forces Flashcards
What is a scalar quantity?
A quantity that only has magnitude (e.g., speed, distance, temperature)
What is a vector?
A quantity with both magnitude and direction (e.g., displacement, force, velocity)
How do arrows represent vector quantities?
The direction of the vector is the direction that the arrow points and the magnitude of the vector are represented by the length of the arrow
What is a force?
A push or pull that acts on an object due to the interaction with another object
What are contact forces?
Forces that occur when objects are physically touching (e.g., friction, air resistance, water resistance)
What are non-contact forces?
Forces that occur when objects are physically separated (e.g., gravity, electrostatic force, magnetism)
What is weight?
The force acting on an object due to gravity
What is an object’s centre of mass?
The point in an object where its mass is concentrated
How are weight and mass correlated?
They are directly proportional
How is weight measured?
Using a newtonmeter (a calibrated spring-balance)
What is a resultant force?
A single force that has the same effect as all the forces acting on an object
What does it mean when work is done on an object?
When a force causes that object to move, and there is a displacement
How many newton-metres in a joule?
1 Nm = 1 J
What happens when work is done against frictional forces?
The object’s temperature rises
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 Newton’s first law of motion?
If the forces acting on an object are balanced, the resultant force is zero and: if the object is at rest, it remains stationary; if the object is moving, it keeps moving at the same velocity and direction
What is deformation?
A change in shape of an object due to the forces acting on it
What is elastic deformation?
A temporary change in shape of an object, when the object can return to its original shape when the forces are removed
What is inelastic deformation?
When an object’s shape has changed permanently, and it doesn’t return to its original shape when the forces are removed
What is the extension of an object?
When an object increases in length due to a force being applied to it
What is compression of an object?
When the object decreases in length due to a force being applied to it
What is Hooke’s law?
Force = spring constant x extension
How are extension and force applied to an object correlated?
They are directly proportional (before the limit of proportionality is exceeded)
What is the moment of a force?
The turning effect the force causes
How can the moment be increased?
Increasing the size of the force applied; increasing the distance from the pivot
What is the effort?
The force applied by the person to a lever
What is the pivot?
The point about which the lever (e.g., a crowbar) turns
What is the line of action of a lever?
The line that a force acts along
How does a lever work as a force multiplier?
The effort needed to lift the object is a small fraction of the object’s weight because of the distance from the pivot from where the force is being applied
How do gears work as force multipliers?
Gear wheels exert equal and opposite forces on each other where their edges meet, so, if there is a larger gear, the force is acting further from its shaft so the turning effect is multiplied
What happens if a low gear is chosen in a car?
A small gear wheel is used to turn a large gear wheel on the output, so the output shaft turns slower than the original shaft; low gear gives low speed and high turning effect
What happens if a high gear is chosen in a car?
The engine shaft drives a large gear wheel, which turns a small gear wheel on the output shaft, so the output turns faster than the engine shaft; high gear gives high speed and low turning effect
What is the principle of moments?
For an object not moving, the sum of all clockwise moments is equal to the sum of all anticlockwise moments
What is a fluid?
A substance that flows (either a liquid or a gas)
How does the pressure of a liquid correlate with the depth of the liquid?
The pressure increases with depth because a point near the bottom of a container has more particles above it, the weight of which results in the increased pressure
How does density of a liquid affect the pressure of that liquid?
Less dense liquids cause less pressure
What does an object experience if it is submerged in a liquid?
Pressure is greater on its bottom surface than its top surface; this causes a resultant force upwards (upthrust)
What is the atmosphere?
A thin layer of air around the earth, which gets less dense with increasing altitude
What causes atmospheric pressure?
Air molecules colliding with a surface
Why does atmospheric pressure decrease as height increases?
There is less air at higher altitudes, so the number of air molecules (and therefore the weight of air) above the surface decreases, causing less pressure being exerted on that surface
What is distance?
How far an object moves (a scalar)
What is displacement?
The length from the start point of a moving object to the finish point (with its overall direction) - it is a vector
What factors can affect the speed at which a person walks/runs/cycles?
Age, terrain, level of fitness, distance
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 the speed of sound in air?
330 m/s
What is velocity?
An object’s speed in a given direction
How can motion in a circle be described?
Constant speed; changing velocity
How can you calculate speed from a distance-time graph?
Its gradient
How can you calculate acceleration from a distance-time graph?
Its gradient
How can you calculate distance from a velocity-time graph?
The area under it
What is a free-falling object’s acceleration?
9.8 m/s²
What is terminal velocity?
The maximum velocity an object can reach while falling through a fluid, such as air.
How is terminal velocity reached?
Because an object falling through a fluid initially accelerates due to gravitational force, but the resultant force of gravity and air resistance eventually becomes zero and acceleration stops
What is inertia?
The tendency of objects to continue in their state of rest or of uniform motion
What is Newton’s second law?
The acceleration of an object is proportional to the resultant force acting on an object, and inversely proportional to the mass of that object; F = m x a
What is the stopping distance of a vehicle?
The sum of the distance the vehicle travels during the driver’s reaction time (thinking time) and the distance it travels under the braking force (braking distance)
What is the thinking distance of a vehicle?
distance travelled by the car during the driver’s reaction time
what is the reaction time
time taken for the driver to spot the obstruction , make a decision and then move their foot to the brake
what is the braking distance
distance the car travels from when the driver applies the brakes to when the car stops
what two factors make up the stopping distance
thinking distance
braking distance
how does the speed affect the stopping distance
The greater the speed, the greater the stopping distance
What is the average person’s reaction time range?
0.2-0.9 s
What can affect a driver’s reaction time?
Tiredness, drugs, alcohol, distractions
What factors can affect the braking distance of a vehicle?
Poor condition of the vehicle e.g worn tires = reduces friction between tyres and road
- weather conditions (e.g., ice on the streets) - wet or icy conditions lessen the friction between the tyres and the road = increases braking distance
What can a large braking force result in?
Greater deceleration, which can lead to brakes overheating and a loss of control of the vehicle
What happens when force is applied to a vehicle’s brakes?
during braking, the brake presses against the wheel = force of friction now acts between the brake and the wheel= kinetic energy of car is now converted to thermal energy in the brakes = the temperature of the brakes increases - at same time the car slows down as it loses kinetic energy
What is momentum?
the objects Mass multiplied by velocity
What is conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after; when a force acts on a moving object, the object’s momentum changes
How does impact time affect impact force?
The longer the impact time, the more the impact force is reduced
How do safety helmets work?
The helmet increases the time it takes for the person’s head to hit the ground, so if the impact time is increased, the rate of change of momentum is reduced, reducing the impact force
How do seat belts work?
Seat belts are worn across the chest, which spreads the impact force out
what is inertial mass
It is a measure of how difficult it is to change the velocity of an object
the ratio of force over acceleration
