Forces & Motion

Motion A change in the position of an object Caused by force (a push or pull)

Force A push or pull on an object Measured in units called newtons (N) Measured with a spring scale Forces act in pairs Types of Force: gravity electric magnetic friction centripetal

Inertia An object at rest will remain at rest until acted upon by an unbalanced force. An object in motion will remain in motion until acted upon by an unbalanced force. When all forces acting on an object are equal (balanced)

Balanced Forces When all the forces acting on an object are equal (net force is zero) Balanced forces do not cause a change in motion. Balanced forces can change the physical properties of an object without changing its motion. In your notes, describe an example of a balanced force.

How Can Balanced Forces Affect Objects? Cause the shape of an object to change without changing its motion Cause an object at rest to stay at rest or an object in motion to stay in motion (inertia) Cause an object moving at a constant speed to continue at a constant speed In your notes, describe an example of a balanced force affecting an object.

Unbalanced Forces When all the forces acting on an object are not equal The forces can be in the same direction or in opposite directions. Unbalanced forces cause a change in motion. In your notes, describe an example of an unbalanced force.

How Can Unbalanced Forces Affect Objects? Acceleration is caused by unbalanced forces: slow down speed up stop start change direction change shape In your notes, describe an example of an unbalanced force affecting an object.

Net Force The total of all forces acting on an object: Forces in the same direction are added. The object will move in the direction of the force. Forces in opposite directions are subtracted. An object will move in the direction of the greater force.

Calculating Net Force 10 N 30 N http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg A child was playing with a jack in the box. The lid pushed down with 10 N of force, while the spring pushed up with 30 N of force. What is the net force applied by the spring? In your notebook, calculate the net force and show all of your work.

Check Your Answer 10 N 30 N http://commons.wikimedia.org/wiki/File:Jack-in-the-box.jpg A child was playing with a jack in the box. The lid pushed down with 10 N of force, while the spring pushed up with 30 N of force. What is the net force applied by the spring? 30 N up - 10 N down= 20 N up

Calculating Net Force Two competitors are playing tug of war. Tug of War Game 30 N 50 N Two competitors are playing tug of war. What is the net force? Which direction will the rope move? In your notebook, calculate the net force and show all of your work.

Check Your Answer Two competitors are playing tug of war. Tug of War Game 50 N 30 N Two competitors are playing tug of war. What is the net force? Which direction will the rope move? 50 N to the right – 30 N to the left = 20 N to the right

Mass and Weight Mass is the amount of stuff (matter) in an object. The mass of an object will remain the same anywhere in the universe. Mass is measured in grams or kilograms. Weight is the mass (kg) of an object multiplied by the acceleration of gravity (9.8 m/s2). The weight of an object depends on the gravitational pull of the location in the universe. The gravitational pull on the Moon is only 1/6 of Earth’s gravitational pull. Weight is measured in newtons (N).

Mass and Weight Example: F=ma A person with a mass of 25 kg x 9.8 m/s2 gravitational acceleration is equal to 245 N of weight force. F= 25 kg x 9.8 m/s2 F = 245 N The person exerts 245 N of force on the ground. If the person went to the Moon, they would only weigh 1/6 of 245 N.

Force Force (N) = mass (kg) x acceleration (m/s2) F ÷ m x a

Force Acceleration (m/s2) = force (N) ÷ mass (kg) F ÷ m x a

Force Mass (kg) = force (N) ÷ acceleration (m/s2) F ÷ m x a

Practice The acceleration of a sprinter is 10 m/s2. The force exerted on the starting blocks is 650 N. What is the mass of the sprinter? Force = Mass = Acceleration =

Check Your Answer The acceleration of a sprinter is 10 m/s2. The force exerted on the starting blocks is 650 N. What is the mass of the sprinter? Force = 650 N Mass = 65 kg Acceleration = 10 m/s2 Mass = force ÷ acceleration Mass = 650 ÷ 10 Mass = 65

Practice A 56 kg cart is accelerating at 15 m/s2. Calculate the force exerted on the cart? Force = Mass = Acceleration =

Check Your Answer A 56 kg cart is accelerating at 15 m/s2. Calculate the force exerted on the cart? Force = 840 N Mass = 56 kg Acceleration = 15 m/s2 Force (N) = mass (kg) x acceleration (m/s2) Force = 56 x 15 Force = 840

Practice A boat is pulling a 54 kg wake boarder. She accelerates at 2 m/s2. Calculate the force that is causing the acceleration. Force = Mass = Acceleration =

Check Your Answer A boat is pulling a 54 kg wake boarder. The force the boat is exerting on her is 108 N. Calculate her acceleration. Force = 108 N Mass = 54 kg Acceleration = 2 m/s2 Acceleration = force ÷ mass Acceleration = 108 ÷ 54 Acceleration = 2