c) Forces, movement, shape and momentum

c) Forces, movement, shape and momentum

1.9 describe the effects of forces between bodies such as changes in speed,
shape or direction

A force is a push or pull that can change the: speed, shape or direction of an object. (Force = mass x acceleration.)
Forces can affect bodies in different ways. 

• Speed - A force can increase or decrease the speed of an object, which will cause acceleration/deceleration. When accelerating, the forward force is larger than the backward force against the object. When decelerating, the backward force is larger than the forward force.  When an object is stationary it has an equal force pushing up and down and forwards and backwards.
• Shape - When a balanced force is acted upon an object, the object becomes stationary. However when a balanced force is acted upon a malleable object, the object changes shape to accommodate the force as it cannot move. 
• Direction - Which ever direction the force is greatest in will be the direction the object travels in.

1.10 identify different types of force such as gravitational or electrostatic

• Gravity or weight  - (Gravity gives everything a weight. It makes everything accelerate towards the ground. The weight of an object corresponds to the pull of its gravity.)

• Electrostatic force - (force between two charged objects, similar charges repel, opposite charges attract.) 

• Thrust - (or push or pull, occurs when a system expels mass in one direction and this mass will cause a force of equal magnitude but opposite direction on the system.)

• Friction (or drag) - (the force that resists movement between two surfaces that are in contact.)

• Tension - (Tension passes through strings, cables, ropes or wires when they are being pulled in opposite directions. The tension force is directed along the length of the wire and pulls equally on the object at the opposite ends of it.)

• Lift - (opposes the weight. Reaction to the surface, not allowing object to sink.)

• Reaction force (or contact force) - (when two objects are pushed together, come into 'contact,' they exert equal and opposite forces on each other. For example, the contact force from the ground pushes up when you stand and your weight pushes down.)

Weight/gravitational, electrostatic and magnetic force can act without touching an object.

                     

1.11 distinguish between vector and scalar quantities

Vectors have magnitude and a direction, whereas scalars just have a magnitude (ie.speed).

1.12 understand that force is a vector quantity

Force has a magnitude (measured in newtons) and acts in a direction. It is therefore a vector quantity.

1.13 find the resultant force of forces that act along a line 

The resultant force is the overall force acting in a direction on an object. This diagram shows how the forces should be added to reach the resultant force.

1.14 understand that friction is a force that opposes motion

Friction (or drag) resists movement between two surfaces that are in contact. It therefore opposes motion. 

1.15 know and use the relationship between unbalanced force, mass and acceleration:

Force = mass x acceleration 
Force (newtons, N) = mass (kg) x acceleration (m/s^2)

Example; What force must be applied to a 1kg bag of sugar to accelerate it by 15m/s^2.

Force = mass x acceleration, 

so force will be = 1 x 15 

 = 15N (newtons.)

What is the acceleration of a 1000 kg car when a force of 2500 N is applied to it?

Acceleration = force/mass 

So acceleration will be = 2500/1000

                                      = 2.5m/s^2.

1.16 know and use the relationship between weight, mass and g

Weight (N) = mass (kg) x gravitational field strength

W = m x g 

1.17 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity. 

The two forces that act on falling objects are; gravity and drag (air resistance.)

When an object first starts to fall it is accelerating, because the downward force acting on it (gravity) is stronger than the opposing upwards force (drag or air resistance.) However, after some time the forces will become equal (balanced) and at this stage, the object will continue moving but will no longer accelerate. It is said to have reached its terminal velocity. 

1.18 describe experiments to investigate the forces acting on falling objects, such as sycamore seeds or parachutes. 

Parachutes

Dropping a parachute from a given height will immediately show gravity at work, as the parachute will begin to fall to the ground. However, if a person then jumped out of a plane with said parachute on, the force of gravity will be much quicker as it increases as the mass of the object does. Additionally, the area of the object will affect the drag force acting upon it. If the area is increased, say the parachute is opened, the object will face much more air resistance, and will therefore go at a slower speed. 


Sycamore seeds

• Collect a few sycamore seeds of different sizes (say 5.) 
• Measure them, and multiply their length by their width to find the surface area of each seed. 
• Hold one at the top of a ruler or another object you can measure length from, eg. a tape measure. 
• Drop each seed separately, but from the same height, and time how long it takes to reach the ground. 
• Repeat this process twice, to make sure you have taken note of the correct measurements. 
• Plot a scatter graph (surface area on y axis and time on x axis) to visually show your results. 
• If plotted correctly, the graph should display a straight line, going up as the graph travels right. This represents a positive correlation and shows you that the larger the surface area of the seed, the longer the seed took to fall (more surface area to experience air resistance, slowed down time it took for gravity to pull it to the ground.) 

1.19 describe the  factors affecting vehicle stopping distance including speed, mass, road condition and reaction time 

Vehicle stopping distance = thinking distance + braking distance 

Factors that effect the thinking and/or braking distance, and therefore the vehicle stopping distance: 

• Speed car is travelling at. The quicker the car is travelling, the larger the thinking/braking distance, therefore the larger the stopping distance. 

• Mass of vehicle (load,) the larger the mass, the longer the car will take to stop, increasing braking distance. 

• Road conditions - If the road is particularly wet, or bumpy or has sleet/snow, then these poorer conditions will increase the thinking/braking distance. 

• Reaction time - The time it takes for the person to see the obstacle and react to it. The longer this reaction time is, the larger the stopping distance will be as it will increase the thinking distance. (The driver's reaction time can be affected if they are drunk, under influence of drugs, have poor visibility etc.)