4 Forces have a magnitude (measured in Newtons) and a direction Forces come in pairsForces have a magnitude (measured in Newtons) and a directionForces are often represented with arrows that show the direction and magnitudeMagnitude: 5NDirection: north (up)5N, north (up)
5 Types of forcesAll 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 notesForces 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?
6 Balanced forcesThere 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.
7 Unbalanced forcesWhat 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 directionUnbalanced forces lead to a change in speed or direction.
8 Because Gravity and Upthrust are equal the ship stays afloat. Balanced ForcesThe ship is floating on the water, the forces are balanced. Which forces are acting on the ship?Gravity(Water’s) UpthrustBecause Gravity and Upthrust are equal the ship stays afloat.
9 Balanced ForcesThe 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.GravityTable’s Upthrust
10 Unbalanced ForcesAir resistance is pushing upwardsThe 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
11 Gravity is pulling to the ground Unbalanced ForcesAir resistance is pushing upwardsThe 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
12 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
13 Unbalanced Forces5 N, right + 10 N, right =15N, right
14 ForcesEach 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?
15 Force pairs Teacher Notes Encourage students to predict the outcome of each simulation.15
16 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 N500 N16
17 Balanced and unbalanced forces A crosswind acting on the car produces a sideways force.500 N500 N100 Ncross windThe 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
18 Resultant forceThe 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 N500 N18
19 Balanced and unbalanced forces – summary If the forces on an object are balanced:and the object is stationary, it will remain stationaryand 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
20 Resultant forces – question 1 1. What is the resultant force on the satellite?5 N 5 NResultant force = 20 N – 10 N = 10 N downThe satellite will accelerate downwards.20 N20
21 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 NThe vertical forces are not balanced, the bird will accelerate in a downwards direction.5 N5 NResultant force = 5 N – 0 N = 5 N down5 N21
22 Resultant forces – question 3 3. What is the resultant force on the yacht?10 N10 N13 N20 N10 NThe 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 right22