- Scalar quantities, e.g. speed, time, have only size. Vector quantities e.g. force, momentum, have size and direction.
- The speed of an object can be found by looking at the gradient of a distance-time graph.
- The acceleration of an object can be found by looking at the gradient of a velocity-time graph.
- Distance travelled by an object is the area under a velocity-time graph.
Acceleration of an object= Final Velocity-Initial Velocity
Velocity of an object= Displacement
When 2 objects interact, the forces exerted on each other are equal and opposite
The resultant force has the same effect on motion as all other forces acting on the object combined.
When an object is stationary:
If the resultant force is zero= Object remains stationary
If it's not zero= Object accelerates in the direction of the resultant force
When an object is moving:
If the resultant force is zero= The object will continue moving at the same speed and same direction
If it's not zero= The object will accelerate in the direction of the resultant force
FORCE=MASS X ACCELERATION
MOMENTUM= MASS X VELOCITY
In a closed system, the total momentum before an event= the total momentum after an event
This is 'conservation of momentum'
FORCE= CHANGE IN MOMENTUM
Forces and Braking
When a vehicle is at a steady speed, driving force=total resistive forces
The greater the speed of the vehicle, the greater the braking force needed to stop it
Stopping distance= Distance travelled during the driver's reaction time+distance travelled under the braking force
Reaction Time: Affected by tiredness, drugs and alcohol
Braking Distance: Affected by icy/wet road conditions and damaged tyres/brakes
When brakes are applied, kinetic energy of the car decreases and temperature of the brakes increases
Forces and Terminal Velocity
The faster an object falls, the greater the frictional force acting on it
An object falling will accelerate due to gravity, then eventually weight and resistive forces will balance, meaning the resultant force=0 and the object will move at its terminal velocity.
Parachutists use parachutes to increase the effect of air resistance and significantly reduce their terminal velocity
WEIGHT=MASS X 10
Forces and Elasticity
Forces acting on an object can change the shape of the object
Elasticity is when the object returns to its original shape when the force is removed
Applying a force to e.g. a spring will result in the object stretching and storing Elastic Potential Energy
Extension is proportional to the force applied (provided the limit of proportionality is not exceeded)
FORCE= CONSTANT X EXTENSION
Forces and Energy
Work is when a force causes an object to move through a distance
WORK= FORCE X DISTANCE MOVED
Energy is transferred when work is done
Work against frictional forces causes energy transfer by heating
POWER= WORK DONE/ENERGY TRANSFERRED
GPE= MASS X 10 X HEIGHT
KINETIC ENERGY = 0.5 X MASS X SPEED(SQUARED)
Centre of Mass
Centre of mass is the point where the mass of an object is considered concentrated
If freely suspended, the object will come to rest with the centre of mass directly below the point of suspension.
For a symmetrical object, the centre of mass is along the axis of symmetry.
TIME PERIOD= 1
The time period for a pendulum depnds on the length
MOMENT=TURNING FORCE X DISTANCE FROM THE PIVOT
If an object isn't turning, the clockwise moment is equal to the anticlockwise moment
Simple levers can be used as force multipliers
If the line of action of the weight of an object is outside the base of the object, the resultant force (imbalance) will cause the body to topple
When an object moves in a circle, it accelerates towards the centre of the circle. This changes the direction of the motion of the body, not the speed.
This resultant force is known as the centripetal force and always is directed towards the centre of the circle.
The centripetal force needed to make an object perform circular motion increases as:
- The mass of the object increases
- The speed of the object increses
- The radius of the circle decreases
Liquids are virtually incompressible, and the pressure in a liquid is transmitted equally in all directions.
This means a force exerted at one point on a liquid will be transmitted to other points in the liquid