Forces & Motion
A push or pull on an object resulting from the object’s interaction with another object.
Force
What happens to an object when the forces acting on it are unbalanced?
Its motion changes. It may start moving, stop moving, speed up, slow down, or change direction.
What is mass?
Amount of matter in an object.
When a car suddenly stops, passengers tend to move forward. What concept explains this motion?
Inertia
Why should passengers wear seat belts when riding in a car?
Seat belts prevent passengers from continuing to move forward due to inertia when the car suddenly stops.
When the sum of all forces acting on an object is zero.
Balanced Forces
A stationary soccer ball is kicked and begins to roll across the field. What caused the ball's motion to change?
An unbalanced force from the kick acted on the ball.
What property of an object causes it to resist changes in its motion?
Inertia
A book remains on a table until someone pushes it. Which characteristic of inertia is demonstrated by the book?
An object at rest tends to stay at rest unless acted upon by an external force.
When a bus suddenly starts moving, passengers often lean backward. How does inertia explain this?
The passengers' bodies tend to remain at rest while the bus moves forward.
The net force is not equal to zero
Unbalanced Forces
A bicycle moving forward slows down and eventually stops when the rider stops pedaling. Which unbalanced forces are mainly responsible for this change in motion?
Friction and air resistance.
A soccer ball and a bowling ball are both at rest. Which object has greater inertia, and why?
The bowling ball has greater inertia because it has greater mass.
A soccer ball lying on the field does not move until a player kicks it. How does this situation demonstrate inertia?
The ball resists changes in its state of rest and remains at rest until an external force (the kick) acts on it.
A student quickly pulls a sheet of paper from under a coin, causing the coin to drop straight into a cup. What property of matter is demonstrated?
Inertia. The coin tends to remain at rest while the paper is pulled away.
A vehicle has a constant velocity of 55 mph heading east. Are the net forces balanced or unbalanced in this state?
Balanced force
Two students are playing tug-of-war. One team pulls with a force of 500 N, while the other team pulls with a force of 350 N. What is the net force, and what will happen to the rope?
The net force is 150 N toward the stronger team, so the rope will move in that direction.
A loaded shopping cart and an empty shopping cart are pushed with the same force. Which cart will be harder to start moving, and what concept explains this?
The loaded shopping cart will be harder to start moving because it has greater mass and therefore greater inertia.
A passenger standing in a moving jeepney falls backward when the vehicle suddenly accelerates forward. How does inertia explain what happened?
The passenger’s body tends to remain at rest while the jeepney moves forward, causing the passenger to appear to fall backward relative to the vehicle.
Why is it generally harder to stop a fully loaded shopping cart than an empty one when both are moving at the same speed?
The loaded cart has more mass, giving it greater inertia and making it resist changes in motion more strongly.
Is the change with time of the position or orientation of an object.
Motion
A 2 kg cart is pushed to the right with a force of 20 N while friction exerts a force of 5 N to the left. Determine the net force acting on the cart and predict how its motion will change.
The net force is 15 N to the right (20 N − 5 N). The cart will accelerate to the right because the forces are unbalanced.
Two students are riding identical skateboards. Student A has a mass of 45 kg, while Student B has a mass of 65 kg. If both are moving at the same speed and try to stop suddenly, which student will be more difficult to stop? Explain using the relationship between mass and inertia.
Student B will be more difficult to stop because greater mass means greater inertia. Objects with greater inertia resist changes in their motion more strongly.
A magician quickly pulls a tablecloth from under several dishes without knocking them over. Explain why the dishes remain nearly in place using the concept of inertia.
The dishes tend to remain at rest due to inertia. Because the tablecloth is pulled quickly, only a small force acts on the dishes for a short time, so they stay nearly in their original position.
A delivery truck and a motorcycle are traveling at the same speed. Both drivers apply the brakes at the same time. Which vehicle will generally be more difficult to stop, and how does inertia explain this?
The delivery truck will generally be more difficult to stop because it has greater mass and therefore greater inertia, causing it to resist changes in motion more than the motorcycle.