Post on 25-Dec-2015
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What are forces?
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Push and pull
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All forces involve interactions between objects. There are several different types.
Types of forces
What forces are acting during this rocket launch?
All types of forces can occur whether objects are still or moving.
Gravity and magnetism are forces that can act over distances.
Friction and upthrust are forces that involve direct contact between objects.
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Forces affecting objects
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Forces changing an object’s shape
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There are two wind machines applying forces to the ice skater. The forces acting on the skater are equal in magnitude and opposite in direction.
Balanced forces
The forces are balanced, so they cancel each other out. The skater does not move.
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Unbalanced forces
Unbalanced forces lead to a change in speed or direction.
What if only one wind machine is blowing on the skater?
The forces acting on her are no longer balanced so she will start to move to the left. Her speed will change – this is called acceleration.
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Measuring forces
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Force pairs
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500 N 500 N
Imagine a car travelling at a constant speed of 50 km/h.
Balanced and unbalanced forces
The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed.
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A crosswind acting on the car produces a sideways force.
The crosswind causes the direction of the car to change – this happens because the sideways forces on the car are not balanced. The car will veer sideways.
Balanced and unbalanced forces
If the car turns right so that the wind is now behind the car, what will happen to the speed?
100 Ncross wind
500 N 500 N
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The resultant force is calculated by working out the difference between opposing forces in each direction.What is the resultant force on this truck?
The sum effect of more than one force is called the resultant force.
A resultant force of 100 N is accelerating the truck.
Resultant force
500 N400 N
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Balanced and unbalanced forces – summary
and the object is stationary, it will remain stationary
and the object is moving, it will continue to move at the same speed and in a straight line.
If the forces are unbalanced, two things can happen:
If the forces on an object are balanced:
In other words, the object will continue to do what it is already doing without any change.
The speed can change. This is called acceleration.
The direction of motion can change.
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20 N
1. What is the resultant force on the satellite?
Resultant force = 20 N – 10 N= 10 N down
The satellite will accelerate downwards.
Resultant forces – question 1
5 N 5 N
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5 N
2. What is the resultant force on the bird?
5 N 5 N
The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction.
Resultant force = 5 N – 5 N = 0 N
Resultant forces – question 2
The vertical forces are not balanced, the bird will accelerate in a downwards direction.
Resultant force = 5 N – 0 N = 5 N down
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The vertical forces are equal in size and opposite in direction so there is no resultant force in the vertical direction.
The horizontal forces are not balanced, so the yacht will accelerate to the right.
10 N
13 N
3. What is the resultant force on the yacht?
10 N
10 N
Resultant force = (20 N +10 N) – 13 N = 17 N right
Resultant forces – question 3
20 N
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When the netball is thrown, why does it fall back down?
What is gravity?
The Earth and the ball are pulling each other together.
There is a gravitational force pulling it towards the Earth.
Gravity is a force that occurs between all objects.
Gravity always acts to pull objects towards each other.
However, the ball moves much more than the Earth because it has a much smaller mass.
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Sir Isaac Newton is a very famous physicist who lived in England 1643–1727.
Sir Isaac Newton
Newton wrote down his ideas in the Philosophiae Naturalis Principia Mathematica; a very important book about forces and gravity.
Some accounts suggest that one of Newton’s greatest discoveries occurred when an apple fell on his head and it made him think about the reason it fell…
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Gravity and Newton
He wrote in the Philosophiae Naturalis, “It is an attractive force that makes apples fall from trees and the planets orbit the Sun.”
Other scientists had already noted the effects of gravity but Newton was the first to calculate the force of gravity on objects.
Newton realized that the motion of falling objects and objects orbiting in space must be caused by the same force – Gravity!
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Gravity is an attractive force that acts between all objects that have mass. The size of the force depends on the masses of the objects.
When you jump, the gravitational force of the Earth pulls you down. Your gravitational force also pulls the Earth towards you, but you don’t notice it because the Earth is too heavy to be visibly affected by your gravity.
What is gravity?
All objects produce a gravitational force. This is very large for huge masses such as planets.
Earth’s gravity
Satellite’s gravity
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The force of gravity between two objects depends on their masses and the distance between them.
Gravity and distance
Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface?
As a spacecraft gets further away from Earth, the force of gravity gets smaller.
Spacecraft produce a very large force, called thrust, to overcome the force of gravity.
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Gravity during a rocket launch
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Mass is the amount of matter in an object and is measured in kilograms.
What are mass and weight?
Mass is not a force.
An object, such as this satellite, has the same mass at any point in the Universe.
Weight is a force and is caused by the pull of gravity acting on a mass.
Weight is measured in newtons and has both magnitude and direction. An object’s weight changes depending on where it is in the Universe.
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Investigating mass and weight
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The force of gravity on the Moon is only one-sixth of that on Earth because the Moon has a much smaller mass.
Mass and weight on the Moon
However, the astronaut still has the same mass – they just weigh less because gravity is weaker.
Any object on the Moon weighs one-sixth of the amount it would weigh on Earth.
Astronauts can jump up 20 feet on the Moon due to there being such a low gravitational force.
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Mass and weight on different planets
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Weight and mass activity
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Missing words about gravity
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More on forces…
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If you rub your hands together, they get warm because there is resistance to the rubbing motion.
What is friction?
What is the name of this resistive force?
Your hands might look smooth, but on a microscopic level they have rough surfaces, as shown in the photograph.
What causes this force?
It is called friction.
When you rub your hands together you feel the resistive force of friction.
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Friction always tries to slow moving objects down – it opposes motion. Frictional forces occur when two touching surfaces move past each other, in this case the box moving across the ground.
Friction and motion
Friction also occurs when things move through air. This is called air resistance.
pull of rope on boxfriction
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Label all sources of friction acting on this bike.
What are the sources of friction?
pedalbearing
links inchain
tyre androad
airresistance
brake pad
wheelbearing
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Sources of friction
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Friction experiment
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Friction is a very important force for the movement of cars. It acts in the opposite direction to the movement of the car.
Cars stopping
The time it takes for a car to brake is affected by the frictional forces between the car’s tyres and the road surface.
thrust from engine friction
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One of the most important sources of friction in cars is that between the tyres and the road.
The friction between the tyres and road is affected by the:
inflation pressure of the tyres
road surface
surface condition caused by the weather (rain, ice, etc).
When the car brakes, it’s important to have as much friction as possible so that the car does not skid.
Factors affecting stopping distance
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Explaining air resistance
Air resistance is a type of friction that occurs whenever an object moves through the air, and is caused by the frictional forces acting between the air and the object.
If the area of contact between the air and object is reduced, the object issaid to be streamlined.
400 N
300 N
Which of these cars is more streamlined?
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Air resistance and drag
The air resistance is determined by:
the size of the car
the shape of the car
the speed of the car.
The photo shows a streamlined car which has been shaped so that the flow of air around the body is made as smooth as possible. This minimizes the air resistance.
If you looked at the car from the front, how much surface area would you see?
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Effects of frictional forces
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How does a plane take-off?
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What forces are acting on the fish?
Forces in water
Upthrust is a force that pushes objects upwards in a liquid or a gas. This balances the weight of the fish, and is also known as buoyancy.
In order to move forward, the fish uses its tail to produce thrust. This force drives it through the water and is always opposed by friction.
thrust friction
weight
upthrust
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Weight and upthrust
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Upthrust quiz
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Forces – summary
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Glossary
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Anagrams
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Multiple-choice quiz