Science: Free Body Diagrams
Identifying forces acting on objects
Science: Free Body Diagrams
Identifying forces acting on objects
Science - Grade 9-12
- 1
A book rests on a table. Identify all forces acting on the book and state the direction of each force.
Include only forces acting on the book itself.
The book has two forces acting on it. Gravity pulls the book downward, and the normal force from the table pushes the book upward. - 2
A box is pushed to the right across a rough floor at constant speed. Identify all forces acting on the box.
The box has four forces acting on it. Gravity acts downward, the normal force acts upward, the applied push acts to the right, and friction acts to the left. - 3
A ball is falling straight down through the air. Identify the forces acting on the ball before it reaches terminal velocity.
Think about the effect of air on a moving object.
The ball has gravity acting downward and air resistance acting upward. Before terminal velocity, gravity is greater than air resistance, so the ball accelerates downward. - 4
A sled is pulled to the right by a rope across snow. Friction is small but not zero. Identify all forces acting on the sled.
The sled has gravity acting downward, the normal force acting upward, tension from the rope pulling to the right, and friction acting to the left. - 5
A lamp hangs motionless from the ceiling by a cord. Identify all forces acting on the lamp.
A hanging object is usually pulled in opposite vertical directions.
The lamp has gravity acting downward and tension in the cord acting upward. Because the lamp is motionless, these forces are balanced. - 6
A car moves around a flat circular track at constant speed. Identify the horizontal force needed for this motion and its direction.
The horizontal force is static friction between the tires and the road, and it points toward the center of the circle. This inward force is the centripetal force. - 7
A crate is pulled upward by a rope and accelerates upward. Identify the forces acting on the crate and compare their sizes.
Acceleration points in the direction of the net force.
The crate has tension acting upward and gravity acting downward. Because the crate accelerates upward, the tension force is greater than the gravitational force. - 8
A skydiver falls at terminal velocity. Identify the forces acting on the skydiver and explain their relationship.
The skydiver has gravity acting downward and air resistance acting upward. At terminal velocity, these forces are equal in size, so the net force is zero. - 9
A block sits on a ramp and does not slide. Identify the forces acting on the block.
The normal force is not always straight up.
The block has gravity acting downward, the normal force acting perpendicular to the surface of the ramp, and static friction acting along the surface of the ramp to prevent sliding. - 10
A hockey puck slides to the right on nearly frictionless ice after being hit. Identify the forces acting on the puck after it leaves the stick.
After the puck leaves the stick, gravity acts downward and the normal force from the ice acts upward. If friction is negligible, there is no significant horizontal force acting on the puck. - 11
An elevator moves upward at constant speed. Identify the forces acting on the elevator and describe their balance.
Constant speed means no acceleration.
The elevator has tension from the cable acting upward and gravity acting downward. Because it moves at constant speed, these forces are balanced and the net force is zero. - 12
A person pushes a wall, but the wall does not move. Identify the forces acting on the wall due to the push and its support.
The wall experiences an applied push from the person in one horizontal direction and support forces from the building structure that balance that push. It also has gravity downward and support forces upward from the ground or structure. - 13
A satellite orbits Earth in nearly circular motion. Identify the main force acting on the satellite.
There is no normal force because the satellite is not touching a surface.
The main force acting on the satellite is Earth's gravitational force directed toward the center of Earth. This force provides the centripetal force for the orbit. - 14
A block is pulled left across a rough surface and slows down. Identify the forces acting on the block and explain the slowing motion.
The block has gravity downward, the normal force upward, an applied force to the left, and friction to the right. Because the block slows while moving left, the net force must be to the right, so friction is greater than the applied force. - 15
A backpack rests on the floor of a bus that is accelerating forward. If the backpack does not slide relative to the floor, identify the horizontal force acting on the backpack.
If the backpack stays in place relative to the bus, some horizontal force must accelerate it.
The horizontal force acting on the backpack is static friction from the floor, and it acts forward. This force accelerates the backpack along with the bus.