement
- the gradient
- the area under the graph
What does a velocity time graph look like for a bouncing ball?
A /I/I/I shape alternating between -ve and +ve velocity, with (near) vertical lines representing changes in direction of velocity
How do you approach a two stage Suvat question?
Sketch a graph of v/t, and work out changes in gradient separately.
What is the acceleration of objects falling or decelerating vertically?
g - 9.81 ms^-2 . You can decide to view this as -ve or positive depending on what if you consider upwards as +ve.
Describe the velocity and motion of horizontal projection.
Horizontal and vertical motions and vertical motions are independent of each other, and together are parabolic.
Horizontal motion is constant velocity of uₓ = vₓ , a = 0.
Vertical motion is constant acceleration due to gravity.
These vectors can be resolved to find the final velocity v, and the angle.
Describe the velocity and motion of non-horizontal projection.
Horizontal and vertical motions and vertical motions are independent of each other, and together are parabolic.
The initial velocity, u, needs to be resolved into uₓ and uᵧ using trig.
Horizontal motion is constant velocity of uₓ = vₓ
Vertical motion is constant acceleration due to gravity.
How do you find the following from non-horizontal projection?
- The max. height reached
- The time of flight
- find sᵧ when vᵧ = 0
- find t when sᵧ = 0
What are the following terms?
- displacement
- velocity
- acceleration
- The distance moved in a particular direction.
- The rate of change of displacement
- When the motion of an object changes, either in velocity or direction.
What affects the drag force of fluid resistance? [3]
- The velocity of the object: F ∝ v
- The viscosity of the fluid
- The shape of the object
What is Newton’s First law?
- If there is no resultant force acting on an object, it will continue to move in uniform motion, or be at rest. In this case, the object is in equilibrium (no resultant force: ∑F = 0)
- A resultant force will cause acceleration.
What is Newton’s Second law?
- For a constant mass?
- In terms of momentum?
- The resultant force is proportional to the acceleration, with a constant mass. F = ma.
- The resultant force is equal to the rate of change of momentum. F = Δp / Δt
What is Newton’s Third law?
For every force exerted on an object, it will always exert an equal and opposite force on the other object.
What is a third law pair of forces?
- Forces that are the same size
- Forces that are in opposite directions
- Forces that act on different objects. Because of this, the forces do not have a resultant of 0.
- The same type of force.
How do you work out:
- The tension in a rope?
- The acceleration of two objects, different masses, being pushed together?
- [find out]
- [find out]
How do you draw a free body diagram? [3]
- Draw only a single object.
- Show all the forces as vectors on that object.
- When one object touches another, there is a perpendicular contact/reaction force.
How do you solve problems involving ropes at angles or inclined planes?
- Draw a free body diagram with all the forces.
- Resolve forces at angles into perpendicular vectors. If two forces are already perpendicular, resolve the third.
- Set the sum of the parallel forces equal to 0 and solve.
What is static friction? How can you work it out.
- Static friction happens between two surfaces not moving relative to each other, and is the tendency to resist motion.
- The force increases up to a maximum value, at which point the object starts to move.
- At the instant the object starts to move, Fmax = μs . R (Fmax is friction force, μs is coefficient of static friction, and R is the normal reaction force). Before it starts to move, Ff ≤ μs . R
What is the coefficient of friction? [3]
- It’s the ratio between two forces on two surfaces - it has no units.
- 0 ≤ μ ≤ 1, with 0 being frictionless.
- μd < μs
How can you work out dynamic friction?
Ff = μd . R (Ff is friction force, μd is coefficient of static friction, and R is the normal reaction force)
What is work? How can you work it out?
- Work is how much energy is transferred in moving an object.
- work done = force x distance moved in direction of force. Measured in J or Nm.
- Work done is also the area under a displacement force graph.
How can you work out the following:
- Gravitational Potential Energy
- Kinetic energy
- Elastic Potential energy > how is this derived?
- Ek = 1/2 x mv²
- Ep = mgΔh
- EPE = 1/2 kΔx², where k is the spring constant (stiffness), and Δx is the extension. F = kΔx, the area under a force-extension graph is the work done in stretching = 1/2 FΔx = EPE. Put the two together, EPE = 1/2 kΔx²
What is power? What is efficiency?
- The rate at which energy is transferred = the rate at which work is done. Measured in J/s or W.
- The ratio of useful energy to total energy transferred. Can be calculated using work, energy, or power.
