# What are forces?. What are forces? Push and pull.

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What are forces?

Push and pull

Forces have a magnitude (measured in Newtons) and a direction
Forces come in pairs Forces have a magnitude (measured in Newtons) and a direction Forces are often represented with arrows that show the direction and magnitude Magnitude: 5N Direction: north (up) 5N, north (up)

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. Teacher notes Forces during take off include gravity, thrust and air resistance (friction). Photo credit: NASA Marshall Space Flight Center (NASA-MSFC) 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. Balanced forces do not cause a change in motion.

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 are forces that are not equal in size or are not opposite in direction Unbalanced forces lead to a change in speed or direction.

Because Gravity and Upthrust are equal the ship stays afloat.
Balanced Forces The ship is floating on the water, the forces are balanced. Which forces are acting on the ship? Gravity (Water’s) Upthrust Because Gravity and Upthrust are equal the ship stays afloat.

Balanced Forces The vase is resting on the table. It is not moving, therefore the forces must be balanced. Which forces are acting on the vase? Have a look around the classroom and name some of the balanced forces in action on different objects. Gravity Table’s Upthrust

Unbalanced Forces Air resistance is pushing upwards The man and the parachute are slowly falling to the ground. Which forces are acting on the parachute? Which force is bigger? How can you tell? Gravity is the bigger force. We can tell this because the direction of movement of the man and the parachute is the same as the direction of the force of gravity. Gravity is pulling the man to the ground

Gravity is pulling to the ground
Unbalanced Forces Air resistance is pushing upwards The same forces are acting on the elephant but without a parachute there is much less air resistance. Gravity is a much bigger force. Because there is a bigger difference between the force pulling downwards and the force pushing upwards the elephant falls much more quickly than a man with a parachute. Gravity is pulling to the ground

Unbalanced Forces The magnetic force is pulling the paperclip upwards
The paperclip is jumping up to the magnet. Which forces are acting on the paper clip? Which is the greater force? The magnetic force is greater than the force of gravity therefore the paperclip moves in the direction of the biggest force. Gravity is pulling the paperclip to the ground

Unbalanced Forces 5 N, right + 10 N, right = 15N, right

Forces Each team is exerting a force – what happens if the pulling force produced by each team is equal? What would happen if the team on the left were able to produce a bigger pulling force than the team on the right?

Force pairs Teacher Notes
Encourage students to predict the outcome of each simulation. 15

Balanced and unbalanced forces
Imagine a car traveling at a constant speed of 50 km/h. The engine provides sufficient force to balance all the frictional forces that are acting to decrease the speed. 500 N 500 N 16

Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force. 500 N 500 N 100 N 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? 17

Resultant force The sum effect of more than one force is called the resultant force. The resultant force is calculated by finding the difference between opposing forces in each direction. What is the resultant force on this truck? A resultant force of 100 N is accelerating the truck. 400 N 500 N 18

Balanced and unbalanced forces – summary
If the forces on an object are balanced: 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. In other words, the object will continue to do what it is already doing without any change. If the forces are unbalanced, four things can happen: The speed can change. It can start moving. It can stop moving. It can change direction. 19

Resultant forces – question 1
1. What is the resultant force on the satellite? 5 N 5 N Resultant force = 20 N – 10 N = 10 N down The satellite will accelerate downwards. 20 N 20

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 = 5 N – 5 N = 0 N The vertical forces are not balanced, the bird will accelerate in a downwards direction. 5 N 5 N Resultant force = 5 N – 0 N = 5 N down 5 N 21

Resultant forces – question 3
3. What is the resultant force on the yacht? 10 N 10 N 13 N 20 N 10 N 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 = (20 N +10 N) – 13 N = 17 N right 22

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