1 c) forces, movement, shape and momentum Flashcards
1.9 describe the effects of forces between bodies such as changes in speed, shape or direction
Force is that which can change the state of rest or uniform motion of an object. Force is simply pushes and pulls of one thing on another.
If a body is thrown up in the air, what is the effect of gravity on the body? At first gravity reduces the speed of upward movement of the body and at a certain height it stops. So Force effects the speed.
Take a sponge and squeeze it will change its shape.
Throw a ball at a person in one direction. That person will hit the ball again i.e. apply force to the ball and it will change its direction.
To sum up the examples, the effects that occur when a force is applied to an object are:
- The object may start to move or stop moving.
- The object may speed up or slow down.
- The object may change its shape
- The object may change its direction of movement.
1.10 identify different types of force such as gravitational or electrostatic
Different sorts of Force:
- Gravitional force or weight: The pull of earth due to gravity.
- Normal Reaction: Simple reaction that stops something when to apply force to it.
E.g.: A book is kept on the table which has a normal reaction on it. Otherwise the book would fall down. - Air Resistance: The resistivity or drag in the air while an object moves is called Air Resistance.
E.g.: When a parachutist open the parachute the movement slows down for the opposite force acting in it. - Upthrust: Upthrust force acts only on liquid or air. It pushes an object upwards inspite of gravity.
E.g.: A helium balloon moves upwards due to up thrust force. - Magnetic: Magnetic force is the attraction force between the poles of magnets. N=S
- Electrostatic: Electrostatic force is the attraction force between charges. +=-
- Tension: The pull at both ends of a stretched spring ,string, or rope
- Frictional force: the force produced when two objects slide one over another is called frictional force.
1.11 understand that friction is a force that opposes motion
Friction is the force that causes moving objects to slow down and finally stop. The kinetic energy of the moving object is converted to heat as work is done by the friction force. Friction occurs when solid objects rub against other solid object and also when objects move through fluids (liquids and gases).
Friction reduces efficiency of machines and cause wastage. It also wears and tears the surface.
It can be reduced by making the surface smooth using lubricating oils.
However, friction is the reason we can walk, or write. It is helping us in various ways.
1.12 know and use the relationship between unbalanced force, mass and acceleration:
Balanced and Unbalanced force:
- When a force acting on an object is equal to the force opposing the object the forces are “balanced.”In this case the object will not move.
- If a force acting on an object is “NOT” equal to the force opposing, then the object the forces are “unbalanced.” In this case the object will move to the direction at which force is acting higher.
Force= mass x acceleration
In equation, F=ma (where, m=mass and a=acceleration)
F α a
Force is directly proportional to acceleration. If force increases acceleration increases.
1.13 know and use the relationship between weight, mass and g:
Weight is the pull of earth. To calculate it, use the formula:
Weight = mass x gravitional acceleration
W =mg
In earth g= 10 m/s2 if there is no opposite force.
1.14 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity
http://www.shawonnotes.com/IGCSE_Physics/physics_images/drag.png
In a free falling object two types of force acts: Drag and Weight. The size of the drag force acting on an object depends on its shape and its speed. If the drag force of an object increase to a point which is equal to Weight, then the acceleration stops. It falls in a constant velocity known as terminal velocity.
Reaching terminal velocity on a parachute:
http://www.shawonnotes.com/IGCSE_Physics/physics_images/terminal-velocity.png
When a skydiver jumps from a plane at high altitude he will accelerate for a time and eventually reach terminal velocity. When he will open her parachute this will cause a sudden increase in the drag force. At that time drag force will be higher than the weight and he will decelerate for sometime. Later those forces will become equal and reach a new terminal velocity.
1.15 describe experiments to investigate the forces acting on falling objects, such as sycamore seeds or parachutes
Experiment: Measuring the force of a falling ball using light gate
Apparatus required: Cylinder, light gate, data logger, electric balance
First, we measure the weight of the ball using an electric balance. This is the force acting downwards all the time.
Set up the cylinder using light gate at different points keeping the same distance difference between each of them. Fill the cylinder with oil or any other liquid. For more accurate results, we will be using a cylinder with a diameter close to the diameter of the ball.
Now release the ball from the top of the cylinder. After it reaches the bottom, we will notice that the time taken between each light gates increases as it go downwards. We can calculate the acceleration from that. Since, F = ma, we can calculate the resultant force.
That means, the resistance acting on the increases. At a time, the resistance will equal the weight, and the forces will be balanced. It will then fall with a constant velocity.
1.16 describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time
The stopping distance is the sum of Thinking distance and Braking distance.
Thinking Distance: The distance travelled after seeing an obstacle and till reaction.
Braking Distance: The distance travelled after the brakes are applied.
The thinking distance depends on the following factors -
Whether the driver is tired or has taken alcohol or drugs.
On the visibility power of the driver.
On the speed of the car.
The braking distance depends on the following factors -
Speed of the car: The more the speed is, the more the braking distance will be; S α V2.
Mass of the car: As acceleration is equal to F/m, for constant braking force, the more is the mass, the less is the deceleration, the more is the braking distance.
Road condition: If the road is rough, the braking distance will be less.
Tyre condition: If the tyre is new (rough), there will be less braking distance.
Braking system: For loose braking system, the braking distance will be more.
1.17 know and use the relationship between the moment of a force and its distance from the pivot:
moment = force × perpendicular distance from the pivot
moment =F x d
The turning effect of a force about a hinge or pivot is called its moment. It is measured in Newton meter (Nm).
1.18 recall that the weight of a body acts through its centre of gravity
The centre of gravity of an object is the point where the whole weight appears to act. So if we support the centre of gravity of the object, the object wont fall no matter how wide it is. Because the moment of the all sides are balanced and there will be no clockwise or anti-clockwise movement.
1.19 describe experiments to investigate how extension varies with applied force for helical springs, metal wires and rubber bands
Experiment: Investigating extension with applied force in spring
Apparatus: Spring/Wire/Rubber-band, Scale, Some masses, Clamp and stand, mass hanger
http://www.shawonnotes.com/IGCSE_Physics/physics_images/experiment-spring-rubber-wire.jpg
Working procedure:
1) Take the length of the normal condition.
2) Add a mass in the mass hanger and determine the extension by using the porter and the scale.
3) Add another mass gradually and determine the extension in all cases.
4) Plot a graph of extension and relevant loads.
1.20 understand that the initial linear region of a force-extension graph is associated with Hooke’s law
Hooke’s law, “Within the elastic limit, extension is directly proportional to the load i.e. e α f”
Hooke measured the increase in length (extension) produced by different load forces on springs. The graph he obtained by plotting force against extension looked like that below. This straight line passing through the origin shows that the extension of the spring is proportional to the force. The relationship is known as Hooke’s law.
Hooke’s Law only applies if you do not stretch a spring to far. At a point the elastic limit it starts to stretch more for each successive increase in the load force. Once you have stretch a spring beyond this limit it has changed shape permanently and will not return to its original shape.
1.21 describe elastic behaviour as the ability of a material to recover its original shape after the forces causing deformation have been removed.
Objects showing elastic behaviour has the ability to return to its original shape after the forces causing its shape are removed. Examples of objects showing elastic behaviour are coiled springs.
