Sci Chap 9 Flashcards by ditii g

What is a force?

A force can be a ‘push’ or a ‘pull’. There are two types of forces:
-Contact forces
-Non-contact forces

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Define contact forces with examples.

Contact forces refer to forces acting between two objects that are in physical contact with each other. Examples include:

- Elastic force
Elastic force is the force acting on a stretched or compressed elastic object to return it to its original shape. When elastic material is stretched or compressed, it resists the change in shape by exerting a force in the opposite direction. e.g. include hairties, rubber bands etc.

- Frictional force
Frictional force is the force that opposes or leads to oppose motion between surfaces in contact. It provides resistance to the motion of a body.

The friction between two surfaces depends on:
- The type of surfaces in contact
- The force exerted by one surface on the other (ie, the normal force)

Friction is independent of:
- The area of the surfaces in contact
- The relative speed between the two surfaces

Friction can be reduced by adding lubricants such as water or lubricating oil.

applications: Lighting a fire: The head of the matchstick rubs against the rough sides of the matchstick box, causing heat. This enables the head to catch fire.

- Normal force
The normal force is the supportive force that is exerted perpendicularly outwards by a surface on an object pressing on it. e.g., On the tyres of a car at rest on the road. The road exerts a normal force on the tyres.

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Define non-contact forces with examples.

Non-contact forces refer to forces acting between two objects that are not touching each other. Examples include:

- Gravitational force
The gravitational force is a force that attracts two objects towards each other due to their masses. It keeps the moon orbiting around Earth. Earth’s gravitational force pulls all things towards its centre.
G.F. leads to ocean tides.
~ G.F. is responsible for the movement of high and low tides in harbours, ports and coastal areas. The tides are caused partly by the moon’s gravity pulling on the water of the ocean.

G.F. leads to weight.
~ When a planet exerts gravitational force on an object and pulls it towards its centre, the object is known to have weight. The G.F. pulls objects to the ground and prevents them from floating away.

- Mangetic force
Magnetic force, or magnetism, is the force exerted by a magnet on magnetic materials such as iron and steel. Every magnet has a South and a North pole. Unlike poles attract each other, aka the magnetic force of attraction. Like poles repel each other, aka the magnetic force of repulsion.

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What is Mass?

Mass is the measure of the amount of matter in a body. The SI unit of mass is KG. The amount of mass does not change regardless of the location (Outer space, on Earth, etc). Therefore, mass always remains the same.
A beam balance or electronic balance can be used to measure mass.

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What is weight?

Weight is the measure of the gravitational force acting on an object. It is a type of force. The SI unit of weight is N. The weight of an object depends on the gravitational force exerted on the object, which may change with location. e.g. less gravitational force acts on an object on the moon than on Earth, as the effect of gravity is weaker on the moon than on Earth, ∴ the weight of an object is greater on Earth than its weight on the moon. A spring balance can be used to measure force. The two types include: extension spring balance, and compression spring balance.

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learn how to draw free body diagrams 💔

Things to note:
Weight is ALWAYS present, draw the arrow from the middle of the object downwards/upwards facing the Earth/ground.

Tension may be present even when friction isn’t.

Normal reaction force arrow points up away from the surface.

Direction of friction is opposite to the direction of motion, shorter than tension arrow.

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What is a resultant force?

The overall force acting on an object (when 2 or more forces act on it) is called the resultant force.

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What are balanced forces?

Balanced force is when the resultant force = 0. It will not change the motion of a body. ∴ A stationary body will remain stationary, and a moving body will continue moving with Constance speed in the same direction.

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What are unbalanced forces?

Unbalanced force is when the resultant force = >0. ∴ A stationary body will start moving, and a moving body will change its speed.

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What are the changes in the states of rest or motion of an object?

moving a stationary object
changing the direction of a moving object (turning effect)
changing the speed of a moving object
stopping a moving object
change in the state in shape/size of the object
change in the pressure exerted.

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Define speed.

Speed is the rate of change of distance, or the distance travelled per unit time. Forces can change the speed of a moving object, causing it to move faster or slower. Speed can be calculated using the formula:

speed = ᵈⁱˢᵗᵃⁿᶜᵉ ᵗʳᵃᵛᵉˡˡᵉᵈ⁄ₜᵢₘₑ ₜₐₖₑₙ

Speed is measured in metres per second m/s or ms^-1. Distance is measured in m, and time taken is measured in **seconds*.

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What is the speed of a rocket that travels 9000m in 12.12 seconds?

743 m/s (3 s.f.)

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Define the changes due to the turning effect on an object.

A force applied to an object can cause a turning effect about a fixed point called the pivot.
The object will turn in a clockwise or anticlockwise direction.

application:
~ When opening a door, the force applied on the door knob produces a turning effect on the door, causing it to open.

~ Levers: using a crowbar. The force is exerted as far away as possible to maximise the turning effect of the force.

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Define the changes in the size and/or shape of an object.

e.g. When moulding a lump of wet clay, the size and shape of the clay changes.

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Define the changes in pressure on an object.

Pressure is defined as the force acting on an object per unit area. It is related to force and area by the formula:
Pressure= ᶠᵒʳᶜᵉ⁄ₐᵣₑₐ
Pressure is measured in Pa (pascal), or N/m^2, force is measured in N, and area is measured in m^2.

As the area of object x in contact with object y increases, the amount of force applied by x on y increases, and the pressure exerted by x on y decreases.

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What is the relationship between pressure and force?

As the force acting on a surface/obj decreases, the pressure exerted decreases.

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What is the relationship between pressure and area?

As the area of the surface increases, the pressure exerted decreases.

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What is an advantage of pressure?

Some advantages of pressure include:
- Cutting food/objects with a knife. less area in contact, more pressure exerted = easier to cut

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What is a disadvantage of pressure?

Disadvantages:
- tyre puncture: car tyre rolls over the sharp pointed end of a nail, tip of nail has a very small area = which exerts large pressure on the tyre.

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Define pressure in liquids:

Liquid pressure increases as the depth of the liquid increases. This is because the water particles push downwards due to gravity. Hence, the base of a dam has a thicker wall to withstand high liquid pressure at the bottom as compared to the wall at the top of the dam.

Submarines have rigid metal bodies to withstand the very high pressure deep underwater.

Water is delivered to and stored in tanks at the top of most high-rise buildings to ensure that water flows to the units below.

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Define atmospheric pressure:

Earth is surrounded by a thick layer of air. Air particles are light, occupy space, and have mass. When the layer of air particles pushes down on Earth’s surface due to gravitational force, it exerts a pressure of approximately 100,000 Pa at sea level on all objects on Earth. The pressure exerted by the air in the Earth’s atmosphere is called atmospheric pressure. It decreases as we move to a higher altitude.

When drinking liquid using a straw, we suck through the straw. This lowers the air pressure in the straw. The higher atmospheric pressure outside the straw pushes the liquid up from the drinking straw.

When using suction cups, as the air pressure in the suction cup than the atmospheric pressure outside, the atmospheric pressure pushes the suction cup towards the surface the suction cup is placed on.

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Why is it that one does not feel the force of atmospheric Pa when it is enough to crush and kill him/her?

The atmospheric Pa outside our bodies is balanced by the air pressure Pa in our bodies. We can only feel air Pa in our body at higher altitudes. Our ears pop bcs the atmospheric Pa outside our body decreases with altitude.

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Define energy.

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Energy is the ability to do work. The SI unit for energy is joule, J.

Define work done.

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When a force is applied to move an object over a distance and causes qa transfer of energy, work is done through the application of the force. In order for work to be done, both of the following conditions must be met:
~ A force acts on the object
~ The object moves in the same direction as the force applied on it.
Work is not always done when a force is applied. Work done is calculated by the following formula:

work done = force (N) x distance (m) moved in the direction of the force.

The unit of work done is joule, J, and the unit of force is N, and the unit of distance is m.

1J= 1Nm (newton metre)

State the types of energy stores.

Kinetic store, gravitational potential store, chemical potential store, internal (thermal) store, nuclear store, elastic potential store.

Define kinetic energy store.

Any object in motion has energy in the kinetic energy store. e.g. moving car. The energy in the kinetic energy store of a moving body is calculated as: Eₖ = 1/2 mv^2 **Where Eₖ is the kinetic energy of the body (J) m is the mass of the body (kg) v is the speed of the body (m/s)**

Define gravitational potential energy store.

An object above ground has energy in the gravitational potential energy store. It is calculated as such: Eₚ = mgh **Where Eₚ is gravitational potential energy in J, m is the mass of the body in kg, g is the gravitational field strength (10 N/kg or 10 m/s^2), and h is the vertical height of the body from the ground in m.**

Define chemical potential energy store.

Energy is stored due to chemical bonds, such as those between molecules. e.g. batteries, fuels, food etc.

Define elastic potential energy store.

An object has energy in the elastic potential store when it is stretched or compressed. e.g. stretched rubber band.

Define internal (thermal) energy store.

The total energy in the kinetic and potential energy stores of the particles in an object, due to the vibrations and the positions of the particles in the object. e.g. A hotter body has a greater amount of internal (thermal) store (the particles in The object vibrates faster).

Define nuclear energy store.

The energy that nuclei possess due to the force between the particles in the nuclei. e.g. A large amount of energy is released during a nuclear fission process or a nuclear fusion process.

State the pathways through which energy can be transferred from one store to another

- electromagnetic radiation or mechanical sound waves (propagation of waves) - electric current - application of a force through a distance - heating due to temp diff

Provide examples for the transfer of energy from one store to another through electromagnetic radiation or mechanical sound waves (propagation of waves)

Energy from the sun is transferred through radiation to one’s skin, making it warm. When a sound is made, energy is transferred to one’s ear by sound waves, making the eardrums vibrate, and the sound is detected.

Provide examples for the transfer of energy from one store to another electrically by electric current.

Energy is transferred from the chemical potential energy store of the battery to the internal (thermal) store of the filament of the light bulb, causing it to become hotter and glow.

Provide examples for the transfer of energy from one store to another through the application of force over a distance.

When a boy applies a pushing force on a table, energy is transferred from the chemical potential store of the boy to the kinetic store of the table, causing it to move.

Provide examples for the transfer of energy from one store to another by heating.

When we touch a hot surface, energy is transferred by heating from the internal (thermal) store of the surface to the internal (thermal) store of our hand, causing the temperature of the hand to increase.

What is the principle of conservation of energy?

The principle of conservation of energy states that energy cannot be created nor destroyed, but can be transferred from one store to another.

Explain the conservation of energy of a pendulum bob.

Moving from top left to middle, as the height x decreases from top to middle, energy is transferred mechanically from the gravitational potential store at top left to kinetic store at middle. Moving from middle to top right, as the pendulum bob moves from middle to top right, energy is transferred mechanically from the kinetic store at middle to the gravitational potential store at top right. The energy of the pendulum bob continuously changes between kinetic and gravitational potential energy stores by mechanical means of transfer of energy. However, the total amount of energy remains constant at all points during the swinging.

Define power

Power is the rate of work done or the rate of energy transferred. The unit of power is W (watt) or J/s. P is calculated as such: P= ʷᵈ⁄ₜ or P= ᵉ⁄ₜ Where power = power (W), WD= work done (J), E= energy (J), and t = time (s).

Girl A reaches the rooftop before Girl B. Both girls are 40 kg, A took 50s to reach the rooftop vs B took 60s to reach the rooftop. Who does more work, who is more powerful, and who is faster?

both do same amount of work. a is more faster and more powerful than b.

Boys A and B reach the rooftop at the same time. A is 80kg, vs B is 60kg. Who does more work, who is more powerful, and who is faster?

A does more work, Both are the same speed and C is more powerful.

How can energy sources be divided? Provide 1-2 examples from each category.

Non-renewable and renewable. Non-renewable Fossil fuels. Most non-renewable energy sources, such as coal and crude oil, are produced from fossil fuels. Fossil fuels are formed by the remains of dead plants and animals buried in the Earth millions of years ago, and are commonly used to generate electrical power in power stations. The burning of fossil fuels in power stations involves the following energy transfer: Chemical potential energy (f.f.) -> heat energy -> kinetic energy (turbine) -> electrical energy (generator) energy from burning fuel is used to boil water to produce steam to turn turbines and generate electricity. When fossil fuels are burnt, air pollutants and greenhouse gases such as CO2 are produced. They contribute to global warming and increase the rate of climate change, increasing the intensity of natural disasters. It can also bring about erratic + destructive weather patterns. Renewable source of energy: Solar power comes from the sun. The sun has energy in its nuclear store. It is commonly used to power devices such as LED street lights and weather recording instruments. Energy transfer: light energy (sun) -> electrical (solar cell)

Three pupils and 2 teachers are pulling in opposite directions on a rope. The pupils each can with a force of 350 N, and each teacher pulls with a force of 500 N. What is the resultant force on the rope?

50 N to the **right**.

If friction is absent, which of the following can successfully still take part? a. Bouncing a basketball. b. Leaning against a wall. c. Running along a track. d. Stopping a speeding car.

A box exerts a pressure of 10 N/m^2 on the ground. Given that the contact area between the box and the ground is 2.5 m^2 and that the gravitational field strength is 10 N/kg, calculate the weight of the box.

10 x 2.5 = 25 N

A person exerts a horizontal force of 500 N on a box, which also experiences a friction force of 100 N. How much work is done **against** friction when the box moves in a horizontal distance of 3 m?

100 N x 3 m = 300 J

A fruit seller uses a knife with a sharp edge and a cross-sectional area of 0.5 cm^2 to cut open a watermelon. If the force applied on the knife is 18 N, what is the pressure exerted by the knife on the watermelon?

p = f/a 0.5cm^2 to m^2: 0.5 x 10^4 m^2 Pa= 18 N divided by (0.5 x 10^4) m^2 = 360 000 Pa

note: 1m = 100 cm 1 m^2 = 100^2 cm^2

ok bud

A fruit seller uses a knife with a sharp edge and a cross-sectional area of 0.5 cm^2 to cut open a papaya, exerting a Pa of 270,000. Calculate the magnitude of the force applied to cut the papaya.

F = Pa x a = 270 000 x (0.5 divided by 10^4) = 13.5 N

A waiter carries a tray full of meals above his head by one arm straight across the room at a constant speed. Determine whether work is done.

A vertically upward force is applied to the tray, but the tray moves in a horizontal distance, not in the same direction as the force applied. Therefore, work is not done.

A ball with a weight of 4 N rolls on a table along a straight length of 1m before falling off vertically from the edge of the table at a height of 0.5 m. Calculate the work done when the ball rolls on the table.

When the ball moves horizontally on the table, it is not moving in the same direction as the 4 N force, which acts vertically downwards. Hence, there is no work done; 4 N x 0 m = 0 J.

Boy holds a pack of chips and lifts it vertically from the floor thru a height of 0.6 m. The work done by Boy on the pack is determined to be 3 J. What is the magnitude of the vertical upward lifting force applied by Boy?

WD = f x d 3 j = F - 0.6 M f = 5 N

A bullet of mass 10 g moves at the speed of 500 m/s aft it is fired from a pistol. Determine the energy in the kinetic store of the moving object.

E_k = 1/2 mv^2 E_k = 1/2 [(10 divided by 1000) x 500^2] = 1250 J

A climber of mass 80 kg climbs to the top of a 25 m high outdoor rock climbing wall. Determine the G.P.E. of the body.

E_p = mgh E_p = 80 x 10 x 25 = 20 000 J

A child of 20 kg climbs up the ladder of 3.0 m as shown in the picture below and moves down a slide of 3.5 m in length. The vertical length from the ground to the top of the slide is 2.5 m. What is G.P.E. gained by the child at the top of the ladder, and what is G.P.E. lost by the child at the end of his journey down the slide?

E_p = mgh E_p = 20 x 10 x 2.5 = 500J

A 5 kg box is pushed horizontally on a smooth surface by a 10 N force. The box moves 4 m horizontally in 20s. Find the average power.

P = WD/t = F x D / t = 10 x 4 / 20 = 2.0 W

always put **.0** after power measurement e.g. 2.0 W, 10.0 W

okboss

Sci Chap 9 Flashcards

(57 cards)