Force Force Force Force

8th Grade8th Grade

Force and MotionForce and Motion

Force• When you push on an object, you are

exerting a force on that object.

• Force: a pushing interaction between two objects.

• A force can only happen when an object pushes on another object.

Measuring Force• Newton: the unit of force in the metric

system.

• The symbol for newton is N.

Force - Big Ideas• The greater the mass, the more force to exert

to move it; the pulling force needed to move the mass is the same as the pushing force.

• The magnitude and direction of the forces determine if and in what direction an object will move.

Force - Big Ideas• Forces acting in the same direction add

together.

• Forces acting in opposite directions subtract.

Force - Big Ideas• If the force exerted on one side of an

object is greater than the force of exerted on the other side of an object, the object will move toward the smaller force.

Force - Big Ideas• If equal forces are exerted on an object

in opposite directions, the object’s motion will not change.

Force - Practice Question #1

• What happens when one pusher is held stationary on one side of a Dotcar, and a force is exerted on the other side with a second pusher?

Force - Practice Question#1

• Answer: The stationary pusher pushes with a force equal to the force exerted by the pusher on the other side of the Dotcar. The force is the same on both sides, so the sled doesn’t move.

Force - Practice Question #2

• What happens when a pusher pushes on one side of a Dotcar and a puller pulls on the other side of the Dotcar?

Force - Practice Question #2

• The forces add because they are acting in the same direction. The Dotcar will move toward the puller because both forces are being applied in that direction.

Force - Practice Question #3

• What will happen when one pusher exerts 2N of force on one side of a Dotcar and another pusher exerts 4N of force on the other side of the dot car?

Force - Practice Question #3

• Answer: The Dotcar will move toward the 2N pusher because more force is making it go in that direction.

Force - Practice Question #4

• What causes an object to start moving?

Force - Practice Question #4

• Answer: A force exerted on the object.

Force - Practice Question #5

• What happens when equal forces are exerted on opposite sides of an unloaded Dotcar?

Force - Practice Question #5

• Answer: Equal forces try to move the car in opposite directions and cancel each other. The cars do not move.

Friction• Friction: the force resisting motion.

– Friction is an interaction between surfaces.– Friction is a force resisting motion.

• Until there is force to overcome an object, the object will not move.

Net Force• A net force applied to a mass produces

change of motion.• Definition 1: If two opposite forces act on an

object, the difference between the strength of the two forces is the net force. – Example: If F1 and F2 are equal, the net force is

zero and the cart will not move. – If F1 is 100N and F2 is 90N, there is a net force of

10 N to the right, so the cart will move to the right.

Picture of Net Force Definition 1

+ F (Right)

- F (Left)

Net Force• Definition 2: The sum of the forces acting

on an object is the net force. – If the net force is positive, the object moves to

the right. – If the net force is negative, the object moves to

the left. – If the net force is zero, no change of motion

results.

Position• Changes of position toward the

destination is motion in a positive direction.

• Changes of position toward home is motion in a negative direction.

• By adding the positive and negative positions, position could be determined.

Resting Objects• Question: If an object is resting on a

table, are there any forces acting on the objects?

Resting Objects• Answer: Gravity is a force pulling the

book and the table down. The table is also exerting a force up that is equal to the force pulling the book down.

Net Force Example #1• If a person is pushing on a cart against

the wall with 600N of force. How can you explain why the cart did not move?

Net Force Example #1• Answer: The forces will cancel each

other. The cart will not move because the wall pushed back with a force of 500 N. The net force acting on the cart was zero.

Net Force Example #2 • If an Force A is larger in magnitude

than Force B , what will happen to the cart?

Net Force Example #2• Answer: Force A will “overpower”

Force B and the cart will move to the right.