Newton’s Second Law. The net force on a body is equal to the product of the body’s mass and its acceleration. Units.
The net force on a body is equal to the product of the body’s mass and its acceleration.
Problem 20: A car traveling at 53 km/h hits a bridge abutment. A passenger in the car moves forward a distance of 65 cm (with respect to the road) while being brought to rest by an inflated air bag. What magnitude of force (assumed constant) acts on the passenger’s upper torso, which has a mass of 41 kg?
Gravitational force is the force that the Earth exerts on any object. It is directed toward the center of the Earth.
The magnitude of the gravitational force is equal to the product of mass and acceleration due to gravity.
The weightW of a body is the magnitude of the net force required to prevent the body from falling freely, as measured by someone on the ground.
The weightW of a body is equal to the magnitude of the gravitational force on the body. A body’s weight is related to the body’s mass by,
Normal Force: When a body presses against a surface, the surface deforms and pushes on the body with a normal force perpendicular to the contact surface. An example is shown in the picture to the left. A block of mass m rests on a table.
Note: In this case FN = mg. This is not always the case.
Friction: If we slide or attempt to slide an object over a surface, the motion is resisted by a bonding between the object and the surface. This force is known as “friction.” More on friction in Chapter 6.
When two bodies interact by exerting forces on each other, the forces are equal in magnitude and opposite in direction.
1. Choose the system to be studied.
2. Make a simple sketch of the system.
3. Choose a convenient coordinate system.
4. Identify all the forces that act on the system. Label them on the diagram.
5. Apply Newton’s laws of motion to the system.
P 17, page 109:In the figure , let the mass of the block be 8.5 kg and the angle θ be 30°. Find (a) the tension in the cord and (b) the normal force acting on the block. (c) If the cord is cut, find the magnitude of the resulting acceleration of the block.
Figure 5-12 shows a block S (the sliding block) with mass M = 3.3 kg. The block is free to move along a horizontal frictionless surface and connected, by a cord that wraps over a frictionless pulley, to a second block H (the hanging block), with mass m = 2.1 kg. The cord and pulley have negligible masses compared to the blocks (they are “massless”). The hanging block H falls as the sliding block S accelerates to the right. Find (a) the acceleration of block S, (b) the acceleration of block H, and (c) the tension in the cord.