P5 - Forces Flashcards
Define a Force
A force is a push or pull that acts on an object due to the interaction with another object.
Define contact force
The objects are physically touching
Define non-contact force
The objects are physically separated
Name 4 examples of contact forces
Friction
Air resistance
Tension
Normal contact force
Name 3 examples of non-contact forces
Gravitational force
Electrostatic force
Magnetic force
Define scalar quantity and give examples
Scalar quantities have magnitude only.
(Mass, Time, temperature, speed, energy, distance)
Define vector quantity and give examples
Vector quantities have magnitude and an associated direction.
(Displacement, weight, force, velocity, acceleration, momentum)
When using an arrow to represent a vector quantity, what does the length and direction of the arrow represent?
Length - represents magnitude.
Direction - represents the direction of the vector quantity.
Define weight
The force acting on an object due to gravity.
Define centre of mass
The weight of an object can be considered to act at a single point.
What is the equation to calculate the weight of an object?
Weight (N) = mass (kg) x gravitational field strength (N/kg)
The weight of an object is (…) to the mass of the object
Directly proportional
Define resultant force.
A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together.
Define work done.
Work is done on an object when a force causes an object to move through a distance.
What is the equation for work done by a force on an object?
Work done (J) =
force (N) x distance (m) (moved along the line of action of the force)
What does work done against the frictional forces acting on an object cause?
A rise in the temperature of the object
Describe elastic deformation.
Elastic materials will always return to their original length or shape if we take away the forces acting on them.
Describe inelastic deformation.
When inelastic materials are stretched, they do not return to their original length or shape when the forces are removed.
What is the equation for calculating the force needed to stretch or compress an object?
Force (N) = spring constant (N/m) x extension (m)
Describe the energy transfers taking place when an elastic object undergoes stretching, compression or bending.
When we stretch or compress an elastic object, we are using a force to do work.
Elastic potential energy is stored in the object.
The work done is equal to the elastic potential energy, however this is only true if the object is not inelastically deformed.
What is meant by a ‘moment’
The turning effect of a force
Unit for a moment
Newton-metre (Nm)
A balanced moment is…
When the total clockwise moment about a pivot = the total anti-clockwise moment about that pivot
Use the idea of moments to explain why unbalanced objects may topple
When an object is titled, there comes a
point where -
line of action of the weight lies outside the base of the object.
There’s now a resultant (unbalanced) moment acting on the block, causing it to topple.
How can a lever be used as a force multiplier?
If distance between effort & pivot is greater than distance between pivot & load, the force applied on the load is greater than the effort force.
This is because the moment on the effort & load must be the same.
What determines the moment of a gear wheel?
The size of the wheel
Describe the moment & speed of a larger gear being driven by a smaller gear
The larger gear will rotate more slowly but produce a larger moment
A fluid is…
A liquid or a gas
What angle do the forces due to pressure act on a given surface?
At right angles (normal) to the surface
Explain why atmospheric pressure decreases with an increase in height
• Pressure is created by collisions of air molecules
• Quantity of molecules decrease as height increases
• This means atmospheric pressure decreases with an increase of height
What is the earths atmosphere?
A thin (relative to the magnitude of the Earth) layer of gas surrounding the earth
What factors influence floating & sinking?
• Upthrust
• Weight
• Density of liquid
How does pressure in liquids increase with depth?
As depth increases, there is a greater weight of liquid acting downwards
Why does pressure in liquids increase with density?
Liquids with greater density have a greater weight acting downwards
Why are objects in a liquid subject to an upthrust?
There is a higher pressure below the object than above it, as there is more weight above it at the bottom than at the top.
This leads to an upward force called upthrust.
You float or sink depending on whether the upthrust is more or less than your weight.
Describe the relation between size of upthrust & weight of water displaced by object
Size of the upthrust acting on the object is the same as the weight of water displaced by the object.
Describe the difference between distance & displacement
• Distance:
How far an object moves - Scalar quantity - no direction
• Displacement:
Distance moved in a straight line from start point to finish point - Vector quantity
State what is meant by speed
The speed of an object tells us the distance travelled by an object in a given time
The (…) of a moving object is (…)
1) Speed
2) Rarely constant
Walking speed:
1.5m/s
Running:
3m/s
Cycling:
6m/s
Car on main road:
13m/s
Fast train in UK:
50m/s
Cruising aeroplane:
250m/s
Speed of sound:
330m/s
What is meant by velocity?
Velocity of an object is its speed in a given direction
Why is velocity a vector quantity?
It included both magnitude & direction
Explain why motion in a circle involves constant speed but changing velocity
Even though the speed is constant, the direction is constantly changing, meaning the velocity is constantly changing.
Describe what is meant by acceleration
The acceleration of an object tells us the change in its velocity over a given time
Describe what happens to an object falling through a fluid
The object initially accelerates due to the force of gravity.
As the object falls, it experiences an upward force of friction with the air particles.
Eventually the resultant force will be zero and the object will move at its terminal velocity.
Newton’s first law of motion:
1) Stationary objects
2) moving objects
1) If resultant force acting on a stationary object is 0, then the object remains stationary
2) If resultant force acting on a moving object is 0, the object will continue moving in the same direction at the same speed (same velocity)
Describe Newton’s second law of motion
The acceleration of an object is proportional to the resultant force acting on the object & inversely proportional to the mass of the object.
Describe what is meant by inertial mass
• A measure of how difficult it is to change the velocity of an object
• Inertial mass is defined as the ratio of the force needed to accelerate an object over the acceleration produced
What is inertia?
Objects will stay stationary or continue moving at the same speed & direction unless a resultant force is applied
Describe Newton’s third law of motion
Whenever two objects interact, the forces they exert on each other are equal & opposite.
What is meant by the stopping distance of a vehicle?
The distance the vehicle travels during the driver’s reaction time (thinking distance) & the distance it travels under the braking force (braking distance).
Factors that affect thinking distance
• Tiredness
• Alcohol & certain drugs
• Distractions in the car
Factors that affect braking distance
• Wet or icy conditions - increase braking distance
• Worn tyres
• Worn brakes
Describe the energy changes taking place when a vehicle brakes
• Brake presses against wheel - kinetic energy converts to thermal energy - temp of brakes increases
• Car slows as kinetic energy lost
What happens if a car travelling at high speeds needs to brake
• Large braking force has to be applied - car decelerates rapidly but a large amount of kinetic energy is transferred to thermal energy in the brakes.
• Brakes can overheat/ driver to lose control of the vehicle.
Key facts of momentum are:
• All moving objects have momentum.
• Objects that are not moving have 0 momentum.
What is Conservation of momentum?
In a closed system, the total momentum before an event is equal to the total momentum after an event.
Describe how safety features such as car seatbelts reduce the dangers of changes in momentum
The safety feature slows down momentum change which reduces the forces acting & reduces the risk of serious injury.
