The effects of forces between bodies such as changes in speed, shape or direction
Effect of changes in speed
Effect of changes in speed
When an object is stationary it has an equal force pushing down and up. The downward force, being gravity, and the upward force being the surface the object is on. The object is not floating but it is not sinking into the ground either.
When an object is accelerating it has the same upwards and downwards forces but it also has forwards and backwards forces (drag and friction). The forward force is larger than the backward force when an object is accelerating.
When an object is going at a constant speed it has downward and upward forces as well as forward and backward forces. The forward and backward forces are equal, so the speed doesn't change even though the object is moving.
When an object is decelerating it has the equal upward and downward forces as well as forward and backward forces, but the backward force is larger than the forward one, slowing the object down.
When an object is accelerating it has the same upwards and downwards forces but it also has forwards and backwards forces (drag and friction). The forward force is larger than the backward force when an object is accelerating.
When an object is going at a constant speed it has downward and upward forces as well as forward and backward forces. The forward and backward forces are equal, so the speed doesn't change even though the object is moving.
When an object is decelerating it has the equal upward and downward forces as well as forward and backward forces, but the backward force is larger than the forward one, slowing the object down.
Effect of changes in Shape
Effects momentum. Force = change in momentum / time taken. An example of this is crumple zones in car decrease the force on the passengers.
Effect of changes in Direction
Which ever direction the force is greatest in will be the direction the object travels in.
Different types of force such as gravitational or electrostatic
Gravity: Acts downwards
Up thrust: Acts upwards
Drag: Acts against the movement
Various types of force:
Up thrust: Acts upwards
Drag: Acts against the movement
Various types of force:
- push/pull (contact force)
- tension-the pull at both ends of a stretch spring, string or rope
- compression
- thrust/upthrust
- load
- effort
- *weight/gravitational
- *electrical/electrostatic
- *magnetic
Scalars have magnitude (size) while vectors have both magnitude and a direction. For example velocity is a speed in a given direction (Vector).
Examples of Scalars: Speed, Distance, Mass, Temperature, Energy, Charge, Volume and Area.
Examples of Vectors: Velocity, Displacement, Weight/tension/compression, Thrust, Drag, Upthrust, Acceleration, Field strength - magnetic/electrical/gravitational
Examples of Scalars: Speed, Distance, Mass, Temperature, Energy, Charge, Volume and Area.
Examples of Vectors: Velocity, Displacement, Weight/tension/compression, Thrust, Drag, Upthrust, Acceleration, Field strength - magnetic/electrical/gravitational
1.11 distinguish between vector and scalar quantities
Force is a vector quantity
Force is a vector quantity because it has magnitude, it is measured in newtons but it acts in a direction. For example 3N drag is an amount of force acting backwards.
Another example is, weight is a force with magnitude, and it acts downwards. It is a vector, and you would normally use an arrow pointing downwards with its magnitude ("xN" N being the unit-Newtons) to represent it in a diagram.
Another example is, weight is a force with magnitude, and it acts downwards. It is a vector, and you would normally use an arrow pointing downwards with its magnitude ("xN" N being the unit-Newtons) to represent it in a diagram.
The resultant force of forces that act along a line
Resultant force is the entire force acting in a direction on an object. It is best explained by this diagram that shows that the resultant force is the overall force given individual forces acting along a line.
Force forces act along a line, meaning they are collinear. You can add the forces together like scalars. For instance if two forces are acting on a box, both pushing towards the right side, one with a force of 4N and the other with 6N, then 4N+6N=10N (resultant force). So a total of 10N is acting on the box, pushing it to the right.
However, if one of the forces was acting to the left with say, 2N and the other to the right with 7N, then the resultant is 5N to the right. The 2N to the left cancels out 2N from the right. You can think of the force acting to the left as a negative value, like the reverse direction, so 7N + (-2N) = 5N. It's good to use diagrams to help you out.
Force forces act along a line, meaning they are collinear. You can add the forces together like scalars. For instance if two forces are acting on a box, both pushing towards the right side, one with a force of 4N and the other with 6N, then 4N+6N=10N (resultant force). So a total of 10N is acting on the box, pushing it to the right.
However, if one of the forces was acting to the left with say, 2N and the other to the right with 7N, then the resultant is 5N to the right. The 2N to the left cancels out 2N from the right. You can think of the force acting to the left as a negative value, like the reverse direction, so 7N + (-2N) = 5N. It's good to use diagrams to help you out.
Friction is a force that opposes motion
Friction is a force that acts in the opposite direction to motion. It is a force that always opposes / resists motion between two surfaces in contact.
1.15 Know and use the relationship between unbalanced force, mass and acceleration.
FORCE = MASS × ACCELERATION (F=M×A)
1.16 know and use the relationship between weight, mass and g
weight = mass x g (W= M x G)
g = gravitational field strength, the gravitational force exerted per unit mass at a point in the field, it is a vector quantity. unit= N/kg
on earth, we consider g as 10N/kg, or g = 9.8 m/s2 but we use g = 10 m/s2 this is how an object in the Earth's gravitational field would accelerate if it was free fall-without friction (this doesn't happen in the real world).
( to explain how we get to this: a= F/m = mg/m = g )
g = gravitational field strength, the gravitational force exerted per unit mass at a point in the field, it is a vector quantity. unit= N/kg
on earth, we consider g as 10N/kg, or g = 9.8 m/s2 but we use g = 10 m/s2 this is how an object in the Earth's gravitational field would accelerate if it was free fall-without friction (this doesn't happen in the real world).
( to explain how we get to this: a= F/m = mg/m = g )
1.17 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity
When first an object is falling it is accelerating towards the ground - the force acting downwards (gravity) is larger than the force acting upwards (air resistance). But when air resistance and gravity become equal the object (the air resistance increases until it equals the object's weight.) ill have reached its maximum speed; its terminal velocity. Acceleration will no longer happen at this point.
- Earth's gravity- Weight: has direction (vector quantity), pulls object downwards towards the centre of the Earth
- Air resistance/Drag: upwards force, pushes object upwards
1.18 describe experiments to investigate the forces acting on falling objects, such as sycamore seeds or parachutes
Dropping parachutes from a given height; this shows us that gravity is acting on them. By increasing the size of the parachute and recording the results we can see that air resistance also has an effect on falling objects; plotting a graph should reveal that bigger surface area takes more time, from which we can infer that air resistance acts on the falling objects.
Example:
Example:
- Get five different sizes of sycamore seeds
- Roughly measure, and label them with, their surface area by multiplying the length by width
- Hold one at the top of a meter ruler
- Drop it and time how long it takes to reach the ground
- Repeat this three times for each of the five seeds
- Then plot a scatter graph with surface area on the y axis (mm/cm) and time on the x axis (s)
- You should find that the line of best fit is a diagonal line pointing away from 0, this represents a positive correlation meaning the larger the surface area the longer it took to fall
- This is because the larger seeds had more surface area to experience air resistance, this is a type of friction that opposes gravity, therefore slowing down the time taken for gravity to bring them to the ground.
1.19 describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time
Vehicle stopping distance = ( reaction time (a.k.a thinking time) x constant speed ) + breaking distance vehicle stopping distance
= thinking distance + braking distance
distance = speed x time, so it becomes thinking distance
Thinking distance: How far the car travels at constant speed before the driver reacts by applying the car brakes
Braking distance: Distance travelled by the car as it decelerates to a stop
Factors that affect the stopping distance are:
= thinking distance + braking distance
distance = speed x time, so it becomes thinking distance
Thinking distance: How far the car travels at constant speed before the driver reacts by applying the car brakes
Braking distance: Distance travelled by the car as it decelerates to a stop
Factors that affect the stopping distance are:
- The condition of the driver; drugs/ tiredness (thinking distance)
- How worn the brakes/ tyres are
- If the weather conditions are poor
- How heavy the car is
- The speed the car is travelling at
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