Motion and Force What is a Force Force

Motion and Force

What is a Force? • Force- any action that can change the state of motion of an object • Four fundamental forces in nature – Gravity – Electromagnetic force – Strong nuclear force – Weak nuclear force • F=mxa – Force equals mass times acceleration

Fundamental Forces • Vary in strength • How do forces act? – Contact forces- push/pull – Field forces- do not require objects to touch each other (ex: gravity) • Both types of forces can cause an object to move or stop moving

Balanced vs. Unbalanced Forces • Net force- combination of all forces acting on an object – Object accelerates in the direction of the net force (unbalanced) – Object will not accelerate if the net force is zero (balanced)

Newton’s First Law • An object at rest remains at rest and an object in motion stays in motion unless it experiences a net force. • Inertia: the tendency of an object to resist change in motion – Related to the mass of an object – Objects with greater mass have a greater inertia (Is it harder to move a baseball or a car? ) • Newton’s First Law is also called the Law of Inertia

Newton’s Second Law • The unbalanced force acting on an object equals the object’s mass times its acceleration. • Force is measured in Newtons (N) – 1 N = 0. 225 pounds

How do We Calculate Net Force? • If forces are in the same direction, you add them. What is the net force acting on this box? • If forces are in opposite directions, you subtract. What is the net force?

Calculating Acceleration •

Multi-Step Problems 1. A 2000 kg car moving at 10 m/s accelerates to 15 m/s in 3 seconds. What is the force applied to the car? 2. A 2 kg ball is thrown from rest to a speed of 60 m/s. How much time is needed if the net force applied is 300 N? 3. A force of 45 N is applied to a 15 kg cart for 6 seconds. If it starts from rest, what will be its final velocity?

Practice Problems 1. A 1300 kg car moving at 25 m/s accelerates to 40 m/s in 3 seconds. What is the force applied to the car? 2. A 1. 4 kg ball is thrown from rest to a speed of 85 m/s. How much time is needed if the net force applied is 500 N? 3. A force of 300 N is applied to a 40 kg cart for 1. 5 seconds. If it starts at 20 m/s, what will be its final velocity?

Friction • Friction- force that opposes motion – Produces heat • Static friction is between stationary surfaces • Kinetic friction is between moving surfaces

Newton’s Third Law • For every action force, there is an equal and opposite reaction force. • Forces always occur in pairs!

Momentum • Defined as the product of an object’s mass and velocity p = mv • Momentum increases as mass and velocity increase.

Calculating Momentum • Calculate the momentum of a 6. 0 kg bowling ball moving at 10. 0 m/s down the alley toward the pins. p = mv p = (6. 0 kg)(10. 0 m/s) = 60. 0 kg m/s