P5: Forces Flashcards
What is a contact force?
When objects are physically touching
What are 4 examples of contact forces?
-Normal reaction force
-Friction
-Air resistance
-Tension
What is a non-contact force?
When the force is not touching the object
What are 3 examples of non-contact forces?
-Magnetism
-Electrostatic force
-Gravity
What is scalar?
A quantity that only has magnitude
What is vector?
A quantity that has magnitude and direction
How can we find the resultant vector?
-By adding them
-If at right angles, make a triangle and use pythagoras
What are 5 examples of scalar quantities?
-Distance
-Speed
-Mass
-Energy
-Temperature
What are 5 examples of vector quantities?
-Displacement
-Velocity
-Acceleration
-Force
-Momentum
What does it mean if there is no resultant force on a vector?
There is no acceleration
What is weight?
The force that gravity pulls an object down with
What is the equation for weight (N)?
Weight (N) = mass (kg) x gravitational field strength, g (N/kg)
W=mg
What is the gravitational field strength on earth?
g = 9.8 N/kg
What is the gravitational field strength on the moon?
g = 1.6 N/kg
What does work done mean?
Energy transferred by a force
What happens to the force of you hold an object or lift at a constant speed?
You must be pushing upwards with a force that’s the same size as it’s weight
What is elastic deformation?
When an object returns to it’s original shape after being deformed
What is Hooke’s law?
Force and extension are directly proportional
What is the equation involving Dorcas, spring constant and extension?
Force (N) = spring constant (N/m) x extension (m)
F = ke
What does a force, extension graph look like?
Straight line through the origin: (directly) proportional relationship
What does the gradient on a force extension graph show?
Spring constant
What is the equation to find elastic potential energy?
R = 1/2 ke^2
What are the steps involved in the stretching a spring practical?
-Add masses into spring to change force (e.g. 100g = 1N)
-Measure extension with ruler at each weight, plot F against e
-Gradient of graph = spring constant
-Avoid systematic error by lining up 0cm with bottom of spring
-Avoid parallax error by getting on eye level when measuring
What is the equation to find the moment?
Moment (Nm) = Force (N) x distance from pivot (m)
What is the principle of moments?
When the sum of (total) clockwise moments = sun of anti-clockwise moments, a system is in equilibrium (it will not turn)
What creates pressure underwater?
The weight of the column of water above an object pushing down on it
How can gears be used to increase moments?
By driving a large gear with a smaller one (they work on ratios: double the diameter, double the moment or ‘torque’)
What is the equation for pressure underwater?
Pressure (Pa) = height (m) x density (kg/m^3) x g (N/kg)
P = hpg
What creates pressure in gas?
Particles colliding with the walls of it’s container, exerting a force on them
How do we increase pressure in a gas?
-Decrease volume
-Add more gas
-Increase temperature
What happens to the atmosphere in higher altitudes?
The atmosphere becomes less dense, meaning there is lower pressure
What is the equation for pressure?
Pressure (N/m^2) = Force (N) / Area (m^2)
What does the gradient of a distance/displacement-time graph show you?
The speed/velocity
What does the gradient of a speed/velocity-time graph show you?
The acceleration
How do you work out speed or velocity?
Speed or velocity (m/s) = distance or displacement (m) / time (s)
v = d/t
How do you work out acceleration?
Acceleration (m/s^2) = change in velocity (m/s) / time (s)
a = v/t
What does the area under the graph give us?
The displacement
What is the average walking speed?
1.5 m/s
What is the average running speed?
3 m/s
What is the average cycling speed?
6 m/s
What is Newton’s 1st law?
If no resultant force acts on an object, it’s motion will be constant (velocity will not change)
What is Newton’s 2nd law?
F = ma
Force (N) = mass (kg) x acceleration (m/s^2)
What is Newton’s 3rd law?
For every action (force), there is an equal and opposite reaction (force) (this is NOT balanced forces)
What is the acceleration practical?
-Accelerate trolley on track with slotted mass on string over pulley
-Use light/photogates to measure acceleration
-Change force by removing masses and placing on trolley (to keep the total mass the same)
-Plot F against a, gradient = total mass
What is inertia?
The tendency for an object’s motion to stay constant if no F
What is the stopping distance equal to?
Thinking distance + braking distance
What are 5 things that the thinking distance can be affected by?
-Speed
-Distractions
-Alcohol
-Drugs
-Tiredness
What are 3 things breaking distance can be affected by?
-Speed: if vx2, braking distance x4 as Ek = 1/2mv^2
-Condition of brakes, tyres & road
-Weather conditions
What is the equation linking momentum, mass and velocity?
momentum (kgm/s) = mass (kg) x velocity (m/s
momentum = mv
What happens to the total momentum in any collision?
It is always conserved
What happens to the momentum when it rebounds?
m1u1 = m1v1 +m2v2
What happens to the momentum when coupling?
m1u1 = mv
What happens to the momentum when it recoils?
0 = m1v1 + m2v2
What can force measure with regards to momentum?
The rate of change of momentum
What is the equation for change in momentum?
F = ma = m🔺v/t = 🔺(mv)/t = change in momentum / time
What happens to the force when the time over which momentum is lost or gained increases?
The force decreases
What are 3 things that increase collision time?
-Seat belts
-Air bags
-Crumple zones
Momentum is lost over a longer time so the force is reduced
What does s stand for in Newton’s equations of motion?
Displacement (m)
What does u stand for in Newton’s equations of motion?
Initial velocity (m/s)
What does v stand for in Newton’s equations of motion?
Final velocity (m/s)
What does a stand for in Newton’s equations of motion?
Acceleration (m/s^2)
What does t stand for in Newton’s equations of motion?
Time (s)
What is the initial velocity (u) if starting at rest?
0m/s
What is the final velocity (v) if stopping?
0m/s
What is the acceleration (a) if the only force involved is gravity?
9.8m/s^2
v =
u+at
s = (with final velocity)
(u+v/2)t
s = (with acceleration)
ut+1/2at^2
v^2-u^2 =
2as
