Science Focus Lesson SC.5.P.13.2 Net Forces Polk County Public Schools

SC.5.P.13.2 Benchmark: Investigate and describe that the greater the force applied to it, the greater the change in motion of a given object. Essential Question: What is the difference between balanced and unbalanced forces? Vocabulary: balanced forces force unbalanced forces net force

What is Force? A force is a push or pull on an object Every force has a STRENGTH Every force has a DIRECTION Forces work in pairs, every force has an opposing force working in the opposite direction

How do forces affect objects? Forces can affect objects in several ways Forces can set an object in motion Forces can change an moving object’s speed and/or direction A force can also affect an object without making it move!

More than one force usually acts on an object at the same time Sometimes forces are applied in the same direction Sometimes forces are applied in different directions Balanced forces produce no change in the motion of an object Unbalanced forces produce a change in the motion of an object How do forces affect objects?

Balanced Forces Balanced forces are opposing forces of the same strength acting on an object Balanced forces result in NO CHANGE in the motion of an object Think about a game of tug-of-war. If there is equal pull on both sides, then each side is balanced and neither side will fall forward!

Balanced Forces Balanced forces do not cause a change in the motion of objects

Balanced forces at work: The window arches of the Roman Coliseum. The blocks in the window arches work together, exerting force against each other, thereby holding each other up. This is an example of balanced forces.

Unbalanced Forces Unbalanced forces are opposing forces of unequal strength acting on an object Unbalanced forces always CAUSE A CHANGE in motion. Think about a game of tug-of-war. If there is an unequal pull, then there are unbalanced forces and one side will fall forward!

Unbalanced Forces Unbalanced Forces happen when unequal forces act on an object

When you ride a bike the force of the friction of the tires against the road is greater than the opposing force of friction with the air. The bicycle moves forward. This is an example of unbalanced forces. Unbalanced forces at work: A bicycle rider

Summarizing Think-Pair-Share Think: How do you know when forces are balanced? unbalanced? Pair: Turn to your shoulder partner Share: Partner A describe balanced forces to partner B. Partner B describe unbalanced forces to partner A.

What is Net Force? A net force is the sum of all forces acting on an object. A net force is capable of accelerating a mass.

When net forces on each side are not equal, the object moves in the direction of the greater force. Example: The force exerted by Linus, on the right, is greater than the force exerted by Charlie Brown, on the left. Therefore the net force is greater on the right and the see-saw moves in the direction of the greater net force.

Forces can act in the same direction If two forces act on an object in opposite directions and the same force is not applied on each side, the difference in the forces is the net force. 12 N – 10 N = 2 N of force to the right If two(or more) forces act on an object in the same direction, the net force is the sum of the forces. 25 N + 20 N = 45 N of force to the right Forces can act in opposite directions

Whenever the net force is equal to zero, the object does not change its motion. When forces cancel each other out, the net force is zero. The object does not change its motion. The car has zero motion. The forces acting on it are of equal strength and opposite in direction. There is a net force of zero and the car does not change its motion. A space probe maintains both its speed and direction. The forces acting on it are of equal strength and opposite in direction. There is a net force of zero and the space probe does not change its motion.

Summarizing Explain how net forces determine the change in the motion of an object.

Guided Practice 1. A 3 Newton force is applied to the left side of a moose, and a 7 Newton force is applied to the right side of a moose. In which direction would the moose move based on the forces applied? A. right B. left C. no movement D. up

The moose would move to the left because there is a greater force being applied on the right side.

2. A woman at a grocery store applies an unbalanced force to a shopping cart. Which of the following things would happen? A.The shopping cart will start to move. B.The shopping cart does not move. C.The shopping cart gets smaller. D.The shopping cart gets bigger.

Unbalanced forces always cause a change in motion.

3. A car is not moving. A balanced force is applied to it. What happens to the car? A. It starts to move slowly, then faster B. It does not move C. It moves backwards D. It starts to accelerate

Balanced forces do NOT cause a change in the motion of objects.

4. In a tug of war contest, one person pulls with a force of 5 Newtons. Another person on the same team pulls with a force of 3 Newtons. In order for the force of the rope to be balanced, what force would the other team be pulling with? A. 2 Newtons B. 5 Newtons C. 8 Newtons D. 15 Newtons

For forces to be balanced they must be equal and opposite. If one side pulls with a combined force of 8 Newtons, the other side would also need to pull with a combined force of 8 Newtons in order for the forces to be balanced.

Turn to your shoulder partner and give examples of balanced and unbalanced forces. Switch roles. Summary Question

Check Your Understanding 1. When an object is at rest, which of the following is true? a. There are no forces acting on it. b. There are forces acting on it, but these forces are balanced. c. There is just one force acting on it, which is gravity. d. The forces acting on it are unequal in strength

Check Your Understanding 2. A girl is standing still on a tennis court, with her racket resting on her shoulder. Her tennis partner just hit a ball to her. Which object is showing the effect of an unbalanced force? a. The girl standing still b. The tennis racket c. The ball d. The ground

Check Your Understanding 3. Jennifer used a basketball to show how energy can be applied to the ball to produce motion. Which action would NOT produce motion by applying energy to the ball? A. She threw the ball to the floor and it bounced back into the air. B. She pitched the ball through the air and it flew into the hoop. C. She held the ball steady on the gym floor with her right foot. D. She placed the ball on the top shelf of a locker when she was finished with it.

Check Your Understanding 4. Why does it take more force to move a heavy box up a ramp than to move the same box down the ramp? A. There is more friction on the bottom of the box when going up a ramp. B. Gravity is pushing the box up the ramp at the same time you are pushing the box up the ramp. C. You are exerting force from both your arms and legs when you are pushing the box up the ram. D. When you push an object up a ramp, you are working against gravity which pulls the box towards the ground.

Check Your Answers 1. B - There are forces acting on it, but these forces are balanced. 2. C- The ball 3. C- She held the ball steady on the gym floor with her right foot. 4. D- In order to move a box up a ramp you must use a force greater than the force of gravity pulling it toward the ground

Summarizing Answer the Essential Question in your Science Notebook. Essential Question: When are forces balanced or unbalanced? Give examples of each.