Forces Flashcards
Newtons third law
Every action has an equal and opposite reaction
Weight/mass units
Weight = Newtons (N)
Mass = kg
What is a moment
If a force causes an object to rotate then it has a turning effect. We call this turning effect a moment.
Principle of moments
If an object is balanced then the sum of all the clockwise moments acting on it equals the sum of all the anticlockwise moments acting on it.
Gears
devices that are used to transmit the rotational effects of forces
How do gears work
Cog A will have a certain moment. The force from this moment is transmitted to cog B at the point of contact. Because cog B is a different size (so has a different perpendicular distance from the pivot) the turning effect of cog B will be different to cog A
Why does the moon orbit the Earth
There is a resultant force acting on the moon towards the centre of the earth due to the earth’s gravitational field. Because this is at right angles to the direction the moon is travelling it causes circular motion. There is also the same sized force acting on the earth, due to the moon’s gravitational field.
Walking/ Running/ cycling speeds
Walking 1.5m/s
Running 3 m/s
Cycling 6 m/s
Speed of sound in air
330m/s
Non-uniform motion
Speed isn’t constant
How can things be traveling at constant speed but changing velocity
An object moving around a circular path (e.g. the Earth moving around the sun) can be moving at a steady speed (i.e. covering the same distance each second) but will not have constant velocity because velocity is a vector quantity and the direction of the journey is constantly changing. If velocity changes then the object is also accelerating since acceleration = change in velocity / time.
Newton’s first law
if no resultant force acts on an object then that object’s motion will not change; it will either remain stationary, or if it is already moving, then it will carry on moving at a steady speed. If there is a resultant force acting on an object then the object’s motion will change: it will accelerate.
Newtons second law
acceleration of an object is directly proportional to the resultant force acting on it and inversely proportional to the mass.
F=MA
Inertial mass
measure of how difficult it is to change an object’s velocity. It is defined as the resultant force acting on an object divided by the acceleration that force generates.
Terminal velocity in air
as an object falls through the air (a fluid) air resistance it experiences increases (because it is colliding with more fluid particles in a certain period of time).
The resultant force on the object decreases and therefore its acceleration decreases.
Its speed continues to increase, but by smaller amounts until eventually the resultant force on the object is zero and the object stops accelerating. It will be travelling at a constant velocity called the terminal velocity.
Stopping distance
Thinking distance + braking distance
Things that affect thinking distance
Speed
Drugs/alcohol
Tiredness/illness
Distraction
Things that affect braking distance
Speed
Road conditions
Brake/tyre conditions
Energy transfers during braking
kinetic store of the vehicle into the thermal store of the brakes.
If this transfer happens too quickly the brakes can overheat and fail. There is also the risk that control of the car can be lost and the car can skid.
Why do we try stop cars over a long period of time
smaller deceleration. Smaller decelerations require smaller resultant forces.
Airbags, seatbelts and crumple zone function
Airbags, seatbelts and crumple zones increase the impact time in a collision. This reduces the rate of change of momentum and hence reduces the impact force on the person in the car.
