P5 - Forces Flashcards
What is a scalar?
A magnitude.
What is an example of a scalar?
Mass, speed, distance.
What is a vector?
A magnitude and a direction.
What is an example of a vector?
Velocity, weight, acceleration.
What is a contact force?
A force that only occurs when objects are in contact.
What is a non-contact force?
A force that doesn’t require contact for a force to be acted on it.
What is the resultant force?
The sum of all the forces acting on an object.
How do you calculate the force acting on a spring?
Force (N) = spring constant (N/m) x extension (m)
What is the limit of proportionality?
The point at which Hooke’s law no longer occurs and the length of the spring gradually increases without a force being added.
What is Hooke’s law?
Weight is proportional to the amount an object is being stretched.
What is elastic/plastic deformation?
When an object is stretched beyond the limit of proportionality, so that it can’t return to it’s original state.
What is inelastic deformation?
When an object is stretched within the limit of proportionality, so that it can return to it’s original shape.
What is a moment?
The turning effect caused by applying a force at a distance from the pivot point.
How do you calculate a moment?
Moment (N/m) = force (N) x distance (m)
What distance measurement is used when calculating a moment?
The perpendicular distance from the pivot, to the line of action from the force.
What is an equation that links pressure, force and area?
Pressure = force / area
What equation shows the magnitude of pressure in liquids at different depths?
Pressure = column height x density of liquid x gravitational field strength
What happens when the upthrust = weight of object?
Up and down forces are balanced. Object will stay still.
What happens when upthrust is less than the weight of the object?
Down force is greater than up force. Object will sink.
Explain upthrust in terms of density.
It is more dense towards the bottom.
More particles are colliding with object.
More force exerted on object. Object forced up.
What is an average speed?
The speed of an object measured over the whole journey.
What is an instantaneous speed?
The speed of an object at the very instance of being measured.
How do you calculate average speed?
Average speed = distance travelled / time taken
What does the gradient of a distance time graph give you?
The speed.
What is the symbol equation for calculating speed?
v = s / t
What is the word equation for calculating speed?
speed = distance / time
What is the equation for acceleration?
Acceleration = change in velocity / time taken
If an object is travelling in a circular motion at a constant speed is it accelerating?
Yes, because the change in direction is a change in velocity.
What is acceleration?
The rate of change of velocity with time.
Describe the changes of velocity a person experiences when skydiving.
Person accelerates (weight is larger than drag). Air resistance increases as person gains speed. Air resistance matches weight so the person reaches a terminal velocity.
Parachute pulled.
Drag more than weight. Person decelerates. Second terminal velocity reached, as person travels slower so drag gets smaller.
What is Newton’s first law?
If resultant force is zero, the forces acting on it are balanced. Meaning acceleration will be zero and it’s velocity will be constant. (stationary or moving)
What is Newton’s second law?
resultant force = mass x acceleration
F = m x a
Meaning the acceleration of an object depends on its mass and the force acting on it.
What is Newton’s third law?
When an object exerts a force on another object, the second object will exert the same amount of force as the first object.
The forces they exert are equal and opposite.
How do you calculate stopping distance?
Stopping distance = thinking distance + braking distnace
What factors effect thinking distance?
Drugs, alcohol, tiredness
What factors effect braking distance?
Road surface, tire quality, brake quality, mass
What factor effects both braking and thinking distance?
Speed
What is momentum?
A property of moving objects. It is a vector. The greater the velocity and mass the greater the momentum.
How do you calculate momentum?
momentum (kg m/s) = mass x velocity
What is the law of conservation of momentum?
In a closed system, the overall momentum before an event is the same as the overall momentum after an event.
How do safety features help reduce the force of impact?
They increase the time it takes to change momentum
What is an elastic collison?
No loss of kinetic energy or momentum.
What is an inelastic collision?
Energy is transferred to other forms of energy, momentum is not lost.
What is the centre of mass?
The single point through which the weight of the object can be said to act.
What equation links force acting on an object, change in velocity, mass of object and time it takes for object to change velocity?
F = m x change in velocity / time taken
How do safety features such as helmets impact momentum?
They increase the time it takes to change momentum.
What is a force?
A push or pull acting on an object due to an interaction with another object.
Give 3 examples of contact forces.
Air resistance.
Friction.
Tension.
Give 3 examples of non-contact forces.
Gravitational forces.
Electrostatic forces.
Magnetic forces.
Is force a vector or scalar quantity?
Vector - as it has both a magnitude and a direction.
Give 3 examples of vector quantities.
Force.
Displacement.
Velocity.
What is weight?
A force that acts on an object due to gravity and an objects mass.
How do you calculate weight?
Weight = mass x GFS
What is the unit used for weight?
Newtons (N)
What is the unit used for GFS?
N/kg
What piece of equipment can be used to measure an object’s weight?
A calibrated spring balance or a newton-meter.
What does it mean if a force is said to do work?
The force causes an object to be displaced through a distance.
How do you calculate work done?
Force x distance
Under what circumstances is one joule of work done?
When a force of one newton causes a displacement of 1 metre.
How many Newton-metres are equal to 1 joule of energy?
1 Nm = 1 J
What occurs when work is done under frictional forces?
A rise in temperature of the object occurs as kinetic energy is converted into heat.
Why does air resistance slow down an object?
The object does work against the air resistance. Kinetic energy is converted into heat, slowing down the object.
State the equation linking force spring constant and extension.
Force = spring constant x extension
What can extension be replaced with in the equation for spring force?
Compression
If an object is in equilibrium what can be said about the moments acting on the objects?
The clockwise moments is equal to the anti-clockwise moments.
What 3 parts make up a lever system?
Load, effort, pivot
Give an example of when a lever is used to multiply a force.
A wrench - has a long handle so the force applied by the user is multiplied.
What determines the moment of a gear wheel?
The size of the wheel
Describe the speed and moment of a larger gear being driven by a smaller gear.
The wheel will rotate more slowly but the moment produced will be larger.
In what unit do you measure moments in?
Nm (newton-metres)
Why do longer spanner’s work better?
Because there is a bigger distance between your force and the pivot so a larger moment is produced.
In a fluid, at what angle do the forces due to pressure act on a surface?
At right-angles
What happens to density of the atmosphere as altitude increases?
Becomes less dense
Why does atmospheric pressure decrease with an increase in height?
Pressure is created by collisions of air molecules.
Less air molecules means less collisions.
Meaning pressure decreases.
How does pressure in fluids increase with depth?
As depth increases, the mass of liquid above that depth increases.
Meaning the force due to mass increases.
Resulting in a pressure increase.
Explain upthrust.
Higher pressure on its bottom surface than top surface, causing a resultant force upwards.
What 3 factors influence whether an object will float or sink?
Upthrust
Weight
Density of fluid
Why does the pressure of a liquid increase with the density of a liquid?
Liquids with a greater density have a greater weight acting downwards.
What is needed for an object to float?
The upthrust must equal the object’s weight.
What is needed for an object to sink?
Upthrust is less than the weight of the object.
How does the size of the upthrust relate to the weight of the water displaced?
The size of the upthrust is equal to the weight of the displaced fluid.
Explain why an object less dense than water floats.
The object only has to displace a small volume of water before the weight of the water displaced equals the weight of the object.
So, the upthrust then equals the weight of the object and the object floats.
The objects sits high in the water, as only a small amount of water is displaced.
Explain why an object with the same density as water floats while touching the surface of the water.
Object displaces it’s own volume of water, in order for the weight of the object to equal the weight of the water displaced.
So therefore, the weight of the object is equal to the upthrust and it floats.
Explain why an object more dense than water sinks.
The object cannot displace a volume of water that is equal to it in weight. So, there is not enough volume displaced to produce an upthrust larger than the object’s weight.
Is displacement a vector or scalar quantity?
Vector - as it also involves a direction
What is the typical value for the speed of sound?
330 m/s
What is the typical value for human walking speed?
1.5 m/s
What is the typical value for human running speed?
3 m/s
What is the typical value for human cycling speed?
6 m/s
Link distance, speed and time in an equation.
Distance = speed x time
Why can an object travelling a constant speed in a circle not have a constant velocity?
Speed is a scalar quantity. Velocity is a vector quantity which means it can only be constant if the direction is constant. Direction is constantly changing in a circle.
How can speed be calculated in a distance-time graph?
Speed is the gradient of the graph.
How do you calculate speed at a given time from a distance-time graph for an accelerating object?
Drawing a tangent at the required time.
Calculating the gradient of the tangent.
What is the resultant force acting on an object when it is falling at terminal velocity?
It is zero. When at terminal velocity the object is moving at a constant speed and so isn’t accelerating.
What can be said about the braking forces and the driving forces when a car is travelling at a constant velocity?
Braking forces are equal to driving forces.
If there is a change in direction but remains at a constant speed is there a resultant force?
Yes, there is a change in direction, therefore a change in velocity, so there must be a resultant force.
What is inertia?
The tendency of an object to continue in it’s state of rest or uniform motion.
What is inertial mass?
A measure of how difficult it is to change an objects velocity.
Give a typical range of values for human reaction time.
0.2 - 0.9 seconds
Describe the energy changes that occur when a car applies it’s brakes.
Work is done by the friction force between the brakes and the wheel.
Kinetic energy of the wheel is converted to heat and is dissipated to the surrounding through the brake discs.
State two consequences of a vehicle undergoing very large decelerations.
Kinetic energy converted to heat is very high causing brakes to overheat.
Loss of control of the vehicle.
State an equation linking change in momentum, force and time.
Change in momentum = force x time
What quantity is equal to the force experienced in a collision?
The rate of change of momentum.
Explain how a seat belt improves a passengers safety during a collision.
It increases the time at which the force is applied, reducing the rate of change of momentum and therefore reducing the force experienced.
What is a eureka can?
Measures displacement of water.
What is spring constant?
Force needed to stretch or compress a spring.
What is it known as when an object accelerates at a constant speed?
Uniform acceleration
How do you know something is directly proportional on a graph?
Line does not curve.
Passes through the origin.
Why is there a higher pressure the deeper you go in water?
Greater volume above.
Increasing weight force acting on you.
Explain the link between weight of water displaced and upthrust.
The upthrust force is equal in size to the weight of the fluid displaced by the object.
