Topic 2 Motion and Forces

• A scalar quantity has magnitude but no specific direction
• A vector quantity has both magnitude and a specific direction
• Vector Quantities include displacement, velocity, acceleration, force, weight and momentum
• Scalar Quantities include distance, speed, mass and energy
• Velocity is speed in a stated direction
• speed(m/s) = distance (m)/time (s)
• On a distance/time graph determine speed by the gradient
• acceleration (m/s^2) = change in velocity (m/s) / time (s)
• v^2(m/s^2) - u^2(m/s^2) = 2 x a(m/s^2) x X(m)
• In Velocity time graphs calculate the acceleration from the gradient and distance travelled by the area under the graph
• To measure speed in the lab you need to measure distance and time, light gates are used as they are more accurate
• Typical speeds: walking - 1.4m/s running - 4m/s Cycling - 6m/s Ferry 18m/s airliner - 250 m/s sound - 330 m/s light breeze - 3m/s gale 20m/s
• The acceleration of free fall is 10m/s^2
• pt 1 First Law : Where the resultant force on a body is zero, the body is moving at a constant velocity or is at rest
• pt 2 First Law: Where the resultant force is not zero, eg the speed and/or direction of the body changes
• Second Law : F=ma Force = N Mass = kg acceleration= m/s^2
• Weight is a force due to the pull of gravity on an object.
• Weight(N) = Mass(kg) x Gravitational Field Strength(N/kg)
• Weight is measured in Newtons
• The Greater the Gravitational field strength the greater the mass and weight of the object
• An object moving in a circular orbit at constant speed has a changing velocity because it is constantly changing direction
• For motion in a circle there must be a resultant force known as centripetal force that acts towards the centre of the circle
• Inertial mass is a measure of how difficult it is to change the velocity of an object and it is defined as the ratio of force over acceleration
• Third Law : whenever two objects interact, the exert equal and opposite forces on each other
• An object in equlibrium will not turn or accelerate
• Momentum (kg m/s) = mass (kg) x velocity (m/s)
• Conservation of Momentum is when moving objects collide the total momentum of both objects is the same before the collision as it is after the collision as long as there is no external forces acting on it.
• Force(N) = change in momentum(kgm/s) / time (s). F = mv-mu/t
• Human reaction times are usually measured by asking people to respond as quickly as possible to a stimulus
• The stopping distance of a vehicle is made up of the sum of the thinking distance and the braking distance
• Stopping distance of a vehicle is affected by: Mass and speed of the vehicle, drivers reaction time, state of the brakes, state of the road and the amount of friction between the tyre and the road
• The factors affecting a drivers reaction time include: drugs, alcohol, distractions, tiredness, illness,
• A typical reaction time for a human is 0.25 seconds
• Large decelerations may cause brakes to overheat, and drivers losing control of the vehicle
• When a car comes to a stop the work done by the brakes must equal the initial kinetic energy of the car F*d = 1/2mv^2
• The faster the speed of the vehicle the larger the distance needed to stop
• Momentum is a quality relating to moving an object and is calculated by its mass * velocity