FORCES AND MOTION Flashcards
(30 cards)
What can forces change/transfer?
- Forces can change the SPEED of objects – either getting it to start moving, making it stop or making it go faster or slower.
- Forces can change the DIRECTION of objects - stop it moving in a straight line, or getting it to deflect/bend away from the direction it was going.
- Forces can change the SHAPE or SIZE of objects – stretching, bending, twisting or squashing.
- Forces transfer energy.
Contact forces?
-MECHANICAL – pushes or pulls, causing objects to move or change speed, often called THRUST.
-DRAG – a force that slows objects down – which can be called FRICTION, AIR RESISTANCE or WATER RESISTANCE.
-UPTHRUST – the force that pushes up from water.
-NORMAL or REACTION – the perpendicular force
that pushes up from the ground or solid surface.
-TENSION – the ELASTIC force in a rope or cable
Non-contact forces?
- WEIGHT or GRAVITATIONAL – the force due to gravity causing objects to be pulled downwards
- ELECTROSTATIC – the force between two electrically charged objects (which can be a force of attraction between unlike charges or repulsion for like charges)
- MAGNETIC – the force between two magnetic objects (which can be a force of attraction between unlike poles or repulsion for like poles)
Friction?
• Friction is a force that opposes motion.
• Friction from fluids (such as liquids or gases, like air) is
normally called resistance or ‘drag’.
• Air resistance and water resistance are examples of these.
• By making the shape of moving objects streamlined, the size of these forces can be reduced.
• Friction (all types) always INCREASES as speed INCREASES
What is a scalar and an example of it?
-A scalar quantity only has MAGNITUDE (or size)
- volume
- density
- speed
What is a vector and examples?
- A vector quantity has MAGNITUDE and DIRECTION
- A VECTOR can be represented by an ARROW
-FORCE is a VECTOR QUALITY – it has a size (magnitude) and direction
- force
- position
- velocity
Speed equation?
Speed = distance/time
Velocity equation?
Velocity= displacement/time
Acceleration equation ?
-Acceleration=change in velocity/time
- a= acceleration (m/s2)
- u= initial velocity (m/s)
- v = final velocity (m/s)
- t = time (s)
What does a steeper straight gradient show on a velocity time graph?
A faster constant acceleration
What does a flat gradient show on a velocity time graph?
Constantly velocity (zero acceleration)
What does a negative gradient show on a velocity time graph?
Negative acceleration or deceleration
Resultant force?
- The overall effect of a lot of forces acting together – is called the RESULTANT FORCE
- It is the one single force that would affect the object the same way as all the other forces combined
- Forces in the SAME direction ADD UP
- Forces in the OPPOSITE direction SUBTRACT from each other
What is Newtons 1st law?
• An object will remain at rest, or continue to move at constant velocity, unless it is acted upon by an external force.
Newtons 1st law explanation?
• If the resultant force acting on an object is zero and:
- the object is at rest - the object remains at rest.
- the object is moving - the object continues to move at the at the same velocity (the same speed and the same direction)
• If the resultant force is non-zero, you get a change in velocity (it changes speed and/or direction) – whether it was at rest before that or was already moving
Balanced forces?
-When the resultant force is zero – the forces acting on the
object are ‘balanced’ or in ‘equilibrium’
-No change in velocity (speed in a certain direction) takes place e.g,
A stationary object stays stationary
A moving object stays moving at the same velocity
Unbalanced forces?
-When the resultant force is non-zero – the forces acting on the object are ‘unbalanced’
- A change in velocity takes place (acceleration)
e. g, A stationary object begins to move (accelerate)
-A moving object starts to accelerate in the direction of the resultant force
Newtons 2nd law?
- When a force acts on an object, the object accelerates in the direction of the force.
- The size of the acceleration depends on the size of the force and the mass of the object
Force equation?
-Force = mass x acceleration
- F = force (N)
- m = mass (kg)
- a = acceleration (m/s2)
Weight equation?
-weight=mass x gravitational field strength
- w=weight (N)
- m=mass (kg)
- g=gravitational field strength (N/kg)
Terminal velocity?
- Any object moving through a fluid (such as air) experiences a resistive force opposite to its motion. This is called air resistance.
- The greater the speed of motion – the greater the air resistance force.
- When air resistance is equal and opposite to the force causing the motion – the object has balanced forces (no resultant force) and it is now at a constant velocity – called the TERMINAL VELOCITY
- A skydiver reaches this when their weight (W = mg) moving them down, is balanced by the air resistance (FAIR) acting up!
Sky diver?
- At first a skydiver increases in velocity as his weight is greater than the air resistance opposing his motion
- As he speeds up, the air resistance increases
- When he reaches terminal velocity his weight (downwards force) is balanced by the air resistance (upwards force) – he is now at a constant velocity!
Parachutes?
• The large surface area of a parachute provides lots of air resistance!
• When opened, the upwards force will be much greater than the downwards force (weight) – so the skydiver slows down
• As they slow down, air resistance FAI
reduces – until we have balanced forces again – but at a much lower terminal velocity - about 30 km/h (around 18 mph)
• They stop on the ground, as weight down = normal/reaction force up!
Stopping distance equation?
Stopping distance=thinking distance + braking distance
