Forces, Energy and Power Flashcards
Describe what is meant by ‘friction’.
A force which opposes the motion of an object.
Name one type of friction.
Air resistance/drag
If two forces act in the same direction, should you add or subtract them to find the resultant force?
Add
If two forces act in opposite directions, should you add or subtract them to find the resultant force?
Subtract
Describe what is meant by an object having ‘balanced forces’ acting on it.
The object has the same force acting on it in two directions.
Draw a free-body diagram of a block with balanced forces acting on it.
Any object with the same value of force on opposing sides.
Describe what is meant by an object having ‘unbalanced forces’ acting on it.
The object has a larger force in one direction than the other.
Draw a free-body diagram of a block with unbalanced forces acting on it.
Any object with different values of force on opposing sides.
Define ‘Newton’s 1st Law’.
An object will remain at rest or move at a constant speed in a straight line unless acted on by an unbalanced force.
What can be said about the forces acting on an object if it moves at a constant speed?
The forces are balanced.
In terms of Newton’s 1st Law, describe what would happen to a passenger not wearing a seatbelt in the event of a crash.
They would continue to move forward, at the original speed before the crash, into the dashboard/steering wheel etc.
Define ‘Newton’s 2nd Law’.
When the forces acting on an object are unbalanced, the object will accelerate in the direction of the unbalanced force.
Write down the equation relating unbalanced force, mass and acceleration.
Fu=ma
State the symbol and units for unbalanced force.
F, N
State the symbol and units for mass.
m, kg
Draw a free-body diagram to show the four forces acting on an aeroplane.
Weight, drag, thrust, lift
Define ‘Newton’s 3rd Law’.
If an object A exerts a force on object B, then B exerts an equal but opposite force on A. OR For every action there is an equal and opposite reaction.
The action and reaction forces for Newton’s 3rd Law are also known as ___________ ________.
Newton pairs
Give two examples of newton pairs (i.e. action and reaction forces).
Force of driver on seatbelt and force of seatbelt on driver/balloon pushing air out and air pushing back on balloon/rocket engines pushing hot gases downwards and hot gases pushing back on rocket/cannon firing cannonball outwards and cannonball exerting a force back on cannon/force of foot kicking ball and force of ball back on foot
When a car crashes, the driver moves against the seatbelt, exerting a force on it (action force). State the reaction force for this newton pair.
The seatbelt locks in place and exerts a force back on the driver.
When a rocket launches, the rocket engines push hot gases downwards (action force). State the reaction force for this newton pair.
Hot gases push back on the rocket (upwards).
Explain, in terms of forces, why a rocket will eventually move upwards during launch.
When the thrust from the engines becomes greater than the weight, an unbalanced force upwards is produced, causing the rocket to accelerate upwards.
As a rocket travels upwards, what happens to the air resistance acting on it as its speed increases?
It increases
As a rocket uses up fuel, what happens to its weight?
It decreases
As a rocket gets further away from the Earth’s surface, what happens to its weight?
It decreases
Draw a free-body diagram to show the forces acting on a rocket during launch.
Drag, thrust, lift, weight
Describe what is meant by a ‘connected system’.
When two or more objects are physically joined in some way.
Describe what is meant by ‘tension’.
The pulling force of a string, rope, cable or chain.
If a mass is suspended (hanging) from a point via some string, in which direction will the tension in the string act?
Towards the point it is being suspended from.
Describe what is meant by ‘free-fall’.
An object that is accelerating towards the ground due to the influence of gravity alone.
Explain how a skydiver in free-fall will reach terminal velocity.
When a skydiver jumps out of a plane, they are in free-fall. Air resistance/drag increases as they accelerate downwards, and eventually the weight of the skydiver downwards and the drag force upwards become balanced. The skydiver now travels at a constant speed i.e. terminal velocity.
Define the term ‘terminal velocity’.
For an object in free-fall, a constant speed is reached when the upward force on the object (air resistance) is balanced by the downward force on the object (weight).
Sketch a velocity-time graph for a free-falling skydiver. No numerical values are required on the axes.
Axes with a sloping acceleration till it reaches a constant speed then a rapid deceleration till steady speed.
Describe what is meant by ‘mass’.
The quantity of particles that make up an object. It is a scalar quantity.
Describe what is meant by ‘weight’.
The force due to gravity acting on an object. It is a vector quantity.
Explain the difference between mass and weight, including their units.
Mass is the quantity of particles that make up an object, measured in kilograms, whereas weight is the force due to gravity acting on an object, measured in newtons.
Write down the equation relating weight, mass and gravitational field strength.
w=mg
State the symbol and units for weight.
W, N
State the symbol and units for gravitational field strength.
g, N/kg
Name the two forces acting on a person who is standing on a set of scales in a lift.
Weight, reaction force
For a person standing on a set of scales in a lift, what is the reaction force equivalent to?
The reading on the scales.
Sketch a free-body diagram for a person standing on a set of scales in a stationary lift.
