P5-Forces Flashcards
What is displacement?
A distance in a given direction
What is a vector quantity?
A physical quantity that has both a magnitude and direction
What is a scalar quantity?
A physical quantity that has magnitude but no direction
What are some examples of vector quantities?
- Force
- Displacement
- Velocity
- Acceleration
- Momentum
What are some examples of scalar quantities?
- Temperature
- Mass
- Energy
- Distance
- Speed
- Density
How can a vector quantity be represented?
Vector quantities can be represented by an arrow in the direction of the vector and of length in proportion to the magnitude of the vector
What are examples of contact forces and how do they work?
- Reaction force-An object at rest on a surface experiences reaction force
- Tension-An object being stretched
- Friction-Experienced by 2 objects sliding past each other
- Air resistance-An object moving through the air experiences air resistance
- Bouyancy-The force of water pushing an object upwards
- Spring force-The force built up by compressing or stretching a spring
- Applied force-Force exerted on an object by another object
What are examples of non-contact forces and how do they work?
- Magnetic force-experienced by any magnetic material (iron,cobalt,nickel) in a magnetic field
- Electrostatic force-experienced by any charged particle in an electric field
- Gravitational force-experienced by any mass in a gravitational field
What is Newton’s third law of motion?
When two objects interact with each other, they exert equal and opposite forces on each other.
What is the unit of force?
Newton (N)
What is a resultant force?
A single force that has the same effect as all of the forces acting on an object
What is Newton’s first law of motion?
If the forces acting on an object are balanced, the resultant force of an object is 0 and:
* If the object is at rest it stays stationary
* If the object is moving, it keeps moving with the same speed and in the same direction
Give 3 examples of unbalanced forces?
- A plane taking off-the thrust force of the engines is greater than the force of air resistance (drag)
- A car driver applying the brakes-the braking force is greater than the force from the engine
- A drop of water accelerating down a waterfall-the air resistance isn’t as strong as gravity
What diagram shows the forces acting on an object?
A free-body diagram
What is the turning effect of a force called?
The moment of the force
What are a crowbar and lever examples of?
Force multipliers
Explain how a force multiplier works
If the pivot is closer to the load than the effort, the force exerted will be multiplied by the length of the distance from the effort to the pivot
What is the equation of the moment of a force?
Moment = Force x Perpendicular distance from the pivot
M = F x d
What are the units for distance?
Metres, m
What are the units for moments?
Newton Metres (Nm)
How do you increase the moment of the force?
By increasing the force or distance
Finish the sentence:
When a lever is used to increase or multiply a force, the force applied to the lever must act …………. from the pivot than the force it has to overcome.
Further
How can gears be used to change the moment of a turning effect?
By having a large gear wheel driving a small gear wheel, the force needed to drive the small wheel is reduced.
What is the centre of mass of the object?
The point where the mass can be thought of as being concentrated
What happens to an object when it is freely suspended?
It comes to rest with its centre of mass directly underneath the point of suspension-to return it to equilibrium
Where is the centre of mass of a symmetrical object?
Along where its axis of symmetry cross
What is the principle of moments?
The sum of all the clockwise moments about any point = the sum of all the anticlockwise moments about that point
What is the parallelogram of forces used to find?
The resultant of 2 forces that don’t act along the same line
How do you resolve a force in 2 perpendicular directions?
By drawing a rectangle with adjacent sides along the two direactions so that the diagonal represents the force vector.
What is the formula for speed?
Speed = distance travelled / time taken
v = s / t
What is the distance time graph for an object that is stationary?
A horizontal line
What is the distance time graph for an object that is moving at a constant speed?
A straight line that slopes upwards
What is the distance time graph for an object that is accelerating?
A line that curves upwards-convex
What is a distance time graph for an object that is accelerating?
A line that curves upwards-concave
What is the difference between speed and velocity?
Velocity is speed in a given direction
e.g. 2 cars travelling at the same speed in different directions would not be said to have the same velocity
Would an object moving round in a circle have a constant velocity?
No-since the direction is constantly changing
What is the definition of displacement?
The distance travelled in a given direction
What is the formula for acceleration?
acceleration = change in velocity / time taken for the change
a = Δv / t
What are the units for acceleration?
Metres per second squared
m/s²
What is deceleration?
The change in velocity when an object slows down
What is the velocity time graph for an object that is moving at a constant speed?
A straight line
What is the velocity time graph for an object that is accelerating at a constant rate?
A line angled upwards
What is the velocity time graph for an object that is decelerating at a constant rate?
A line angled downwards
What does the area underneath the line on a velocity time graph represent?
Distance travelled
How do you find the speed of an object on a certain point of a distance-time graph?
By drawing a triangle under the line and then calculating the gradient of the tangent
What is the equation linking final velocity, initial velocity, acceleration and distance travelled?
Final velocity² - Initial velocity² = 2 x acceleration x distance travelled
What is Newton’s second law?
The acceleration of an object is:
proportional to the resultant force on the object
inversely proportional to the mass of the object
What is the formula that explains Newton’s second law?
resultant force = mass x acceleration
F = m x a
What is inertia?
The tendancy of an object to stay at rest or to continue in uniform motion
What is the inertial mass of an object? (definition+formula)
A measure of the difficulty of changing the object’s’ velocity
Force / Acceleration
What would happen if the resultant force of an object is in the opposite direction to its velocity?
It would decelerate
What is the difference between weight and mass?
The weight of an object is the force acting on it due to gravity while the mass of the object depends on the quantity of matter in it
What is the formula for weight?
weight = mass x gravitational field strength
W = m x g
What are the units for gravitational field strength?
newtons per kilogram, N/kg
What is the acceleration of an object due to gravity?
10m/s²
What is terminal velocity?
The velocity reached when the resultant force is 0 and the frictional force is equal and opposite to its weight
What happens to the acceleration of an object when it falls?
It decreases
What does the stopping distance of a vehicle comprise of?
The thinking distance (distance travelled by the vehicle in the time it takes the drive to react) as well as the braking distance (distance travelled by the vehicle during the time the braking forces act)
What are and explain factors that affect thinking distance?
- Tiredness, alchohol and drugs affect the brain and increase reaction times
- Distractions such as using a mobile phone also increase reaction time
- The speed of the vehicle
What are and explain factors that affect braking distance?
- The road conditions as on wet and icy roads drivers will have to brake with less force to avoid skidding
- If the brakes or tyres are worn it may take longer to stop-these are less effective
- The mass of the vehicle directly increases the force needed to stop it and therefore increase braking distance
- The speed of the vehicle-the faster the vehicle is travelling the longer it will take to brake
What happens to the braking distance if the speed of an object would double?
It would quadruple
What formula can be rearranged to calculate the deceleration of a vehicle?
v² - u² = 2 x a x s
How do you work out the braking force?
F = m x a
What is the formula for momentum?
momentum = mass x velocity
p = m x v
What are the units for momentum?
kg m/s
What is the law of the conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after an event
What happens when two objects push each other apart?
They move in different speeds if they have unequal masses, and with equal and opposite momentum
What safety features in a car are designed to lessen the force of an impact?
Crumple zones of the front and back end of a car-this increases the impact time and decreases the impact force
How do you calculate the impact force of an object with its mass, change in velocity and time taken?
force = (mass x change in velocity) / time taken
F = (m x Δv) / t
How are the following designed to reduce impact forces?
* Safety helmets
* Car seats
* Playgrounds
Safety helmets-By cushioning the head a longer impact time is created resulting in the helmet reducing the force of the impact
Car seats-Seat belts stop people in cars slower and it acts across the chest so spreads the force out. Air bags increase impact time as well as spread the force out across someone’s body
Playgrounds-These are fitted with cushiones surfaces underneath swings in case children fall off-this reduces impact time and overall force
What is an elastic object?
Any object that returns to its original shape after removing the force deforming it
What is the equation for Hooke’s law?
force applied = spring constant x extension
F = k x e
What happens to an object when it has been stretched beyond its limit of proportionality?
It has been inelastically deformed and won’t go back to its original shape when all forces are removed-non-linear relationship
What is pressure?
The force per unit area
What is the pascal equal to?
One newton per square metre (N/m²)
What is the formula to find pressure?
pressure = force / area
p = F / A
What are the units for pressure?
pascals (Pa)
What happens to the pressure in a liquid when the depth increases?
It increases
What is the formula for pressure in a liquid column?
pressure = height x density x gravitational field strength
What are the units for density?
kilograms per cubic metre
kg/m³
How is atmospheric pressure caused?
Air molecules colliding with surfaces
How does a rubber suction cap work?
The atmospheric pressure acts on the outside of the cap as all of the air in the cap is squeezed out
What happens to atmospheric pressure at high altitude?
Atmospheric pressure decreases
What is the atmospheric pressure at sea level?
100kPa
What is the formula for atmospheric pressure?
pressure = height x density x gravity
p = hpg
What is the upwards force of water on the body?
Upthrust
What causes an object to float?
If its weight is equal to the upthrust
What would cause an object to sink?
If its weight is greater than the upthrust on it when fully immersed
