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. 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 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 Acceleration: • Acceleration is caused by unbalanced forces: – slow down – speed up – stop – start – change direction – change shape 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. Object will move 7N to the right 4N 3N N N – Forces in opposite directions are subtracted. • An object will move in the direction of the greater force. 8N 3N Object will move 5N to the left N N Calculating Net Force: 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 upward - 10 N downward = 20 N upward Two competitors are playing tug of war. What is the net force? Which direction will the rope move? Tug of War Game 50N to the right – 30N to the left = 20N 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). 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. F m a Force (N) = mass (kg) x acceleration (m/s2) Acceleration (m/s2) = force (N) ÷ mass (kg) Mass (kg) = force (N) ÷ acceleration (m/s2) Practice: 1. 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 = ? Acceleration = 10 m/s2 Mass = force ÷ acceleration Mass = 650 ÷ 10 Mass = 65 2. A 56 kg cart is accelerating at 15 m/s2. Calculate the force exerted on the cart? Force = ? Mass = 56 kg Acceleration = 15 m/s2 Force (N) = mass (kg) x acceleration (m/s2) Force = 56 x 15 Force = 840 N 3. 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 = ? Acceleration = force ÷ mass Acceleration = 108 ÷ 54 Acceleration = 2 .