Forces - pushes or pulls

Contact forces - requires contact to act.Noncontact forces - action at a distance.

Newton’s First Law - objects resist acceleration. Law of inertia. The mass of an object is a quantitative measure of inertia.

An inertial frame of reference is one in which Newton’s law of inertia is valid.

Newton’s second law - F = ma. The unit of force is the Newton.

1 Newton = 1kg•m/s2.

Free-body diagram A diagram that represents the object and the forces that act on it.

Ex. 1 - Two people are pushing a stalled car. The mass of the car is 1850 kg. One person applies a force of 275 N to the car, while the other applies a force of 395 N. Both forces act in the same direction. Frictional forces of 560 N act in the opposite direction. Find the acceleration of the car.

Ex. 2 - A man is stranded on a raft (mass of man and raft = 1300 kg). By paddling, he causes an average force P of 17 N to be applied to the raft in a direction due east . The wind also exerts a force A on the raft. This force has a magnitude of 15 N and points 67° north of east. Ignoring water resistance, find the x and y components of the raft’s acceleration.

Ex. 3 - At the moment the forces P and A begin acting on the raft, the velocity of the raft is 0.15 m/s, in a direction due east. Assuming that the forces are maintained for 65 s, find the x and y components of the raft’s displacement during this time interval.

Newton’s Third Law - for every action (force) there is an equal and opposite reaction. The third law describes two different forces being applied to two different objects.

Ex. 4 - The mass of a spacecraft is mS = 11 000 kg and that of an astronaut is mA = 92 kg. The astronaut exerts a force of F = +36 N on the spacecraft from outside. Find the accelerations of the spacecraft and the astronaut.

Two general types of forces: Fundamental forces and Nonfundamental forces. Fundamental forces are unique and are used to describe other forces.

Three fundamental forces:

1. Gravitational force2. Strong nuclear force3. Electroweak force.