Work in Physics Explained

Can you push against a wall for an hour and do zero work? In physics, yes, because nothing moved. Work only happens when a force moves an object through a distance. The amount depends on three things: how hard you push, how far the object goes, and the angle between the push and the motion. This guide explains each piece and shows why work can even be negative.

What Counts as Work When You Push, Pull or Lift?

Work counts only when a force moves an object, and only the part of the force along the motion counts. A push that moves a cart does work. A push that moves nothing does none, however tired your arms feel.

Picture sliding a heavy box across a garage floor. You push with 100 newtons, and the box slides 5 meters in the same direction. The work you did is 100 times 5, which is 500 joules.

That example uses the everyday meaning of effort, but physics keeps a stricter score. Holding a heavy bag still feels like work to your muscles. Physics says the bag did not move, so your force did no work on it.

The joule is the unit that keeps this score. NIST lists the joule as a newton times a meter, the SI unit for energy, work and heat. That shared unit is no accident, because doing work is how energy moves from one thing to another. For a side-by-side look at the joule and the food calorie, see our guide to joules vs calories.

Why Does the Angle Change How Much Work a Force Does?

Only the part of the force along the direction of motion does work. The cosine of the angle picks out that part. The full formula is W = F d cos(theta), where theta is the angle between force and motion.

Think about pulling a wagon by its handle. The handle tilts up, so part of your pull lifts the front of the wagon. Only the forward part of the pull moves it along the path.

Here is the math with a 50 newton pull over 10 meters. Straight ahead, at 0 degrees, the cosine is 1, so the work is 500 joules. At 60 degrees, the cosine is 0.5, so the same pull does only 250 joules.

To solve these without a table, the work calculator with an angle input finds the work, or the missing force or distance, from the other values.

When Is the Work Exactly Zero?

Work is zero in two cases. The object does not move, or the force points at a right angle to the motion. In both cases the force adds no energy along the path.

The first case is the wall from the opening. You can push with 300 newtons, but zero distance times any force is still zero. Your body burns energy inside the muscles, yet none of it moves the wall.

The second case surprises more people. Carry a 10 kilogram box across a flat room at steady height. Your arms push up with about 98.1 newtons, but the box moves sideways. The angle is 90 degrees, the cosine is 0, and the upward force does no work on the box.

Lifting that same box is different. Raising it 1.5 meters straight up means force and motion line up. The work is 98.1 times 1.5, or about 147 joules. That energy is now stored as height, a topic covered in our guide to kinetic vs potential energy.

What Makes Work Negative?

Work turns negative when the force points against the motion. The angle is then more than 90 degrees, so the cosine is negative. Negative work takes energy away from the moving object.

Friction is the most common example. A sliding crate feels a 20 newton friction force pointing backward. Over 5 meters of sliding, the angle is 180 degrees, so friction does 20 times 5 times -1, or -100 joules.

Brakes work the same way. The braking force points backward while the car rolls forward, so the brakes do negative work. That removed energy turns into heat in the brake parts.

Negative work is not an error or a missing value. It is a signal about direction, much like a negative bank entry. A positive number means the force fed energy in. A negative number means the force drained energy out.

How Does Work Change an Object’s Speed?

The net work on an object equals its change in kinetic energy. This rule is the work-energy theorem. Positive net work speeds an object up, and negative net work slows it down.

Net work means the total from every force, added with signs. Say you push a 10 kilogram cart and do 600 joules of work. Friction does -100 joules over the same stretch. The net work is 500 joules.

Starting from rest, the cart ends with 500 joules of kinetic energy. Kinetic energy is one half times mass times speed squared. Solving gives a speed of 10 meters per second, since one half of 10 times 10 squared is 500.

The theorem also runs backward. A moving car that stops has lost all its kinetic energy. The brakes and road friction did that much negative work on it, which is why faster cars need longer stopping distances.

How Is Work Different From Power?

Work is the energy a force transfers. Power is how fast that transfer happens. Power equals work divided by time, and its unit is the watt, one joule per second.

Two people can lift the same box to the same shelf and do the same work. The faster lifter uses more power. Lifting the 147 joule box in 3 seconds takes about 49 watts.

Use the earlier 500 joule push as a second check. Done in 10 seconds, it needs 50 watts. Done in 5 seconds, it needs 100 watts, twice the power for the same work.

FAQs About Work in Physics

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This article explains work for a constant force along a straight path; forces that change along a curved path need calculus. Reviewed for accuracy by Prof. Dr. Khalil Mudassar, PhD. Last updated September 27, 2026.