6.5.3 Forces and Elasticity
Elastic and Inelastic Deformation
Elastic deformation is when the object returns to its original shape
Elastic deformation is when an object does not return to its original shape after being deformed.
An object is elastic if it returns to its original shape when the forces deforming it are removed.
To change the shape of a stationary object, more than one force has to be applied to prevent the object from moving as a result of the force.
Hooke's Law: The extension of an elastic object is proportional to the force applied, provided that the limit of proportionality is not exceeded.
F = ke
A force that stretches or compresses a spring does work and elastic potential energy is stored in the spring. Provided the spring is not inelastically deformed, the work done on the spring and the EPE stored are equal.
E = 1/2 k e^2
6.5.2 Work Done and Energy Transfer
When a force causes an object to move through a distance, work is done.
A force does work on an object when the force causes a displacement of the object.
W = Fs
One joule of work is done when a force of one newton causes a displacement of one metre therefore 1 joule = 1 newton-metre.
Work done against the frictional forces acting on an object causes a rise in temperature of the object.
6.5.1 Forces and Their Interactions
22.214.171.124 Scalar and Vector Quantities
Scalar quantities have magnitude only.
Vector quantities have magnitude and an associated direction.
A vector quantity may be represented by an arrow. The length of the arrow represents the magnitude, and the direction of the arrow the direction, of the vector quantity.
126.96.36.199 Contact and Non-Contact Forces
A force is push or a pull that acts on an object due to the interaction with another object. Force is a vector quantity.
Objects are physically touching.
Normal Contact Force
Objects are physically seperated
Weight is the force acting on an object due to gravity.
The forces of gravity close to Earth is due to the gravitational field around the Earth
The weight of an object depends on the gravitational field strength at the point where the object is.
W = mg
The weight of an object may be considered to act at a single point referred to as the object's centre of mass.
Weight is directly proportional to mass.
Weight is measured using a calibrated spring-balance (a newtonmeter)
188.8.131.52 Resultant Forces
A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together, called the resultant force.
A single force can be resolved into two components acting at right angles to each other, having the same effect as the single force.
6.5.4 Forces and Motion
184.108.40.206 Describing Motion Along a Line
220.127.116.11.1 Distance and Displacement
How far an object moves.
Does not involve direction
Both the distance an object moves, measured in a straight line from the start point to the finish point, and the direction of that straight line.
The speed of a moving object is rarely constant.
Typical values for speed
Sound ~330 m/s
s = v t
Speed in a given direction
18.104.22.168.4 The Distance-Time Relationship
If an object travels along a straight line, the distance travelled can be represented by a distance-time graph.
The speed of an object can be calculated from the gradient of its distance-time graph.
If an object is accelerating, its speed at any particular time can be determined by drawing a tangent and measuring the gradient or the distance-time graph at that point.
a = ∆v / t
The acceleration of an object can be calculated from the gradient of a velocity-time graph.
The distance travelled by an object can be calculated from the area under a velocity-time graph.
22.214.171.124 Forces, Accelerations, and Newton's Laws of Motion
126.96.36.199.1 Newton's First Law
If the resultant force acting on an object is zero and...
...the object is stationary, the object remains stationary.
...the object is moving, the object continues to move at a constant speed.
When a vehicle travels at a steady speed, the resistive forces balance the driving force.
The velocity of an object will only change if a resultant force is acting on the object.
The tendency of objects to continue in their state of rest or of uniform motion is called inertia.
188.8.131.52.2 Newton's Second Law
The acceleration of an object is proportional to the resultant force acting on the object, and inversely proportional to the mass of the object.
F = m a
Inertial mass is a measure of how difficult it is to change the velocity of an object.
Inertial mass is defined as the ratio of force over acceleration.
184.108.40.206.3 Newton's Third Law
When two objects interact, the forces they exert on each other are equal and opposite.
220.127.116.11 Forces and Breaking
18.104.22.168.1 Stopping Distance
The distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under a braking force (breaking distance).
For a given breaking force, the greater the speed of the vehicle, the greater the stopping distance.
22.214.171.124.2 Reaction Time
Reaction times vary from person to person but typical values range from 0.2 s to 0.9 s.
A driver's reaction time might be affected by: tiredness; drugs and alcohol; distractions.
Factors Affecting Braking Distance
The breaking distance can be affected by adverse road weather conditions and poor condition of the vehicle.
When a force is applied to the brakes of a vehicle, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brake increases.
The greater the speed of a vehicle, the greater the braking force needed to stop the vehicle in a certain distance.
The greater the braking force the greater the deceleration of the vehicle. Large decelerations may lead to breaks overheating and/or loss of control.