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What are forces?. Push and pull. Types of forces. All forces involve interactions between objects. There are several different types. 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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Types of forces All forces involve interactions between objects. There are several different types. Gravityand magnetism are forces that can act over distances. Frictionandupthrustare forces that involve direct contact between objects. All types of forces can occur whether objects are still or moving. What forces are acting during this rocket launch?
Balanced forces 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. The forces are balanced, so they cancel each other out. The skater does not move.
Unbalanced forces 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. Unbalanced forces lead to a change in speed or direction.
Balanced and unbalanced forces Imagine a car travelling at a constant speed of 50km/h. The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed. 500N 500N
Balanced and unbalanced forces A crosswind acting on the car produces a sideways force. 500N 500N 100N cross wind 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. If the car turns right so that the wind is now behind the car, what will happen to the speed?
Resultant force The sum effect of more than one force is called theresultant force. The resultant force is calculated by working out the difference between opposing forces in each direction. What is the resultant force on this truck? A resultant force of 100N is accelerating the truck. 400N 500N
Balanced and unbalanced forces – summary If the forces on an object are balanced: • and the object isstationary, it will remain stationary • and the object ismoving, it will continuetomove at the same speed and in a straight line. In other words, the object will continue to do what it is already doing without any change. If the forces are unbalanced, two things can happen: • Thespeed can change.This is called acceleration. • Thedirectionof motioncan change.
Resultant forces – question 1 1. What is the resultant force on the satellite? 5N 5N Resultant force = 20N –10N = 10N down The satellite will accelerate downwards. 20N
Resultant forces – question 2 2. What is the resultant force on the bird? The forces acting in each direction horizontally are equal in size, so there is no resultant force in this direction. Resultant force = 5N – 5N = 0N The vertical forces are not balanced, the bird will accelerate in a downwards direction. 5N 5N Resultant force = 5N – 0N = 5N down 5N
Resultant forces – question 3 3. What is the resultant force on the yacht? 10N 10N 13N 20N 10N 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. Resultant force = (20N +10N) – 13N = 17N right
What is gravity? When the netball is thrown, why does it fall back down? There is a gravitational force pulling it towards the Earth. Gravityis a force that occurs between all objects. Gravity always acts to pull objects towards each other. The Earth and the ball are pulling each other together. However, the ball moves much more than the Earth because it has a much smaller mass.
Sir Isaac Newton Sir Isaac Newton is a very famous physicist who lived in England 1643–1727. 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…
Gravity and Newton Newton realized that the motion of falling objects and objects orbiting in space must be caused by the same force – Gravity! 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.
What is gravity? Satellite’s gravity Earth’s gravity Gravityis an attractive force that acts between all objects that have mass. The size of the force depends on the masses of the objects. All objects produce a gravitational force. This is very large for huge masses such as planets. 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.
Gravity and distance The force of gravity between two objects depends on theirmassesand thedistancebetween them. Spacecraft produce a very large force, called thrust, to overcome the force of gravity. As a spacecraft gets further away from Earth, the force of gravity gets smaller. Why do spacecraft lose their large fuel tanks and booster rockets once they have left the Earth’s surface?
What are mass and weight? Massis the amount of matter in an object and is measured in kilograms. Mass is not a force. An object, such as this satellite, has the same mass at any point in the Universe. Weightis 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.
Mass and weight on the Moon The force of gravity on the Moon is only one-sixth of that on Earth because the Moon has a much smaller mass. 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. However, the astronaut still has the same mass – they just weigh less because gravity is weaker.
